Debezium connector for MongoDB

Overview

MongoDB’s replication mechanism provides redundancy and high availability, and is the preferred way to run MongoDB in production. MongoDB connector captures the changes in a replica set or sharded cluster.

A MongoDB replica set consists of a set of servers that all have copies of the same data, and replication ensures that all changes made by clients to documents on the replica set’s primary are correctly applied to the other replica set’s servers, called secondaries. MongoDB replication works by having the primary record the changes in its oplog (or operation log), and then each of the secondaries reads the primary’s oplog and applies in order all of the operations to their own documents. When a new server is added to a replica set, that server first performs an snapshot of all of the databases and collections on the primary, and then reads the primary’s oplog to apply all changes that might have been made since it began the snapshot. This new server becomes a secondary (and able to handle queries) when it catches up to the tail of the primary’s oplog.

Change streams

Although the Debezium MongoDB connector does not become part of a replica set, it uses a similar replication mechanism to obtain oplog data. The main difference is that the connector does not read the oplog directly. Instead, it delegates the capture and decoding of oplog data to the MongoDB change streams feature. With change streams, the MongoDB server exposes the changes that occur in a collection as an event stream. The Debezium connector monitors the stream and then delivers the changes downstream. The first time that the connector detects a replica set, it examines the oplog to obtain the last recorded transaction, and then performs a snapshot of the primary’s databases and collections. After the connector finishes copying the data, it creates a change stream beginning from the oplog position that it read earlier.

As the MongoDB connector processes changes, it periodically records the position at which the event originated in the oplog stream. When the connector stops, it records the last oplog stream position that it processed, so that after a restart it can resume streaming from that position. In other words, the connector can be stopped, upgraded or maintained, and restarted some time later, and always pick up exactly where it left off without losing a single event. Of course, MongoDB oplogs are usually capped at a maximum size, so if the connector is stopped for long periods, operations in the oplog might be purged before the connector has a chance to read them. In this case, after a restart the connector detects the missing oplog operations, performs a snapshot, and then proceeds to stream changes.

The MongoDB connector is also quite tolerant of changes in membership and leadership of the replica sets, of additions or removals of shards within a sharded cluster, and network problems that might cause communication failures. The connector always uses the replica set’s primary node to stream changes, so when the replica set undergoes an election and a different node becomes primary, the connector will immediately stop streaming changes, connect to the new primary, and start streaming changes using the new primary node. Similarly, if connector is unable to communicate with the replica set primary, it attempts to reconnect (using exponential backoff so as to not overwhelm the network or replica set). After connection is reestablished, the connector continues to stream changes from the last event that it captured. In this way the connector dynamically adjusts to changes in replica set membership, and automatically handles communication disruptions.

Additional resources

Read Preference

You specify read preferences for a MongoDB connection by setting the readPreference parameter in the mongodb.connection.string.

How the MongoDB connector works

An overview of the MongoDB topologies that the connector supports is useful for planning your application.

When a MongoDB connector is configured and deployed, it starts by connecting to the MongoDB servers at the seed addresses, and determines the details about each of the available replica sets. Since each replica set has its own independent oplog, the connector will try to use a separate task for each replica set. The connector can limit the maximum number of tasks it will use, and if not enough tasks are available the connector will assign multiple replica sets to each task, although the task will still use a separate thread for each replica set.

When running the connector against a sharded cluster, use a value of tasks.max that is greater than the number of replica sets. This will allow the connector to create one task for each replica set, and will let Kafka Connect coordinate, distribute, and manage the tasks across all of the available worker processes.

Supported MongoDB topologies

The MongoDB connector supports the following MongoDB topologies:

MongoDB replica set

The Debezium MongoDB connector can capture changes from a single MongoDB replica set. Production replica sets require a minimum of at least three members.

To use the MongoDB connector with a replica set, you must set the value of the mongodb.connection.string property in the connector configuration to the replica set connection string. When the connector is ready to begin capturing changes from a MongoDB change stream, it starts a connection task. The connection task then uses the specified connection string to establish a connection to an available replica set member.

MongoDB sharded cluster

A MongoDB sharded cluster consists of:

  • One or more shards, each deployed as a replica set;

  • A separate replica set that acts as the cluster’s configuration server

  • One or more routers (also called mongos) to which clients connect and that routes requests to the appropriate shards

    To use the MongoDB connector with a sharded cluster, in the connector configuration, set the value of the mongodb.connection.string property to the sharded cluster connection string.

MongoDB standalone server

The MongoDB connector is not capable of monitoring the changes of a standalone MongoDB server, since standalone servers do not have an oplog. The connector will work if the standalone server is converted to a replica set with one member.

MongoDB does not recommend running a standalone server in production. For more information, see the MongoDB documentation.

Required user permissions

To capture data from MongoDB, Debezium attaches to the database as a MongoDB user. The MongoDB user account that you create for Debezium requires specific database permissions to read from the database. The connector user requires the following permissions:

  • Read from the database.

  • Run the hello command.

The connector user might also require the following permission:

  • Read from the config.shards system collection.

Database read permissions

The connector user must be able to read from all databases, or to read from a specific database, depending on the value of the connector’s capture.scope property. Assign one of the following permissions to the user, depending on the capture.scope setting:

capture.scope is set to deployment

Grant the user permission to read any database.

capture.scope is set to database

Grant the user permission to read the database specified by the connector’s capture.target property.

Permission to use the MongoDB hello command

Regardless of the capture.scope setting, the user requires permission to run the MongoDB hello command.

Permission to read the config.shards collection

Depending on your Debezium environment, to enable the connector to perform offset consolidation, you must grant the connector user explicit permission to read the config.shards collection. Permission to read the config.shards collection is required for the following connector environments:

  • Connectors upgraded from Debezium 2.5 or earlier.

  • Connectors configured to capture changes from a sharded MongoDB cluster.

Logical connector name

The connector configuration property topic.prefix serves as a logical name for the MongoDB replica set or sharded cluster. The connector uses the logical name in a number of ways: as the prefix for all topic names, and as a unique identifier when recording the change stream position of each replica set.

You should give each MongoDB connector a unique logical name that meaningfully describes the source MongoDB system. We recommend logical names begin with an alphabetic or underscore character, and remaining characters that are alphanumeric or underscore.

Offset consolidation

The Debezium MongoDB connector no longer supports replica_set connections to sharded MongoDB deployments. As a result, the offsets recorded by connector versions that used the replica_set connection mode are incompatible with the current version.

To minimize the affects of the connection mode change, and to prevent the connector from running unnecessary snapshots, when the connector restarts after the upgrade, it runs a procedure to consolidate offsets. During this offset consolidation procedure, the connector completes the following steps to reconcile offsets that were recorded by the earlier connector version:

  1. Offsets that were recorded by connector versions later than 2.5 are used as-is.

  2. Offsets for events that were captured in sharded connection mode from sharded MongoDB deployments, or from MongoDB replica set deployments, are used as-is.

  3. Shard-specific offsets that were recorded by connector versions 2.5.x and earlier are used as-is, if both of the following conditions apply:

    • The offsets exist for all current database shards.

    • Offset invalidation is enabled.
      If offset invalidation is disabled, the connector fails to start.

  4. After the connector processes existing offsets in the preceding steps, it resumes streaming changes, and then commits offsets for new events that it captures.
    If the offset consolidation procedure does not detect any existing offsets, the connector performs an initial snapshot.

Performing a snapshot

When a Debezium task starts to use a replica set, it uses the connector’s logical name and the replica set name to find an offset that describes the position where the connector previously stopped reading changes. If an offset can be found and it still exists in the oplog, then the task immediately proceeds with streaming changes, starting at the recorded offset position.

However, if no offset is found, or if the oplog no longer contains that position, the task must first obtain the current state of the replica set contents by performing a snapshot. This process starts by recording the current position of the oplog and recording that as the offset (along with a flag that denotes a snapshot has been started). The task then proceeds to copy each collection, spawning as many threads as possible (up to the value of the snapshot.max.threads configuration property) to perform this work in parallel. The connector records a separate read event for each document it sees. Each read event contains the object’s identifier, the complete state of the object, and source information about the MongoDB replica set where the object was found. The source information also includes a flag that denotes that the event was produced during a snapshot.

This snapshot will continue until it has copied all collections that match the connector’s filters. If the connector is stopped before the tasks’ snapshots are completed, upon restart the connector begins the snapshot again.

Try to avoid task reassignment and reconfiguration while the connector performs snapshots of any replica sets. The connector generates log messages to report on the progress of the snapshot. To provide for the greatest control, run a separate Kafka Connect cluster for each connector.

Table 1. Settings for snapshot.mode connector configuration property
SettingDescription

always

The connector performs a snapshot every time that it starts. After the snapshot completes, the connector begins to stream event records for subsequent database changes.

initial

The connector performs a database snapshot as described in the default workflow for creating an initial snapshot. After the snapshot completes, the connector begins to stream event records for subsequent database changes.

initial_only

The connector performs a database snapshot. After the snapshot completes, the connector stops, and does not stream event records for subsequent database changes.

never

Deprecated, see no_data.

no_data

The connector captures the structure of all relevant tables, performing all the steps described in the default snapshot workflow, except that it does not create READ events to represent the data set at the point of the connector’s start-up (Step 7.b).

when_needed

After the connector starts, it performs a snapshot only if it detects one of the following circumstances:

  • It cannot detect any topic offsets.

  • A previously recorded offset specifies a log position that is not available on the server.

configuration_based

Set the snapshot mode to configuration_based to control snapshot behavior through the set of connector properties that have the prefix ‘snapshot.mode.configuration.based’.

custom

The custom snapshot mode lets you inject your own implementation of the io.debezium.spi.snapshot.Snapshotter interface. Set the snapshot.mode.custom.name configuration property to the name provided by the name() method of your implementation. The name is specified on the classpath of your Kafka Connect cluster. If you use the Debezium EmbeddedEngine, the name is included in the connector JAR file. For more information, see custom snapshotter SPI.

For more information, see snapshot.mode in the table of connector configuration properties.

Ad hoc snapshots

By default, a connector runs an initial snapshot operation only after it starts for the first time. Following this initial snapshot, under normal circumstances, the connector does not repeat the snapshot process. Any future change event data that the connector captures comes in through the streaming process only.

However, in some situations the data that the connector obtained during the initial snapshot might become stale, lost, or incomplete. To provide a mechanism for recapturing collection data, Debezium includes an option to perform ad hoc snapshots. You might want to perform an ad hoc snapshot after any of the following changes occur in your Debezium environment:

  • The connector configuration is modified to capture a different set of collections.

  • Kafka topics are deleted and must be rebuilt.

  • Data corruption occurs due to a configuration error or some other problem.

You can re-run a snapshot for a collection for which you previously captured a snapshot by initiating a so-called ad-hoc snapshot. Ad hoc snapshots require the use of signaling collections. You initiate an ad hoc snapshot by sending a signal request to the Debezium signaling collection.

When you initiate an ad hoc snapshot of an existing collection, the connector appends content to the topic that already exists for the collection. If a previously existing topic was removed, Debezium can create a topic automatically if automatic topic creation is enabled.

Ad hoc snapshot signals specify the collections to include in the snapshot. The snapshot can capture the entire contents of the database, or capture only a subset of the collections in the database.

You specify the collections to capture by sending an execute-snapshot message to the signaling collection. Set the type of the execute-snapshot signal to incremental or blocking, and provide the names of the collections to include in the snapshot, as described in the following table:

Table 2. Example of an ad hoc execute-snapshot signal record
FieldDefaultValue

type

incremental

Specifies the type of snapshot that you want to run.
Currently, you can request incremental or blocking snapshots.

data-collections

N/A

An array that contains regular expressions matching the fully-qualified names of the collection to be snapshotted.
The format of the names is the same as for the signal.data.collection configuration option.

Triggering an ad hoc incremental snapshot

You initiate an ad hoc incremental snapshot by adding an entry with the execute-snapshot signal type to the signaling collection. After the connector processes the message, it begins the snapshot operation. The snapshot process reads the first and last primary key values and uses those values as the start and end point for each collection. Based on the number of entries in the collection, and the configured chunk size, Debezium divides the collection into chunks, and proceeds to snapshot each chunk, in succession, one at a time.

For more information, see Incremental snapshots.

Triggering an ad hoc blocking snapshot

You initiate an ad hoc blocking snapshot by adding an entry with the execute-snapshot signal type to the signaling collection. After the connector processes the message, it begins the snapshot operation. The connector temporarily stops streaming, and then initiates a snapshot of the specified collection, following the same process that it uses during an initial snapshot. After the snapshot completes, the connector resumes streaming.

For more information, see Blocking snapshots.

Incremental snapshots

To provide flexibility in managing snapshots, Debezium includes a supplementary snapshot mechanism, known as incremental snapshotting. Incremental snapshots rely on the Debezium mechanism for sending signals to a Debezium connector. Incremental snapshots are based on the DDD-3 design document.

In an incremental snapshot, instead of capturing the full state of a database all at once, as in an initial snapshot, Debezium captures each collection in phases, in a series of configurable chunks. You can specify the collections that you want the snapshot to capture and the size of each chunk. The chunk size determines the number of rows that the snapshot collects during each fetch operation on the database. The default chunk size for incremental snapshots is 1024 rows.

As an incremental snapshot proceeds, Debezium uses watermarks to track its progress, maintaining a record of each collection row that it captures. This phased approach to capturing data provides the following advantages over the standard initial snapshot process:

  • You can run incremental snapshots in parallel with streamed data capture, instead of postponing streaming until the snapshot completes. The connector continues to capture near real-time events from the change log throughout the snapshot process, and neither operation blocks the other.

  • If the progress of an incremental snapshot is interrupted, you can resume it without losing any data. After the process resumes, the snapshot begins at the point where it stopped, rather than recapturing the collection from the beginning.

  • You can run an incremental snapshot on demand at any time, and repeat the process as needed to adapt to database updates. For example, you might re-run a snapshot after you modify the connector configuration to add a collection to its collection.include.list property.

Incremental snapshot process

When you run an incremental snapshot, Debezium sorts each collection by primary key and then splits the collection into chunks based on the configured chunk size. Working chunk by chunk, it then captures each collection row in a chunk. For each row that it captures, the snapshot emits a READ event. That event represents the value of the row when the snapshot for the chunk began.

As a snapshot proceeds, it’s likely that other processes continue to access the database, potentially modifying collection records. To reflect such changes, INSERT, UPDATE, or DELETE operations are committed to the transaction log as per usual. Similarly, the ongoing Debezium streaming process continues to detect these change events and emits corresponding change event records to Kafka.

How Debezium resolves collisions among records with the same primary key

In some cases, the UPDATE or DELETE events that the streaming process emits are received out of sequence. That is, the streaming process might emit an event that modifies a collection row before the snapshot captures the chunk that contains the READ event for that row. When the snapshot eventually emits the corresponding READ event for the row, its value is already superseded. To ensure that incremental snapshot events that arrive out of sequence are processed in the correct logical order, Debezium employs a buffering scheme for resolving collisions. Only after collisions between the snapshot events and the streamed events are resolved does Debezium emit an event record to Kafka.

Snapshot window

To assist in resolving collisions between late-arriving READ events and streamed events that modify the same collection row, Debezium employs a so-called snapshot window. The snapshot windows demarcates the interval during which an incremental snapshot captures data for a specified collection chunk. Before the snapshot window for a chunk opens, Debezium follows its usual behavior and emits events from the transaction log directly downstream to the target Kafka topic. But from the moment that the snapshot for a particular chunk opens, until it closes, Debezium performs a de-duplication step to resolve collisions between events that have the same primary key..

For each data collection, the Debezium emits two types of events, and stores the records for them both in a single destination Kafka topic. The snapshot records that it captures directly from a table are emitted as READ operations. Meanwhile, as users continue to update records in the data collection, and the transaction log is updated to reflect each commit, Debezium emits UPDATE or DELETE operations for each change.

As the snapshot window opens, and Debezium begins processing a snapshot chunk, it delivers snapshot records to a memory buffer. During the snapshot windows, the primary keys of the READ events in the buffer are compared to the primary keys of the incoming streamed events. If no match is found, the streamed event record is sent directly to Kafka. If Debezium detects a match, it discards the buffered READ event, and writes the streamed record to the destination topic, because the streamed event logically supersede the static snapshot event. After the snapshot window for the chunk closes, the buffer contains only READ events for which no related transaction log events exist. Debezium emits these remaining READ events to the collection’s Kafka topic.

The connector repeats the process for each snapshot chunk.

Incremental snapshots require that the primary key for each table is stably ordered. Because String fields can include special characters, and are subject to different encodings, string-based primary keys do not lend themselves to sorting in a consistent and predictable order. When performing incremental snapshots, it’s best to set the primary key to a data type other than String.

For more information about BSON string types in MongoDB, see the MongoDB documentation).

Incremental snapshots for sharded clusters

To use incremental snapshots with sharded MongoDB clusters, you must set specific values for the following properties:

Triggering an incremental snapshot

Currently, the only way to initiate an incremental snapshot is to send an ad hoc snapshot signal to the signaling collection on the source database.

You submit a signal to the signaling collection by using the MongoDB insert() method.

After Debezium detects the change in the signaling collection, it reads the signal, and runs the requested snapshot operation.

The query that you submit specifies the collections to include in the snapshot, and, optionally, specifies the type of snapshot operation. Currently, the only valid options for snapshots operations are incremental`and `blocking.

To specify the collections to include in the snapshot, provide a data-collections array that lists the collections or an array of regular expressions used to match collections, for example,
{"data-collections": ["public.Collection1", "public.Collection2"]}

The data-collections array for an incremental snapshot signal has no default value. If the data-collections array is empty, Debezium detects that no action is required and does not perform a snapshot.

If the name of a collection that you want to include in a snapshot contains a dot (.) in the name of the database, schema, or table, to add the collection to the data-collections array, you must escape each part of the name in double quotes.

For example, to include a data collection that exists in the public database, and that has the name My.Collection, use the following format: “public”.”My.Collection”.

Prerequisites

Using a source signaling channel to trigger an incremental snapshot

  1. Insert a snapshot signal document into the signaling collection:

    1. <signalDataCollection>.insert({"id" : _<idNumber>,"type" : <snapshotType>, "data" : {"data-collections" ["<collectionName>", "<collectionName>"],"type": <snapshotType>}});

    For example,

    1. db.debeziumSignal.insert({ (1)
    2. "type" : "execute-snapshot", (2) (3)
    3. "data" : {
    4. "data-collections" ["\"public\".\"Collection1\"", "\"public\".\"Collection2\""], (4)
    5. "type": "incremental"} (5)
    6. });

    The values of the id,type, and data parameters in the command correspond to the fields of the signaling collection.

    The following table describes the parameters in the example:

    Table 3. Descriptions of fields in a MongoDB insert() command for sending an incremental snapshot signal to the signaling collection
    ItemValueDescription

    1

    db.debeziumSignal

    Specifies the fully-qualified name of the signaling collection on the source database.

    2

    null

    The _id parameter specifies an arbitrary string that is assigned as the id identifier for the signal request.
    The insert method in the preceding example omits use of the optional _id parameter. Because the document does not explicitly assign a value for the parameter, the arbitrary id that MongoDB automatically assigns to the document becomes the id identifier for the signal request.
    Use this string to identify logging messages to entries in the signaling collection. Debezium does not use this identifier string. Rather, during the snapshot, Debezium generates its own id string as a watermarking signal.

    3

    execute-snapshot

    Specifies type parameter specifies the operation that the signal is intended to trigger.

    4

    data-collections

    A required component of the data field of a signal that specifies an array of collection names or regular expressions to match collection names to include in the snapshot.
    The array lists regular expressions which match collections by their fully-qualified names, using the same format as you use to specify the name of the connector’s signaling collection in the signal.data.collection configuration property.

    5

    incremental

    An optional type component of the data field of a signal that specifies the type of snapshot operation to run.
    Currently supports the incremental and blocking types.
    If you do not specify a value, the connector runs an incremental snapshot.

The following example, shows the JSON for an incremental snapshot event that is captured by a connector.

Example: Incremental snapshot event message

  1. {
  2. "before":null,
  3. "after": {
  4. "pk":"1",
  5. "value":"New data"
  6. },
  7. "source": {
  8. ...
  9. "snapshot":"incremental" (1)
  10. },
  11. "op":"r", (2)
  12. "ts_ms":"1620393591654",
  13. "ts_us":"1620393591654962",
  14. "ts_ns":"1620393591654962147",
  15. "transaction":null
  16. }
ItemField nameDescription

1

snapshot

Specifies the type of snapshot operation to run.
Currently, the only valid options are blocking and incremental.
Specifying a type value in the SQL query that you submit to the signaling collection is optional.
If you do not specify a value, the connector runs an incremental snapshot.

2

op

Specifies the event type.
The value for snapshot events is r, signifying a READ operation.

Using the Kafka signaling channel to trigger an incremental snapshot

You can send a message to the configured Kafka topic to request the connector to run an ad hoc incremental snapshot.

The key of the Kafka message must match the value of the topic.prefix connector configuration option.

The value of the message is a JSON object with type and data fields.

The signal type is execute-snapshot, and the data field must have the following fields:

Table 4. Execute snapshot data fields
FieldDefaultValue

type

incremental

The type of the snapshot to be executed. Currently Debezium supports the incremental and blocking types.
See the next section for more details.

data-collections

N/A

An array of comma-separated regular expressions that match the fully-qualified names of tables to include in the snapshot.
Specify the names by using the same format as is required for the signal.data.collection configuration option.

additional-condition

N/A

An optional string that specifies a condition that the connector evaluates to designate a subset of records to include in a snapshot.

This property is deprecated and should be replaced by the additional-conditions property.

additional-conditions

N/A

An optional array of additional conditions that specifies criteria that the connector evaluates to designate a subset of records to include in a snapshot.
Each additional condition is an object that specifies the criteria for filtering the data that an ad hoc snapshot captures. You can set the following parameters for each additional condition: data-collection:: The fully-qualified name of the collection that the filter applies to. You can apply different filters to each collection. filter:: Specifies column values that must be present in a database record for the snapshot to include it, for example, “color=’blue’”.

The values that you assign to the filter parameter are the same types of values that you might specify in the WHERE clause of SELECT statements when you set the snapshot.select.statement.overrides property for a blocking snapshot. In earlier Debezium releases, an explicit filter parameter was not defined for snapshot signals; instead, filter criteria were implied by the values that were specified for the now deprecated additional-condition parameter.

An example of the execute-snapshot Kafka message:

  1. Key = `test_connector`
  2. Value = `{"type":"execute-snapshot","data": {"data-collections": ["schema1.table1", "schema1.table2"], "type": "INCREMENTAL"}}`

Ad hoc incremental snapshots with additional-conditions

Debezium uses the additional-conditions field to select a subset of a collection’s content.

Typically, when Debezium runs a snapshot, it runs a SQL query such as:

SELECT * FROM _<tableName>_ …​.

When the snapshot request includes an additional-conditions property, the data-collection and filter parameters of the property are appended to the SQL query, for example:

SELECT * FROM _<data-collection>_ WHERE _<filter>_ …​.

For example, given a products collection with the columns id (primary key), color, and brand, if you want a snapshot to include only content for which color='blue', when you request the snapshot, you could add the additional-conditions property to filter the content:

  1. Key = `test_connector`
  2. Value = `{"type":"execute-snapshot","data": {"data-collections": ["schema1.products"], "type": "INCREMENTAL", "additional-conditions": [{"data-collection": "schema1.products" ,"filter":"color='blue'"}]}}`

You can use the additional-conditions property to pass conditions based on multiple columns. For example, using the same products collection as in the previous example, if you want a snapshot to include only the content from the products collection for which color='blue', and brand='MyBrand', you could send the following request:

  1. Key = `test_connector`
  2. Value = `{"type":"execute-snapshot","data": {"data-collections": ["schema1.products"], "type": "INCREMENTAL", "additional-conditions": [{"data-collection": "schema1.products" ,"filter":"color='blue' AND brand='MyBrand'"}]}}`

Stopping an incremental snapshot

You can also stop an incremental snapshot by sending a signal to the collection on the source database. You submit a stop snapshot signal by inserting a document into the to the signaling collection. After Debezium detects the change in the signaling collection, it reads the signal, and stops the incremental snapshot operation if it’s in progress.

The query that you submit specifies the snapshot operation of incremental, and, optionally, the collections of the current running snapshot to be removed.

Prerequisites

Using a source signaling channel to stop an incremental snapshot

  1. Insert a stop snapshot signal document into the signaling collection:

    1. <signalDataCollection>.insert({"id" : _<idNumber>,"type" : "stop-snapshot", "data" : {"data-collections" ["<collectionName>", "<collectionName>"],"type": "incremental"}});

    For example,

    1. db.debeziumSignal.insert({ (1)
    2. "type" : "stop-snapshot", (2) (3)
    3. "data" : {
    4. "data-collections" ["\"public\".\"Collection1\"", "\"public\".\"Collection2\""], (4)
    5. "type": "incremental"} (5)
    6. });

    The values of the id, type, and data parameters in the signal command correspond to the fields of the signaling collection.

    The following table describes the parameters in the example:

    Table 5. Descriptions of fields in an insert command for sending a stop incremental snapshot document to the signaling collection
    ItemValueDescription

    1

    db.debeziumSignal

    Specifies the fully-qualified name of the signaling collection on the source database.

    2

    null

    The insert method in the preceding example omits use of the optional _id parameter. Because the document does not explicitly assign a value for the parameter, the arbitrary id that MongoDB automatically assigns to the document becomes the id identifier for the signal request.
    Use this string to identify logging messages to entries in the signaling collection. Debezium does not use this identifier string.

    3

    stop-snapshot

    The type parameter specifies the operation that the signal is intended to trigger.

    4

    data-collections

    An optional component of the data field of a signal that specifies an array of collection names or regular expressions to match collection names to remove from the snapshot.
    The array lists regular expressions which match collections by their fully-qualified names, using the same format as you use to specify the name of the connector’s signaling collection in the signal.data.collection configuration property. If this component of the data field is omitted, the signal stops the entire incremental snapshot that is in progress.

    5

    incremental

    A required component of the data field of a signal that specifies the type of snapshot operation that is to be stopped.
    Currently, the only valid option is incremental.
    If you do not specify a type value, the signal fails to stop the incremental snapshot.

Using the Kafka signaling channel to stop an incremental snapshot

You can send a signal message to the configured Kafka signaling topic to stop an ad hoc incremental snapshot.

The key of the Kafka message must match the value of the topic.prefix connector configuration option.

The value of the message is a JSON object with type and data fields.

The signal type is stop-snapshot, and the data field must have the following fields:

Table 6. Execute snapshot data fields
FieldDefaultValue

type

incremental

The type of the snapshot to be executed. Currently Debezium supports only the incremental type.
See the next section for more details.

data-collections

N/A

An optional array of comma-separated regular expressions that match the fully-qualified names of the tables to include in the snapshot.
Specify the names by using the same format as is required for the signal.data.collection configuration option.

The following example shows a typical stop-snapshot Kafka message:

  1. Key = `test_connector`
  2. Value = `{"type":"stop-snapshot","data": {"data-collections": ["schema1.table1", "schema1.table2"], "type": "INCREMENTAL"}}`

Custom snapshotter SPI

For more advanced uses, you can fine-tune control of the snapshot by implementing one of the following interfaces:

io.debezium.snapshot.spi.Snapshotter

Controls whether the connector takes a snapshot.

io.debezium.snapshot.spi.SnapshotQuery

Controls how data is queried during a snapshot.

io.debezium.snapshot.spi.SnapshotLock

Controls whether the connector locks tables when taking a snapshot.

io.debezium.snapshot.spi.Snapshotter interface. All built-in snapshot modes implement this interface.

  1. /**
  2. * {@link Snapshotter} is used to determine the following details about the snapshot process:
  3. * <p>
  4. * - Whether a snapshot occurs. <br>
  5. * - Whether streaming continues during the snapshot. <br>
  6. * - Whether the snapshot includes schema (if supported). <br>
  7. * - Whether to snapshot data or schema following an error.
  8. * <p>
  9. * Although Debezium provides many default snapshot modes,
  10. * to provide more advanced functionality, such as partial snapshots,
  11. * you can customize implementation of the interface.
  12. * For more information, see the documentation.
  13. *
  14. *
  15. *
  16. */
  17. @Incubating
  18. public interface Snapshotter extends Configurable {
  19. /**
  20. * @return the name of the snapshotter.
  21. *
  22. *
  23. */
  24. String name();
  25. /**
  26. * @param offsetExists is {@code true} when the connector has an offset context (i.e. restarted)
  27. * @param snapshotInProgress is {@code true} when the connector is started, but a snapshot is already in progress
  28. *
  29. * @return {@code true} if the snapshotter should take a data snapshot
  30. */
  31. boolean shouldSnapshotData(boolean offsetExists, boolean snapshotInProgress);
  32. /**
  33. * @param offsetExists is {@code true} when the connector has an offset context (i.e. restarted)
  34. * @param snapshotInProgress is {@code true} when the connector is started, but a snapshot is already in progress
  35. *
  36. * @return {@code true} if the snapshotter should take a schema snapshot
  37. */
  38. boolean shouldSnapshotSchema(boolean offsetExists, boolean snapshotInProgress);
  39. /**
  40. * @return {@code true} if the snapshotter should stream after taking a snapshot
  41. */
  42. boolean shouldStream();
  43. /**
  44. * @return {@code true} whether the schema can be recovered if database schema history is corrupted.
  45. */
  46. boolean shouldSnapshotOnSchemaError();
  47. /**
  48. * @return {@code true} whether the snapshot should be re-executed when there is a gap in data stream.
  49. */
  50. boolean shouldSnapshotOnDataError();
  51. /**
  52. *
  53. * @return {@code true} if streaming should resume from the start of the snapshot
  54. * transaction, or {@code false} for when a connector resumes and takes a snapshot,
  55. * streaming should resume from where streaming previously left off.
  56. */
  57. default boolean shouldStreamEventsStartingFromSnapshot() {
  58. return true;
  59. }
  60. /**
  61. * Lifecycle hook called after the snapshot phase is successful.
  62. */
  63. default void snapshotCompleted() {
  64. // no operation
  65. }
  66. /**
  67. * Lifecycle hook called after the snapshot phase is aborted.
  68. */
  69. default void snapshotAborted() {
  70. // no operation
  71. }
  72. }

io.debezium.snapshot.spi.SnapshotQuery interface. All built-in snapshot query modes implement this interface.

  1. /**
  2. * {@link SnapshotQuery} is used to determine the query used during a data snapshot
  3. *
  4. *
  5. */
  6. public interface SnapshotQuery extends Configurable, Service {
  7. /**
  8. * @return the name of the snapshot lock.
  9. *
  10. *
  11. */
  12. String name();
  13. /**
  14. * Generate a valid query string for the specified table, or an empty {@link Optional}
  15. * to skip snapshotting this table (but that table will still be streamed from)
  16. *
  17. * @param tableId the table to generate a query for
  18. * @param snapshotSelectColumns the columns to be used in the snapshot select based on the column
  19. * include/exclude filters
  20. * @return a valid query string, or none to skip snapshotting this table
  21. */
  22. Optional<String> snapshotQuery(String tableId, List<String> snapshotSelectColumns);
  23. }

io.debezium.snapshot.spi.SnapshotLock interface. All built-in snapshot lock modes implement this interface.

  1. /**
  2. * {@link SnapshotLock} is used to determine the table lock mode used during schema snapshot
  3. *
  4. *
  5. */
  6. public interface SnapshotLock extends Configurable, Service {
  7. /**
  8. * @return the name of the snapshot lock.
  9. *
  10. *
  11. */
  12. String name();
  13. /**
  14. * Returns a SQL statement for locking the given table during snapshotting, if required by the specific snapshotter
  15. * implementation.
  16. */
  17. Optional<String> tableLockingStatement(Duration lockTimeout, String tableId);
  18. }

Blocking snapshots

To provide more flexibility in managing snapshots, Debezium includes a supplementary ad hoc snapshot mechanism, known as a blocking snapshot. Blocking snapshots rely on the Debezium mechanism for sending signals to a Debezium connector.

A blocking snapshot behaves just like an initial snapshot, except that you can trigger it at run time.

You might want to run a blocking snapshot rather than use the standard initial snapshot process in the following situations:

  • You add a new collection and you want to complete the snapshot while the connector is running.

  • You add a large collection, and you want the snapshot to complete in less time than is possible with an incremental snapshot.

Blocking snapshot process

When you run a blocking snapshot, Debezium stops streaming, and then initiates a snapshot of the specified collection, following the same process that it uses during an initial snapshot. After the snapshot completes, the streaming is resumed.

Configure snapshot

You can set the following properties in the data component of a signal:

  • data-collections: to specify which collections must be snapshot

  • additional-conditions: You can specify different filters for different collection.

    • The data-collection property is the fully-qualified name of the collection for which the filter will be applied.

    • The filter property will have the same value used in the snapshot.select.statement.overrides

For example:

  1. {"type": "blocking", "data-collections": ["schema1.table1", "schema1.table2"], "additional-conditions": [{"data-collection": "schema1.table1", "filter": "SELECT * FROM [schema1].[table1] WHERE column1 = 0 ORDER BY column2 DESC"}, {"data-collection": "schema1.table2", "filter": "SELECT * FROM [schema1].[table2] WHERE column2 > 0"}]}

Possible duplicates

A delay might exist between the time that you send the signal to trigger the snapshot, and the time when streaming stops and the snapshot starts. As a result of this delay, after the snapshot completes, the connector might emit some event records that duplicate records captured by the snapshot.

Streaming changes

After the connector task for a replica set records an offset, it uses the offset to determine the position in the oplog where it should start streaming changes. The task then (depending on the configuration) either connects to the replica set’s primary node or connects to a replica-set-wide change stream and starts streaming changes from that position. It processes all of create, insert, and delete operations, and converts them into Debezium change events. Each change event includes the position in the oplog where the operation was found, and the connector periodically records this as its most recent offset. The interval at which the offset is recorded is governed by offset.flush.interval.ms, which is a Kafka Connect worker configuration property.

When the connector is stopped gracefully, the last offset processed is recorded so that, upon restart, the connector will continue exactly where it left off. If the connector’s tasks terminate unexpectedly, however, then the tasks may have processed and generated events after it last records the offset but before the last offset is recorded; upon restart, the connector begins at the last recorded offset, possibly generating some the same events that were previously generated just prior to the crash.

When all components in a Kafka pipeline operate nominally, Kafka consumers receive every message exactly once. However, when things go wrong, Kafka can only guarantee that consumers receive every message at least once. To avoid unexpected results, consumers must be able to handle duplicate messages.

As mentioned earlier, the connector tasks always use the replica set’s primary node to stream changes from the oplog, ensuring that the connector sees the most up-to-date operations as possible and can capture the changes with lower latency than if secondaries were to be used instead. When the replica set elects a new primary, the connector immediately stops streaming changes, connects to the new primary, and starts streaming changes from the new primary node at the same position. Likewise, if the connector experiences any problems communicating with the replica set members, it tries to reconnect, by using exponential backoff so as to not overwhelm the replica set, and once connected it continues streaming changes from where it last left off. In this way, the connector is able to dynamically adjust to changes in replica set membership and automatically handle communication failures.

To summarize, the MongoDB connector continues running in most situations. Communication problems might cause the connector to wait until the problems are resolved.

Pre-image support

In MongoDB 6.0 and later, you can configure change streams to emit the pre-image state of a document to populate the before field for MongoDB change events. To enable the use of pre-images in MongoDB, you must set the changeStreamPreAndPostImages for a collection by using db.createCollection(), create, or collMod. To enable the Debezium MongoDB to include pre-images in change events, set the capture.mode for the connector to one of the *_with_pre_image options.

Size limits on MongoDB change stream events

The size of a MongoDB change stream event is limited to 16 megabytes. The use of pre-images thus increases the likelihood of exceeding this threshold, which can lead to failures. For information about how to avoid exceeding the change stream limit, see the MongoDB documentation.

Topic names

The MongoDB connector writes events for all insert, update, and delete operations to documents in each collection to a single Kafka topic. The name of the Kafka topics always takes the form logicalName.databaseName.collectionName, where logicalName is the logical name of the connector as specified with the topic.prefix configuration property, databaseName is the name of the database where the operation occurred, and collectionName is the name of the MongoDB collection in which the affected document existed.

For example, consider a MongoDB replica set with an inventory database that contains four collections: products, products_on_hand, customers, and orders. If the connector monitoring this database were given a logical name of fulfillment, then the connector would produce events on these four Kafka topics:

  • fulfillment.inventory.products

  • fulfillment.inventory.products_on_hand

  • fulfillment.inventory.customers

  • fulfillment.inventory.orders

Notice that the topic names do not incorporate the replica set name or shard name. As a result, all changes to a sharded collection (where each shard contains a subset of the collection’s documents) all go to the same Kafka topic.

You can set up Kafka to auto-create the topics as they are needed. If not, then you must use Kafka administration tools to create the topics before starting the connector.

Partitions

The MongoDB connector does not make any explicit determination about how to partition topics for events. Instead, it allows Kafka to determine how to partition topics based on event keys. You can change Kafka’s partitioning logic by defining the name of the Partitioner implementation in the Kafka Connect worker configuration.

Kafka maintains total order only for events written to a single topic partition. Partitioning the events by key does mean that all events with the same key always go to the same partition. This ensures that all events for a specific document are always totally ordered.

Transaction Metadata

Debezium can generate events that represents transaction metadata boundaries and enrich change data event messages.

Limits on when Debezium receives transaction metadata

Debezium registers and receives metadata only for transactions that occur after you deploy the connector. Metadata for transactions that occur before you deploy the connector is not available.

For every transaction BEGIN and END, Debezium generates an event that contains the following fields:

status

BEGIN or END

id

String representation of unique transaction identifier.

event_count (for END events)

Total number of events emitted by the transaction.

data_collections (for END events)

An array of pairs of data_collection and event_count that provides number of events emitted by changes originating from given data collection.

The following example shows a typical message:

  1. {
  2. "status": "BEGIN",
  3. "id": "1462833718356672513",
  4. "event_count": null,
  5. "data_collections": null
  6. }
  7. {
  8. "status": "END",
  9. "id": "1462833718356672513",
  10. "event_count": 2,
  11. "data_collections": [
  12. {
  13. "data_collection": "rs0.testDB.collectiona",
  14. "event_count": 1
  15. },
  16. {
  17. "data_collection": "rs0.testDB.collectionb",
  18. "event_count": 1
  19. }
  20. ]
  21. }

Unless overridden via the topic.transaction option, transaction events are written to the topic named .transaction.

Change data event enrichment

When transaction metadata is enabled, the data message Envelope is enriched with a new transaction field. This field provides information about every event in the form of a composite of fields:

id

String representation of unique transaction identifier.

total_order

The absolute position of the event among all events generated by the transaction.

data_collection_order

The per-data collection position of the event among all events that were emitted by the transaction.

Following is an example of what a message looks like:

  1. {
  2. "after": "{\"_id\" : {\"$numberLong\" : \"1004\"},\"first_name\" : \"Anne\",\"last_name\" : \"Kretchmar\",\"email\" : \"annek@noanswer.org\"}",
  3. "source": {
  4. ...
  5. },
  6. "op": "c",
  7. "ts_ms": "1580390884335",
  8. "ts_us": "1580390884335486",
  9. "ts_ns": "1580390884335486281",
  10. "transaction": {
  11. "id": "1462833718356672513",
  12. "total_order": "1",
  13. "data_collection_order": "1"
  14. }
  15. }

Data change events

The Debezium MongoDB connector generates a data change event for each document-level operation that inserts, updates, or deletes data. Each event contains a key and a value. The structure of the key and the value depends on the collection that was changed.

Debezium and Kafka Connect are designed around continuous streams of event messages. However, the structure of these events may change over time, which can be difficult for consumers to handle. To address this, each event contains the schema for its content or, if you are using a schema registry, a schema ID that a consumer can use to obtain the schema from the registry. This makes each event self-contained.

The following skeleton JSON shows the basic four parts of a change event. However, how you configure the Kafka Connect converter that you choose to use in your application determines the representation of these four parts in change events. A schema field is in a change event only when you configure the converter to produce it. Likewise, the event key and event payload are in a change event only if you configure a converter to produce it. If you use the JSON converter and you configure it to produce all four basic change event parts, change events have this structure:

  1. {
  2. "schema": { (1)
  3. ...
  4. },
  5. "payload": { (2)
  6. ...
  7. },
  8. "schema": { (3)
  9. ...
  10. },
  11. "payload": { (4)
  12. ...
  13. },
  14. }
Table 7. Overview of change event basic content
ItemField nameDescription

1

schema

The first schema field is part of the event key. It specifies a Kafka Connect schema that describes what is in the event key’s payload portion. In other words, the first schema field describes the structure of the key for the document that was changed.

2

payload

The first payload field is part of the event key. It has the structure described by the previous schema field and it contains the key for the document that was changed.

3

schema

The second schema field is part of the event value. It specifies the Kafka Connect schema that describes what is in the event value’s payload portion. In other words, the second schema describes the structure of the document that was changed. Typically, this schema contains nested schemas.

4

payload

The second payload field is part of the event value. It has the structure described by the previous schema field and it contains the actual data for the document that was changed.

By default, the connector streams change event records to topics with names that are the same as the event’s originating collection. See topic names.

The MongoDB connector ensures that all Kafka Connect schema names adhere to the Avro schema name format. This means that the logical server name must start with a Latin letter or an underscore, that is, a-z, A-Z, or . Each remaining character in the logical server name and each character in the database and collection names must be a Latin letter, a digit, or an underscore, that is, a-z, A-Z, 0-9, or \. If there is an invalid character it is replaced with an underscore character.

This can lead to unexpected conflicts if the logical server name, a database name, or a collection name contains invalid characters, and the only characters that distinguish names from one another are invalid and thus replaced with underscores.

Change event keys

A change event’s key contains the schema for the changed document’s key and the changed document’s actual key. For a given collection, both the schema and its corresponding payload contain a single id field. The value of this field is the document’s identifier represented as a string that is derived from MongoDB extended JSON serialization strict mode.

Consider a connector with a logical name of fulfillment, a replica set containing an inventory database, and a customers collection that contains documents such as the following.

Example document

  1. {
  2. "_id": 1004,
  3. "first_name": "Anne",
  4. "last_name": "Kretchmar",
  5. "email": "annek@noanswer.org"
  6. }

Example change event key

Every change event that captures a change to the customers collection has the same event key schema. For as long as the customers collection has the previous definition, every change event that captures a change to the customers collection has the following key structure. In JSON, it looks like this:

  1. {
  2. "schema": { (1)
  3. "type": "struct",
  4. "name": "fulfillment.inventory.customers.Key", (2)
  5. "optional": false, (3)
  6. "fields": [ (4)
  7. {
  8. "field": "id",
  9. "type": "string",
  10. "optional": false
  11. }
  12. ]
  13. },
  14. "payload": { (5)
  15. "id": "1004"
  16. }
  17. }
Table 8. Description of change event key
ItemField nameDescription

1

schema

The schema portion of the key specifies a Kafka Connect schema that describes what is in the key’s payload portion.

2

fulfillment.inventory.customers.Key

Name of the schema that defines the structure of the key’s payload. This schema describes the structure of the key for the document that was changed. Key schema names have the format connector-name.database-name.collection-name.Key. In this example:

  • fulfillment is the name of the connector that generated this event.

  • inventory is the database that contains the collection that was changed.

  • customers is the collection that contains the document that was updated.

3

optional

Indicates whether the event key must contain a value in its payload field. In this example, a value in the key’s payload is required. A value in the key’s payload field is optional when a document does not have a key.

4

fields

Specifies each field that is expected in the payload, including each field’s name, type, and whether it is required.

5

payload

Contains the key for the document for which this change event was generated. In this example, the key contains a single id field of type string whose value is 1004.

This example uses a document with an integer identifier, but any valid MongoDB document identifier works the same way, including a document identifier. For a document identifier, an event key’s payload.id value is a string that represents the updated document’s original _id field as a MongoDB extended JSON serialization that uses strict mode. The following table provides examples of how different types of _id fields are represented.

Table 9. Examples of representing document _id fields in event key payloads
TypeMongoDB _id ValueKey’s payload

Integer

1234

{ “id” : “1234” }

Float

12.34

{ “id” : “12.34” }

String

“1234”

{ “id” : “\”1234\”” }

Document

{ “hi” : “kafka”, “nums” : [10.0, 100.0, 1000.0] }

{ “id” : “{\”hi\” : \”kafka\”, \”nums\” : [10.0, 100.0, 1000.0]}” }

ObjectId

ObjectId(“596e275826f08b2730779e1f”)

{ “id” : “{\”$oid\” : \”596e275826f08b2730779e1f\”}” }

Binary

BinData(“a2Fma2E=”,0)

{ “id” : “{\”$binary\” : \”a2Fma2E=\”, \”$type\” : \”00\”}” }

Change event values

The value in a change event is a bit more complicated than the key. Like the key, the value has a schema section and a payload section. The schema section contains the schema that describes the Envelope structure of the payload section, including its nested fields. Change events for operations that create, update or delete data all have a value payload with an envelope structure.

Consider the same sample document that was used to show an example of a change event key:

Example document

  1. {
  2. "_id": 1004,
  3. "first_name": "Anne",
  4. "last_name": "Kretchmar",
  5. "email": "annek@noanswer.org"
  6. }

The value portion of a change event for a change to this document is described for each event type:

create events

The following example shows the value portion of a change event that the connector generates for an operation that creates data in the customers collection:

  1. {
  2. "schema": { (1)
  3. "type": "struct",
  4. "fields": [
  5. {
  6. "type": "string",
  7. "optional": true,
  8. "name": "io.debezium.data.Json", (2)
  9. "version": 1,
  10. "field": "after"
  11. },
  12. {
  13. "type": "string",
  14. "optional": true,
  15. "name": "io.debezium.data.Json",
  16. "version": 1,
  17. "field": "patch"
  18. },
  19. {
  20. "type": "struct",
  21. "fields": [
  22. {
  23. "type": "string",
  24. "optional": false,
  25. "field": "version"
  26. },
  27. {
  28. "type": "string",
  29. "optional": false,
  30. "field": "connector"
  31. },
  32. {
  33. "type": "string",
  34. "optional": false,
  35. "field": "name"
  36. },
  37. {
  38. "type": "int64",
  39. "optional": false,
  40. "field": "ts_ms"
  41. },
  42. {
  43. "type": "int64",
  44. "optional": false,
  45. "field": "ts_us"
  46. },
  47. {
  48. "type": "int64",
  49. "optional": false,
  50. "field": "ts_ns"
  51. },
  52. {
  53. "type": "boolean",
  54. "optional": true,
  55. "default": false,
  56. "field": "snapshot"
  57. },
  58. {
  59. "type": "string",
  60. "optional": false,
  61. "field": "db"
  62. },
  63. {
  64. "type": "string",
  65. "optional": false,
  66. "field": "rs"
  67. },
  68. {
  69. "type": "string",
  70. "optional": false,
  71. "field": "collection"
  72. },
  73. {
  74. "type": "int32",
  75. "optional": false,
  76. "field": "ord"
  77. },
  78. {
  79. "type": "int64",
  80. "optional": true,
  81. "field": "h"
  82. }
  83. ],
  84. "optional": false,
  85. "name": "io.debezium.connector.mongo.Source", (3)
  86. "field": "source"
  87. },
  88. {
  89. "type": "string",
  90. "optional": true,
  91. "field": "op"
  92. },
  93. {
  94. "type": "int64",
  95. "optional": true,
  96. "field": "ts_ms"
  97. },
  98. {
  99. "type": "int64",
  100. "optional": true,
  101. "field": "ts_us"
  102. },
  103. {
  104. "type": "int64",
  105. "optional": true,
  106. "field": "ts_ns"
  107. }
  108. ],
  109. "optional": false,
  110. "name": "dbserver1.inventory.customers.Envelope" (4)
  111. },
  112. "payload": { (5)
  113. "after": "{\"_id\" : {\"$numberLong\" : \"1004\"},\"first_name\" : \"Anne\",\"last_name\" : \"Kretchmar\",\"email\" : \"annek@noanswer.org\"}", (6)
  114. "source": { (7)
  115. "version": "2.6.2.Final",
  116. "connector": "mongodb",
  117. "name": "fulfillment",
  118. "ts_ms": 1558965508000,
  119. "ts_ms": 1558965508000000,
  120. "ts_ms": 1558965508000000000,
  121. "snapshot": false,
  122. "db": "inventory",
  123. "rs": "rs0",
  124. "collection": "customers",
  125. "ord": 31,
  126. "h": 1546547425148721999
  127. },
  128. "op": "c", (8)
  129. "ts_ms": 1558965515240, (9)
  130. "ts_us": 1558965515240142, (10)
  131. "ts_ns": 1558965515240142879, (11)
  132. }
  133. }
Table 10. Descriptions of create event value fields
ItemField nameDescription

1

schema

The value’s schema, which describes the structure of the value’s payload. A change event’s value schema is the same in every change event that the connector generates for a particular collection.

2

name

In the schema section, each name field specifies the schema for a field in the value’s payload.

io.debezium.data.Json is the schema for the payload’s after, patch, and filter fields. This schema is specific to the customers collection. A create event is the only kind of event that contains an after field. An update event contains a filter field and a patch field. A delete event contains a filter field, but not an after field nor a patch field.

3

name

io.debezium.connector.mongo.Source is the schema for the payload’s source field. This schema is specific to the MongoDB connector. The connector uses it for all events that it generates.

4

name

dbserver1.inventory.customers.Envelope is the schema for the overall structure of the payload, where dbserver1 is the connector name, inventory is the database, and customers is the collection. This schema is specific to the collection.

5

payload

The value’s actual data. This is the information that the change event is providing.

It may appear that the JSON representations of the events are much larger than the documents they describe. This is because the JSON representation must include the schema and the payload portions of the message. However, by using the Avro converter, you can significantly decrease the size of the messages that the connector streams to Kafka topics.

6

after

An optional field that specifies the state of the document after the event occurred. In this example, the after field contains the values of the new document’s id, first_name, last_name, and email fields. The after value is always a string. By convention, it contains a JSON representation of the document. MongoDB oplog entries contain the full state of a document only for _create events and also for update events, when the capture.mode option is set to change_streams_update_full; in other words, a create event is the only kind of event that contains an after field regardless of capture.mode option.

7

source

Mandatory field that describes the source metadata for the event. This field contains information that you can use to compare this event with other events, with regard to the origin of the events, the order in which the events occurred, and whether events were part of the same transaction. The source metadata includes:

  • Debezium version.

  • Name of the connector that generated the event.

  • Logical name of the MongoDB replica set, which forms a namespace for generated events and is used in Kafka topic names to which the connector writes.

  • Names of the collection and database that contain the new document.

  • If the event was part of a snapshot.

  • Timestamp for when the change was made in the database and ordinal of the event within the timestamp.

  • Unique identifier of the MongoDB operation (the h field in the oplog event).

  • Unique identifiers of the MongoDB session lsid and transaction number txnNumber in case the change was executed inside a transaction (change streams capture mode only).

8

op

Mandatory string that describes the type of operation that caused the connector to generate the event. In this example, c indicates that the operation created a document. Valid values are:

  • c = create

  • u = update

  • d = delete

  • r = read (applies to only snapshots)

9

ts_ms

Optional field that displays the time at which the connector processed the event. The time is based on the system clock in the JVM running the Kafka Connect task.

In the source object, ts_ms indicates the time that the change was made in the database. By comparing the value for payload.source.ts_ms with the value for payload.ts_ms, you can determine the lag between the source database update and Debezium.

10

ts_us

Optional field that displays the time at which the connector processed the event, in microseconds. The time is based on the system clock in the JVM running the Kafka Connect task.

9

ts_ns

Optional field that displays the time at which the connector processed the event, in nanoseconds. The time is based on the system clock in the JVM running the Kafka Connect task.

update events

Change streams capture mode

The value of a change event for an update in the sample customers collection has the same schema as a create event for that collection. Likewise, the event value’s payload has the same structure. However, the event value payload contains different values in an update event. An update event includes an after value only if the capture.mode option is set to change_streams_update_full. A before value is provided if the capture.mode option is set to one of the *_with_pre_image option. There is a new structured field updateDescription with a few additional fields in this case:

  • updatedFields is a string field that contains the JSON representation of the updated document fields with their values

  • removedFields is a list of field names that were removed from the document

  • truncatedArrays is a list of arrays in the document that were truncated

Here is an example of a change event value in an event that the connector generates for an update in the customers collection:

  1. {
  2. "schema": { ... },
  3. "payload": {
  4. "op": "u", (1)
  5. "ts_ms": 1465491461815, (2)
  6. "ts_us": 1465491461815698, (2)
  7. "ts_ns": 1465491461815698142, (2)
  8. "before":"{\"_id\": {\"$numberLong\": \"1004\"},\"first_name\": \"unknown\",\"last_name\": \"Kretchmar\",\"email\": \"annek@noanswer.org\"}", (3)
  9. "after":"{\"_id\": {\"$numberLong\": \"1004\"},\"first_name\": \"Anne Marie\",\"last_name\": \"Kretchmar\",\"email\": \"annek@noanswer.org\"}", (4)
  10. "updateDescription": {
  11. "removedFields": null,
  12. "updatedFields": "{\"first_name\": \"Anne Marie\"}", (5)
  13. "truncatedArrays": null
  14. },
  15. "source": { (6)
  16. "version": "2.6.2.Final",
  17. "connector": "mongodb",
  18. "name": "fulfillment",
  19. "ts_ms": 1558965508000,
  20. "ts_us": 1558965508000000,
  21. "ts_ns": 1558965508000000000,
  22. "snapshot": false,
  23. "db": "inventory",
  24. "rs": "rs0",
  25. "collection": "customers",
  26. "ord": 1,
  27. "h": null,
  28. "tord": null,
  29. "stxnid": null,
  30. "lsid":"{\"id\": {\"$binary\": \"FA7YEzXgQXSX9OxmzllH2w==\",\"$type\": \"04\"},\"uid\": {\"$binary\": \"47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=\",\"$type\": \"00\"}}",
  31. "txnNumber":1
  32. }
  33. }
  34. }
Table 11. Descriptions of update event value fields
ItemField nameDescription

1

op

Mandatory string that describes the type of operation that caused the connector to generate the event. In this example, u indicates that the operation updated a document.

2

ts_ms, ts_us, ts_ns

Optional field that displays the time at which the connector processed the event. The time is based on the system clock in the JVM running the Kafka Connect task.

In the source object, ts_ms indicates the time that the change was made in the database. By comparing the value for payload.source.ts_ms with the value for payload.ts_ms, you can determine the lag between the source database update and Debezium.

3

before

Contains the JSON string representation of the actual MongoDB document before change. + An update event value does not contain an before field if the capture mode is not set to one of the *_with_preimage options.

4

after

Contains the JSON string representation of the actual MongoDB document.
An update event value does not contain an after field if the capture mode is not set to change_streams_update_full

5

updatedFields

Contains the JSON string representation of the updated field values of the document. In this example, the update changed the first_name field to a new value.

6

source

Mandatory field that describes the source metadata for the event. This field contains the same information as a create event for the same collection, but the values are different since this event is from a different position in the oplog. The source metadata includes:

  • Debezium version.

  • Name of the connector that generated the event.

  • Logical name of the MongoDB replica set, which forms a namespace for generated events and is used in Kafka topic names to which the connector writes.

  • Names of the collection and database that contain the updated document.

  • If the event was part of a snapshot.

  • Timestamp for when the change was made in the database and ordinal of the event within the timestamp.

  • Unique identifiers of the MongoDB session lsid and transaction number txnNumber in case the change was executed inside a transaction.

The after value in the event should be handled as the at-point-of-time value of the document. The value is not calculated dynamically but is obtained from the collection. It is thus possible if multiple updates are closely following one after the other, that all update updates events will contain the same after value which will be representing the last value stored in the document.

If your application depends on gradual change evolution then you should rely on updateDescription only.

delete events

The value in a delete change event has the same schema portion as create and update events for the same collection. The payload portion in a delete event contains values that are different from create and update events for the same collection. In particular, a delete event contains neither an after value nor a updateDescription value. Here is an example of a delete event for a document in the customers collection:

  1. {
  2. "schema": { ... },
  3. "payload": {
  4. "op": "d", (1)
  5. "ts_ms": 1465495462115, (2)
  6. "ts_us": 1465495462115748, (2)
  7. "ts_ns": 1465495462115748263, (2)
  8. "before":"{\"_id\": {\"$numberLong\": \"1004\"},\"first_name\": \"Anne Marie\",\"last_name\": \"Kretchmar\",\"email\": \"annek@noanswer.org\"}",(3)
  9. "source": { (4)
  10. "version": "2.6.2.Final",
  11. "connector": "mongodb",
  12. "name": "fulfillment",
  13. "ts_ms": 1558965508000,
  14. "ts_us": 1558965508000000,
  15. "ts_ns": 1558965508000000000,
  16. "snapshot": true,
  17. "db": "inventory",
  18. "rs": "rs0",
  19. "collection": "customers",
  20. "ord": 6,
  21. "h": 1546547425148721999
  22. }
  23. }
  24. }
Table 12. Descriptions of delete event value fields
ItemField nameDescription

1

op

Mandatory string that describes the type of operation. The op field value is d, signifying that this document was deleted.

2

ts_ms, ts_us. ts_ns

Optional field that displays the time at which the connector processed the event. The time is based on the system clock in the JVM running the Kafka Connect task.

In the source object, ts_ms indicates the time that the change was made in the database. By comparing the value for payload.source.ts_ms with the value for payload.ts_ms, you can determine the lag between the source database update and Debezium.

3

before

Contains the JSON string representation of the actual MongoDB document before change. + An update event value does not contain an before field if the capture mode is not set to one of the *_with_preimage options.

4

source

Mandatory field that describes the source metadata for the event. This field contains the same information as a create or update event for the same collection, but the values are different since this event is from a different position in the oplog. The source metadata includes:

  • Debezium version.

  • Name of the connector that generated the event.

  • Logical name of the MongoDB replica set, which forms a namespace for generated events and is used in Kafka topic names to which the connector writes.

  • Names of the collection and database that contained the deleted document.

  • If the event was part of a snapshot.

  • Timestamp for when the change was made in the database and ordinal of the event within the timestamp.

  • Unique identifier of the MongoDB operation (the h field in the oplog event).

  • Unique identifiers of the MongoDB session lsid and transaction number txnNumber in case the change was executed inside a transaction (change streams capture mode only).

MongoDB connector events are designed to work with Kafka log compaction. Log compaction enables removal of some older messages as long as at least the most recent message for every key is kept. This lets Kafka reclaim storage space while ensuring that the topic contains a complete data set and can be used for reloading key-based state.

Tombstone events

All MongoDB connector events for a uniquely identified document have exactly the same key. When a document is deleted, the delete event value still works with log compaction because Kafka can remove all earlier messages that have that same key. However, for Kafka to remove all messages that have that key, the message value must be null. To make this possible, after Debezium’s MongoDB connector emits a delete event, the connector emits a special tombstone event that has the same key but a null value. A tombstone event informs Kafka that all messages with that same key can be removed.

Setting up MongoDB

The MongoDB connector uses MongoDB’s change streams to capture the changes, so the connector works only with MongoDB replica sets or with sharded clusters where each shard is a separate replica set. See the MongoDB documentation for setting up a replica set or sharded cluster. Also, be sure to understand how to enable access control and authentication with replica sets.

You must also have a MongoDB user that has the appropriate roles to read the admin database where the oplog can be read. Additionally, the user must also be able to read the config database in the configuration server of a sharded cluster and must have listDatabases privilege action. When change streams are used (the default) the user also must have cluster-wide privilege actions find and changeStream.

When you intend to utilize pre-image and populate the before field, you need to first enable changeStreamPreAndPostImages for a collection using db.createCollection(), create, or collMod.

MongoDB in the Cloud

You can use the Debezium connector for MongoDB with MongoDB Atlas. Note that MongoDB Atlas only supports secure connections via SSL, i.e. the +mongodb.ssl.enabled connector option must be set to true.

Optimal Oplog Config

The Debezium MongoDB connector reads change streams to obtain oplog data for a replica set. Because the oplog is a fixed-sized, capped collection, if it exceeds its maximum configured size, it begins to overwrite its oldest entries. If the connector is stopped for any reason, when it restarts, it attempts to resume streaming from the last oplog stream position. However, if last stream position was removed from the oplog, depending on the value specified in the connector’s snapshot.mode property, the connector might fail to start, reporting an invalid resume token error. In the event of a failure, you must create a new connector to enable Debezium to continue capturing records from the database. For more information, see Connector fails after it is stopped for a long interval if snapshot.mode is set to initial.

To ensure that the oplog retains the offset values that Debezium requires to resume streaming, you can use either of the following approaches:

  • Increase the size of the oplog. Based on your typical workloads, set the oplog size to a value that is greater than the peak number of oplog entries per hour.

  • Increase the minimum number of hours that an oplog entry is retained (MongoDB 4.4 and greater). This setting is time-based, such that entries in the last n hours are guaranteed to be available even if the oplog reaches its maximum configured size. Although this is generally the preferred option, for clusters with high workloads that are nearing capacity, specify the maximum oplog size.

To help prevent failures that are related to missing oplog entries, it’s important to track metrics that report replication behavior, and to optimize the oplog size to support Debezium. In particular, you should monitor the values of Oplog GB/Hour and Replication Oplog Window. If Debezium is offline for an interval that exceeds the value of the replication oplog window, and the primary oplog grows faster than Debezium can consume entries, a connector failure can result.

For information about how to monitor these metrics, see the MongoDB documentation.

It’s best to set the maximum oplog size to a value that is based on the anticipated hourly growth of the oplog (Oplog GB/Hour), multiplied by the time that might be required to address a Debezium failure.

That is,

_Oplog GB/Hour_ X _average reaction time to Debezium failure_

For example, if the oplog size limit is set to 1GB, and the oplog grows by 3GB per hour, oplog entries are cleared three times per hour. If Debezium were to fail during this time, its last oplog position is likely to be removed.

If the oplog grows at the rate of 3GB/hour, and Debezium is offline for two hours, you would thus set the oplog size to 3GB/hour X 2 hours, or 6GB.

Deployment

To deploy a Debezium MongoDB connector, you install the Debezium MongoDB connector archive, configure the connector, and start the connector by adding its configuration to Kafka Connect.

Prerequisites

Procedure

  1. Download the connector’s plug-in archive,

  2. Extract the JAR files into your Kafka Connect environment.

  3. Add the directory with the JAR files to Kafka Connect’s plugin.path.

  4. Restart your Kafka Connect process to pick up the new JAR files.

If you are working with immutable containers, see Debezium’s Container images for Apache Zookeeper, Apache Kafka, and Kafka Connect with the MongoDB connector already installed and ready to run.

You can also run Debezium on Kubernetes and OpenShift.

The Debezium tutorial walks you through using these images, and this is a great way to learn about Debezium.

MongoDB connector configuration example

Following is an example of the configuration for a connector instance that captures data from a MongoDB replica set rs0 at port 27017 on 192.168.99.100, which we logically name fullfillment. Typically, you configure the Debezium MongoDB connector in a JSON file by setting the configuration properties that are available for the connector.

You can choose to produce events for a particular MongoDB replica set or sharded cluster. Optionally, you can filter out collections that are not needed.

  1. {
  2. "name": "inventory-connector", (1)
  3. "config": {
  4. "connector.class": "io.debezium.connector.mongodb.MongoDbConnector", (2)
  5. "mongodb.connection.string": "mongodb://192.168.99.100:27017/?replicaSet=rs0", (3)
  6. "topic.prefix": "fullfillment", (4)
  7. "collection.include.list": "inventory[.]*" (5)
  8. }
  9. }
1The name of our connector when we register it with a Kafka Connect service.
2The name of the MongoDB connector class.
3The connection string to use to connect to the MongoDB replica set.
4The logical name of the MongoDB replica set, which forms a namespace for generated events and is used in all the names of the Kafka topics to which the connector writes, the Kafka Connect schema names, and the namespaces of the corresponding Avro schema when the Avro converter is used.
5A list of regular expressions that match the collection namespaces (for example, <dbName>.<collectionName>) of all collections to be monitored. This is optional.

For the complete list of the configuration properties that you can set for the Debezium MongoDB connector, see MongoDB connector configuration properties.

You can send this configuration with a POST command to a running Kafka Connect service. The service records the configuration and starts one connector task that performs the following actions:

  • Connects to the MongoDB replica set or sharded cluster.

  • Assigns tasks for each replica set.

  • Performs a snapshot, if necessary.

  • Reads the change stream.

  • Streams change event records to Kafka topics.

Adding connector configuration

To start running a Debezium MongoDB connector, create a connector configuration, and add the configuration to your Kafka Connect cluster.

Prerequisites

Procedure

  1. Create a configuration for the MongoDB connector.

  2. Use the Kafka Connect REST API to add that connector configuration to your Kafka Connect cluster.

Results

After the connector starts, it completes the following actions:

  • Performs a consistent snapshot of the collections in your MongoDB replica sets.

  • Reads the change streams for the replica sets.

  • Produces change events for every inserted, updated, and deleted document.

  • Streams change event records to Kafka topics.

Connector properties

The Debezium MongoDB connector has numerous configuration properties that you can use to achieve the right connector behavior for your application. Many properties have default values. Information about the properties is organized as follows:

The following configuration properties are required unless a default value is available.

Table 13. Required Debezium MongoDB connector configuration properties
PropertyDefaultDescription

false

Set this property to true to enable the connector to invalidate and consolidate shard-specific offsets that were recorded by earlier connector versions.

This property permits you to modify the current default behavior. The property is subject to removal in a future release if the default behavior changes to permit the connector to automatically invalidate and consolidate offsets that are recorded by earlier connector versions.

No default

Unique name for the connector. Attempting to register again with the same name will fail. (This property is required by all Kafka Connect connectors.)

No default

The name of the Java class for the connector. Always use a value of io.debezium.connector.mongodb.MongoDbConnector for the MongoDB connector.

No default

Specifies a connection string that the connector uses to connect to a MongoDB replica set. This property replaces the mongodb.hosts property that was available in previous versions of the MongoDB connector.

No default

A unique name that identifies the connector and/or MongoDB replica set or sharded cluster that this connector monitors. Each server should be monitored by at most one Debezium connector, since this server name prefixes all persisted Kafka topics emanating from the MongoDB replica set or cluster. Use only alphanumeric characters, hyphens, dots and underscores to form the name. The logical name should be unique across all other connectors, because the name is used as the prefix in naming the Kafka topics that receive records from this connector.

Do not change the value of this property. If you change the name value, after a restart, instead of continuing to emit events to the original topics, the connector emits subsequent events to topics whose names are based on the new value.

DefaultMongoDbAuthProvider

A full Java class name that is an implementation of the io.debezium.connector.mongodb.connection.MongoDbAuthProvider interface. This class handles setting the credentials on the MongoDB connection (called on each app boot). Default behavior uses the mongodb.user, mongodb.password, and mongodb.authsource properties according to each of their documentation, but other implementations may use them differently or ignore them altogether. Note that any setting in mongodb.connection.string will override settings set by this class

No default

When using default mongodb.authentication.class: Name of the database user to be used when connecting to MongoDB. This is required only when MongoDB is configured to use authentication.

No default

When using default mongodb.authentication.class: Password to be used when connecting to MongoDB. This is required only when MongoDB is configured to use authentication.

admin

When using default mongodb.authentication.class: Database (authentication source) containing MongoDB credentials. This is required only when MongoDB is configured to use authentication with another authentication database than admin.

false

Connector will use SSL to connect to MongoDB instances.

false

When SSL is enabled this setting controls whether strict hostname checking is disabled during connection phase. If true the connection will not prevent man-in-the-middle attacks.

regex

The mode used to match events based on included/excluded database and collection names. Set the property to one of the following values:

    regex

    Database and collection includes/excludes are evaluated as comma-separated list of regular expressions.

    literal

    Database and collection includes/excludes are evaluated as comma-separated list of string literals. Whitespace characters surrounding these literals are stripped.

empty string

An optional comma-separated list of regular expressions or literals that match database names to be monitored. By default, all databases are monitored.
When database.include.list is set, the connector monitors only the databases that the property specifies. Other databases are excluded from monitoring.

To match the name of a database, Debezium performs one of the following actions based on the value of filters.match.mode property

  • applies the regular expression that you specify as an anchored regular expression. That is, the specified expression is matched against the entire name string of the database; it does not match substrings that might be present in a database name.

  • compares the literals that you specify with the entire name string of the database

If you include this property in the configuration, do not also set the database.exclude.list property.

empty string

An optional comma-separated list of regular expressions or literals that match database names to be excluded from monitoring. When database.exclude.list is set, the connector monitors every database except the ones that the property specifies.

To match the name of a database, Debezium performs one of the following actions based on the value of filters.match.mode property

  • applies the regular expression that you specify as an anchored regular expression. That is, the specified expression is matched against the entire name string of the database; it does not match substrings that might be present in a database name.

  • compares the literals that you specify with the entire name string of the database

If you include this property in the configuration, do not set the database.include.list property.

empty string

An optional comma-separated list of regular expressions or literals that match fully-qualified namespaces for MongoDB collections to be monitored. By default, the connector monitors all collections except those in the local and admin databases. When collection.include.list is set, the connector monitors only the collections that the property specifies. Other collections are excluded from monitoring. Collection identifiers are of the form databaseName.collectionName.

To match the name of a namespace, Debezium performs one of the following actions based on the value of filters.match.mode property

  • applies the regular expression that you specify as an anchored regular expression. That is, the specified expression is matched against the entire name string of the namespace; it does not match substrings in the name.

  • compares the literals that you specify with the entire name string of the namespace

If you include this property in the configuration, do not also set the collection.exclude.list property.

empty string

An optional comma-separated list of regular expressions or literals that match fully-qualified namespaces for MongoDB collections to be excluded from monitoring. When collection.exclude.list is set, the connector monitors every collection except the ones that the property specifies. Collection identifiers are of the form databaseName.collectionName.

To match the name of a namespace, Debezium performs one of the following actions based on the value of filters.match.mode property

  • applies the regular expression that you specify as an anchored regular expression. That is, the specified expression is matched against the entire name string of the namespace; it does not match substrings that might be present in a database name.

  • compares the literals that you specify with the entire name string of the namespace

If you include this property in the configuration, do not set the collection.include.list property.

changestreams_update_full

Specifies the method that the connector uses to capture update event changes from a MongoDB server. Set this property to one of the following values:

    change_streams

    update event messages do not include the full document. Messages do not include a field that represents the state of the document before the change.

    change_streams_update_full

    update event messages include the full document. Messages do not include a before field that represents the state of the document before the update. The event message returns the full state of the document in the after field. Set capture.mode.full.update.type to specify how the connector fetches full documents from the database.

    In some situations, when capture.mode is configured to return full documents, the updateDescription and after fields of the update event message might report inconsistent values. Such discrepancies can result after multiple updates are applied to a document in rapid succession. The connector requests the full document from the MongoDB database only after it receives the update described in the event’s updateDescription field. If a later update modifies the source document before the connector can retrieve it from the database, the connector receives the document that is modified by this later update.

    change_streams_update_full_with_pre_image

    update event event messages include the full document, and include a field that represents the state of the document before the change. Set capture.mode.full.update.type to specify how the connector fetches full documents from the database.

    change_streams_with_pre_image

    update events do not include the full document, but include a field that represents the state of the document before the change.

deployment

Specifies the scope of the change streams that the connector opens. Set this property to one of the following values:

    deployment

    Opens a change stream cursor for a deployment (either a replica set or a sharded cluster) to watch for changes to all non-system collections across all databases, except for admin, local, and config.

    database

    Opens a change stream cursor for a single database to watch for changes to all of its non-system collections.

    To support Debezium signaling, if you set capture.scope to database, the signaling data collection must reside in a database that is specified by the capture.target property.

Specifies the database that the connector monitors for changes. This property applies only if the capture.scope is set to database.

empty string

An optional comma-separated list of the fully-qualified names of fields that should be excluded from change event message values. Fully-qualified names for fields are of the form databaseName.collectionName.fieldName.nestedFieldName, where databaseName and collectionName may contain the wildcard () which matches any characters.

empty string

An optional comma-separated list of the fully-qualified replacements of fields that should be used to rename fields in change event message values. Fully-qualified replacements for fields are of the form databaseName.collectionName.fieldName.nestedFieldName:newNestedFieldName, where databaseName and collectionName may contain the wildcard () which matches any characters, the colon character (:) is used to determine rename mapping of field. The next field replacement is applied to the result of the previous field replacement in the list, so keep this in mind when renaming multiple fields that are in the same path.

true

Controls whether a delete event is followed by a tombstone event.

true - a delete operation is represented by a delete event and a subsequent tombstone event.

false - only a delete event is emitted.

After a source record is deleted, emitting a tombstone event (the default behavior) allows Kafka to completely delete all events that pertain to the key of the deleted row in case log compaction is enabled for the topic.

none

Specifies how schema names should be adjusted for compatibility with the message converter used by the connector. Possible settings:

  • none does not apply any adjustment.

  • avro replaces the characters that cannot be used in the Avro type name with underscore.

  • avro_unicode replaces the underscore or characters that cannot be used in the Avro type name with corresponding unicode like _uxxxx. Note: is an escape sequence like backslash in Java

none

Specifies how field names should be adjusted for compatibility with the message converter used by the connector. Possible settings:

  • none does not apply any adjustment.

  • avro replaces the characters that cannot be used in the Avro type name with underscore.

  • avrounicode replaces the underscore or characters that cannot be used in the Avro type name with corresponding unicode like _uxxxx. Note: is an escape sequence like backslash in Java

See Avro naming for more details.

The following advanced configuration properties have good defaults that will work in most situations and therefore rarely need to be specified in the connector’s configuration.

Table 14. Debezium MongoDB connector advanced configuration properties
PropertyDefaultDescription

lookup

Specifies how the connector looks up the full value of an updated document when the capture.mode is set retrieve full documents. The connector retrieves full documents when its capture.mode is set to one of the following options:

  • change_streams_update_full

  • change_streams_update_full_with_pre-image

To use this option with a MongoDB change streams collection, you must configure the collection to return document pre- and post-images. Pre- and post-images for an operation are available only if the required configuration is in place before the operation occurs.

Set this property to one of the following values:

    lookup

    The connector uses a separate lookup to fetch the updated full MongoDB document.

    post_image

    The connector uses MongoDB post images to populate events with the full MongoDB document. The database must be running MongoDB 6.0 or later to use this option.

2048

Positive integer value that specifies the maximum size of each batch of events that should be processed during each iteration of this connector. Defaults to 2048.

8192

Positive integer value that specifies the maximum number of records that the blocking queue can hold. When Debezium reads events streamed from the database, it places the events in the blocking queue before it writes them to Kafka. The blocking queue can provide backpressure for reading change events from the database in cases where the connector ingests messages faster than it can write them to Kafka, or when Kafka becomes unavailable. Events that are held in the queue are disregarded when the connector periodically records offsets. Always set the value of max.queue.size to be larger than the value of max.batch.size.

0

A long integer value that specifies the maximum volume of the blocking queue in bytes. By default, volume limits are not specified for the blocking queue. To specify the number of bytes that the queue can consume, set this property to a positive long value.
If max.queue.size is also set, writing to the queue is blocked when the size of the queue reaches the limit specified by either property. For example, if you set max.queue.size=1000, and max.queue.size.in.bytes=5000, writing to the queue is blocked after the queue contains 1000 records, or after the volume of the records in the queue reaches 5000 bytes.

1000

Positive integer value that specifies the number of milliseconds the connector should wait during each iteration for new change events to appear. Defaults to 500 milliseconds, or 0.5 second.

1000

Positive integer value that specifies the initial delay when trying to reconnect to a primary after the first failed connection attempt or when no primary is available. Defaults to 1 second (1000 ms).

1000

Positive integer value that specifies the maximum delay when trying to reconnect to a primary after repeated failed connection attempts or when no primary is available. Defaults to 120 seconds (120,000 ms).

16

Positive integer value that specifies the maximum number of failed connection attempts to a replica set primary before an exception occurs and task is aborted. Defaults to 16, which with the defaults for connect.backoff.initial.delay.ms and connect.backoff.max.delay.ms results in just over 20 minutes of attempts before failing.

v2

Schema version for the source block in CDC events. Debezium 0.10 introduced a few breaking
changes to the structure of the source block in order to unify the exposed structure across all the connectors.
By setting this option to v1 the structure used in earlier versions can be produced. Note that this setting is not recommended and is planned for removal in a future Debezium version.

0

Controls how frequently heartbeat messages are sent.
This property contains an interval in milliseconds that defines how frequently the connector sends messages into a heartbeat topic. This can be used to monitor whether the connector is still receiving change events from the database. You also should leverage heartbeat messages in cases where only records in non-captured collections are changed for a longer period of time. In such situation the connector would proceed to read the oplog/change stream from the database but never emit any change messages into Kafka, which in turn means that no offset updates are committed to Kafka. This will cause the oplog files to be rotated out but connector will not notice it so on restart some events are no longer available which leads to the need of re-execution of the initial snapshot.

Set this parameter to 0 to not send heartbeat messages at all.
Disabled by default.

t

A comma-separated list of operation types that will be skipped during streaming. The operations include: c for inserts/create, u for updates/replace, d for deletes, t for truncates, and none to not skip any aforementioned operations. By default, for consistency with other Debezium connectors, truncate operations are skipped (not emitted by this connector). However, since MongoDB does not support truncate change events, this is effectively the same as specifying none.

No default

Controls which collection items are included in snapshot. This property affects snapshots only. Specify a comma-separated list of collection names in the form databaseName.collectionName.

For each collection that you specify, also specify another configuration property: snapshot.collection.filter.overrides.databaseName.collectionName. For example, the name of the other configuration property might be: snapshot.collection.filter.overrides.customers.orders. Set this property to a valid filter expression that retrieves only the items that you want in the snapshot. When the connector performs a snapshot, it retrieves only the items that matches the filter expression.

No default

An interval in milliseconds that the connector should wait before taking a snapshot after starting up;
Can be used to avoid snapshot interruptions when starting multiple connectors in a cluster, which may cause re-balancing of connectors.

0

Specifies the maximum number of documents that should be read in one go from each collection while taking a snapshot. The connector will read the collection contents in multiple batches of this size.
Defaults to 0, which indicates that the server chooses an appropriate fetch size.

All collections specified in collection.include.list

An optional, comma-separated list of regular expressions that match the fully-qualified names (<databaseName>.<collectionName>) of the schemas that you want to include in a snapshot. The specified items must be named in the connectors’s collection.include.list property. This property takes effect only if the connector’s snapshot.mode property is set to a value other than never.
This property does not affect the behavior of incremental snapshots.

To match the name of a schema, Debezium applies the regular expression that you specify as an anchored regular expression. That is, the specified expression is matched against the entire name string of the schema; it does not match substrings that might be present in a schema name.

1

Positive integer value that specifies the maximum number of threads used to perform an intial sync of the collections in a replica set. Defaults to 1.

initial

Specifies the criteria for performing a snapshot when the connector starts. Set the property to one of the following values:

    always

    The connector performs a snapshot every time that it starts. The snapshot includes the structure and data of the captured tables. Specify this value to populate topics with a complete representation of the data from the captured tables every time that the connector starts. After the snapshot completes, the connector begins to stream event records for subsequent database changes.

    initial

    The connector performs a database snapshot as described in the default workflow for creating an initial snapshot. After the snapshot completes, the connector begins to stream event records for subsequent database changes.

    initial_only

    The connector performs a database a snapshot only when no offsets have been recorded for the logical server name. After the snapshot completes, the connector stops. It does not transition to streaming event records for subsequent database changes.

    never

    Deprecated, see no_data.

    no_data

    The connector runs a snapshot that captures the structure of all relevant tables, performing all the steps described in the default snapshot workflow, except that it does not create READ events to represent the data set at the point of the connector’s start-up (Step 7.b).

    when_needed

    After the connector starts, it performs a snapshot only if it detects one of the following circumstances:

    • It cannot detect any topic offsets.

    • A previously recorded offset specifies a log position that is not available on the server.

    configuration_based

    With this option, you control snapshot behavior through a set of connector properties that have the prefix ‘snapshot.mode.configuration.based’.

    custom

    The custom snapshot mode lets you inject your own implementation of the io.debezium.spi.snapshot.Snapshotter interface. Set the snapshot.mode.custom.name configuration property to the name provided by the name() method of your implementation.

For more information, see custom snapshotter SPI.

false

If the snapshot.mode is set to configuration_based, set this property to specify whether the connector includes table data when it performs a snapshot.

false

If the snapshot.mode is set to configuration_based, set this property to specify whether the connector includes the table schema when it performs a snapshot.

false

If the snapshot.mode is set to configuration_based, set this property to specify whether the connector begins to stream change events after a snapshot completes.

false

If the snapshot.mode is set to configuration_based, set this property to specify whether the connector includes table schema in a snapshot if the schema history topic is not available.

false

If the snapshot.mode is set to configuration_based, this property specifies whether the connector attempts to snapshot table data if it does not find the last committed offset in the transaction log.
Set the value to true to instruct the connector to perform a new snapshot.

No default

If snapshot.mode is set to custom, use this setting to specify the name of the custom implementation that is provided in the name() method that is defined in the ‘io.debezium.spi.snapshot.Snapshotter’ interface. After a connector restart, Debezium calls the specified custom implementation to determine whether to perform a snapshot. For more information, see custom snapshotter SPI.

false

When set to true Debezium generates events with transaction boundaries and enriches data events envelope with transaction metadata.

See Transaction Metadata for additional details.

10000 (10 seconds)

The number of milliseconds to wait before restarting a connector after a retriable error occurs.

30000

The interval in which the connector polls for new, removed, or changed replica sets.

10000 (10 seconds)

The number of milliseconds the driver will wait before a new connection attempt is aborted.

10000 (10 seconds)

The frequency that the cluster monitor attempts to reach each server.

0

The number of milliseconds before a send/receive on the socket can take before a timeout occurs. A value of 0 disables this behavior.

30000 (30 seconds)

The number of milliseconds the driver will wait to select a server before it times out and throws an error.

No default

When streaming changes, this setting applies processing to change stream events as part of the standard MongoDB aggregation stream pipeline. A pipeline is a MongoDB aggregation pipeline composed of instructions to the database to filter or transform data. This can be used customize the data that the connector consumes. The value of this property must be an array of permitted aggregation pipeline stages in JSON format. Note that this is appended after the internal pipeline used to support the connector (e.g. filtering operation types, database names, collection names, etc.).

internal_first

The order used to construct the effective MongoDB aggregation stream pipeline. Set the property to one of the following values:

    internal_first

    Internal stages defined by the connector are applied first. This means that only the events which ought to be captured by the connector are fed to the user defined stages (configured by setting cursor.pipeline).

    user_first

    Stages defined by the ‘cursor.pipeline’ property are applied first. In this mode all events, included those not captured by the connector, are fed to user defined pipeline stages. This mode can have negative performance impact if the value of cursor.pipeline contains complex operations.

    user_only

    Stages defined by the ‘cursor.pipeline’ property will replace internal stages defined by the connector. This mode is intended only for expert users since all events are processed only by user defined pipeline stages. This mode can have negative impact on performance and overall functionality of the connector!

fail

The strategy used to handle change events for documents exceeding specified BSON size. Set the property to one of the following values:

    fail

    The connector fails if the total size of change event exceed the maximum BSON size.

    skip

    Any change events for documents exceeding the maximum (specified by the cursor.oversize.skip.threshold property) size will be ignored

    split

    Change events exceeding the maximum BSON size will be split using the $changeStreamSplitLargeEvent aggregation. This option requires MongoDB 6.0.9 or newer.

0

The maximum allowed size in bytes of the stored document for which change events are processed. This includes both, the size before and after database operation, more specifically this limits the size of fullDocument and fullDocumentBeforeChange filed of MongoDB change events.

0

Specifies the maximum number of milliseconds the oplog/change stream cursor will wait for the server to produce a result before causing an execution timeout exception. A value of 0 indicates using the server/driver default wait timeout.

No default

Fully-qualified name of the data collection that is used to send signals to the connector. Use the following format to specify the collection name:
<databaseName>.<collectionName>

source

List of the signaling channel names that are enabled for the connector. By default, the following channels are available:

No default

List of notification channel names that are enabled for the connector. By default, the following channels are available:

1024

The maximum number of documents that the connector fetches and reads into memory during an incremental snapshot chunk. Increasing the chunk size provides greater efficiency, because the snapshot runs fewer snapshot queries of a greater size. However, larger chunk sizes also require more memory to buffer the snapshot data. Adjust the chunk size to a value that provides the best performance in your environment.

insert_insert

Specifies the watermarking mechanism that the connector uses during an incremental snapshot to deduplicate events that might be captured by an incremental snapshot and then recaptured after streaming resumes.
You can specify one of the following options:

    insert_insert

    When you send a signal to initiate an incremental snapshot, for every chunk that Debezium reads during the snapshot, it writes an entry to the signaling data collection to record the signal to open the snapshot window. After the snapshot completes, Debezium inserts a second entry that records the signal to close the window.

    insert_delete

    When you send a signal to initiate an incremental snapshot, for every chunk that Debezium reads, it writes a single entry to the signaling data collection to record the signal to open the snapshot window. After the snapshot completes, this entry is removed. No entry is created for the signal to close the snapshot window. Set this option to prevent rapid growth of the signaling data collection.

io.debezium.schema.DefaultTopicNamingStrategy

The name of the TopicNamingStrategy class that should be used to determine the topic name for data change, schema change, transaction, heartbeat event etc., defaults to DefaultTopicNamingStrategy.

.

Specify the delimiter for topic name, defaults to ..

10000

The size used for holding the topic names in bounded concurrent hash map. This cache will help to determine the topic name corresponding to a given data collection.

debezium-heartbeat

Controls the name of the topic to which the connector sends heartbeat messages. The topic name has this pattern:

topic.heartbeat.prefix.topic.prefix

For example, if the topic prefix is fulfillment, the default topic name is debezium-heartbeat.fulfillment.

transaction

Controls the name of the topic to which the connector sends transaction metadata messages. The topic name has this pattern:

topic.prefix.topic.transaction

For example, if the topic prefix is fulfillment, the default topic name is fulfillment.transaction.

No default

The custom metric tags will accept key-value pairs to customize the MBean object name which should be appended the end of regular name, each key would represent a tag for the MBean object name, and the corresponding value would be the value of that tag the key is. For example: k1=v1,k2=v2.

-1

Specifies how the connector responds after an operation that results in a retriable error, such as a connection error.
Set one of the following options:

    -1

    No limit. The connector always restarts automatically, and retries the operation, regardless of the number of previous failures.

    0

    Disabled. The connector fails immediately, and never retries the operation. User intervention is required to restart the connector.

    > 0

    The connector restarts automatically until it reaches the specified maximum number of retries. After the next failure, the connector stops, and user intervention is required to restart it.

Debezium connector Kafka signals configuration properties

Debezium provides a set of signal.* properties that control how the connector interacts with the Kafka signals topic.

The following table describes the Kafka signal properties.

Table 15. Kafka signals configuration properties
PropertyDefaultDescription

<topic.prefix>-signal

The name of the Kafka topic that the connector monitors for ad hoc signals.

If automatic topic creation is disabled, you must manually create the required signaling topic. A signaling topic is required to preserve signal ordering. The signaling topic must have a single partition.

kafka-signal

The name of the group ID that is used by Kafka consumers.

No default

A list of host/port pairs that the connector uses for establishing an initial connection to the Kafka cluster. Each pair references the Kafka cluster that is used by the Debezium Kafka Connect process.

100

An integer value that specifies the maximum number of milliseconds that the connector waits when polling signals.

false

Enable the offset commit for the signal topic in order to guarantee At-Least-Once delivery. If disabled, only signals received when the consumer is up&running are processed. Any signals received when the consumer is down are lost.

Debezium connector pass-through signals Kafka consumer client configuration properties

The Debezium connector provides for pass-through configuration of the signals Kafka consumer. Pass-through signals properties begin with the prefix signals.consumer.*. For example, the connector passes properties such as signal.consumer.security.protocol=SSL to the Kafka consumer.

Debezium strips the prefixes from the properties before it passes the properties to the Kafka signals consumer.

Debezium connector sink notifications configuration properties

The following table describes the notification properties.

Table 16. Sink notification configuration properties
PropertyDefaultDescription

No default

The name of the topic that receives notifications from Debezium. This property is required when you configure the notification.enabled.channels property to include sink as one of the enabled notification channels.

Monitoring

The Debezium MongoDB connector has two metric types in addition to the built-in support for JMX metrics that Zookeeper, Kafka, and Kafka Connect have.

  • Snapshot metrics provide information about connector operation while performing a snapshot.

  • Streaming metrics provide information about connector operation when the connector is capturing changes and streaming change event records.

The Debezium monitoring documentation provides details about how to expose these metrics by using JMX.

Snapshot Metrics

The MBean is debezium.mongodb:type=connector-metrics,context=snapshot,server=_<topic.prefix>_,task=_<task.id>_.

Snapshot metrics are not exposed unless a snapshot operation is active, or if a snapshot has occurred since the last connector start.

The following table lists the shapshot metrics that are available.

AttributesTypeDescription

string

The last snapshot event that the connector has read.

long

The number of milliseconds since the connector has read and processed the most recent event.

long

The total number of events that this connector has seen since last started or reset.

long

The number of events that have been filtered by include/exclude list filtering rules configured on the connector.

string[]

The list of tables that are captured by the connector.

int

The length the queue used to pass events between the snapshotter and the main Kafka Connect loop.

int

The free capacity of the queue used to pass events between the snapshotter and the main Kafka Connect loop.

int

The total number of tables that are being included in the snapshot.

int

The number of tables that the snapshot has yet to copy.

boolean

Whether the snapshot was started.

boolean

Whether the snapshot was paused.

boolean

Whether the snapshot was aborted.

boolean

Whether the snapshot completed.

long

The total number of seconds that the snapshot has taken so far, even if not complete. Includes also time when snapshot was paused.

long

The total number of seconds that the snapshot was paused. If the snapshot was paused several times, the paused time adds up.

Map<String, Long>

Map containing the number of rows scanned for each table in the snapshot. Tables are incrementally added to the Map during processing. Updates every 10,000 rows scanned and upon completing a table.

long

The maximum buffer of the queue in bytes. This metric is available if max.queue.size.in.bytes is set to a positive long value.

long

The current volume, in bytes, of records in the queue.

The Debezium MongoDB connector also provides the following custom snapshot metrics:

AttributeTypeDescription

NumberOfDisconnects

long

Number of database disconnects.

Streaming Metrics

The MBean is debezium.mongodb:type=connector-metrics,context=streaming,server=_<topic.prefix>_,task=_<task.id>_.

The following table lists the streaming metrics that are available.

AttributesTypeDescription

string

The last streaming event that the connector has read.

long

The number of milliseconds since the connector has read and processed the most recent event.

long

The total number of events that this connector has seen since the last start or metrics reset.

long

The total number of create events that this connector has seen since the last start or metrics reset.

long

The total number of update events that this connector has seen since the last start or metrics reset.

long

The total number of delete events that this connector has seen since the last start or metrics reset.

long

The number of events that have been filtered by include/exclude list filtering rules configured on the connector.

string[]

The list of tables that are captured by the connector.

int

The length the queue used to pass events between the streamer and the main Kafka Connect loop.

int

The free capacity of the queue used to pass events between the streamer and the main Kafka Connect loop.

boolean

Flag that denotes whether the connector is currently connected to the database server.

long

The number of milliseconds between the last change event’s timestamp and the connector processing it. The values will incoporate any differences between the clocks on the machines where the database server and the connector are running.

long

The number of processed transactions that were committed.

Map<String, String>

The coordinates of the last received event.

string

Transaction identifier of the last processed transaction.

long

The maximum buffer of the queue in bytes. This metric is available if max.queue.size.in.bytes is set to a positive long value.

long

The current volume, in bytes, of records in the queue.

The Debezium MongoDB connector also provides the following custom streaming metrics:

AttributeTypeDescription

NumberOfDisconnects

long

Number of database disconnects.

NumberOfPrimaryElections

long

Number of primary node elections.

MongoDB connector common issues

Debezium is a distributed system that captures all changes in multiple upstream databases, and will never miss or lose an event. When the system is operating normally and is managed carefully, then Debezium provides exactly once delivery of every change event.

If a fault occurs, the system does not lose any events. However, while it is recovering from the fault, it might repeat some change events. In such situations, Debezium, like Kafka, provides at least once delivery of change events.

The rest of this section describes how Debezium handles various kinds of faults and problems.

Configuration and startup errors

In the following situations, the connector fails when trying to start, reports an error or exception in the log, and stops running:

  • The connector’s configuration is invalid.

  • The connector cannot successfully connect to MongoDB by using the specified connection parameters.

After a failure, the connector attempts to reconnect by using exponential backoff. You can configure the maximum number of reconnection attempts.

In these cases, the error will have more details about the problem and possibly a suggested work around. The connector can be restarted when the configuration has been corrected or the MongoDB problem has been addressed.

MongoDB becomes unavailable

Once the connector is running, if the primary node of any of the MongoDB replica sets become unavailable or unreachable, the connector will repeatedly attempt to reconnect to the primary node, using exponential backoff to prevent saturating the network or servers. If the primary remains unavailable after the configurable number of connection attempts, the connector will fail.

The attempts to reconnect are controlled by three properties:

  • connect.backoff.initial.delay.ms - The delay before attempting to reconnect for the first time, with a default of 1 second (1000 milliseconds).

  • connect.backoff.max.delay.ms - The maximum delay before attempting to reconnect, with a default of 120 seconds (120,000 milliseconds).

  • connect.max.attempts - The maximum number of attempts before an error is produced, with a default of 16.

Each delay is double that of the prior delay, up to the maximum delay. Given the default values, the following table shows the delay for each failed connection attempt and the total accumulated time before failure.

Reconnection attempt numberDelay before attempt, in secondsTotal delay before attempt, in minutes and seconds

1

1

00:01

2

2

00:03

3

4

00:07

4

8

00:15

5

16

00:31

6

32

01:03

7

64

02:07

8

120

04:07

9

120

06:07

10

120

08:07

11

120

10:07

12

120

12:07

13

120

14:07

14

120

16:07

15

120

18:07

16

120

20:07

Connector Unable to Start - InvalidResumeToken or ChangeStreamHistoryLost

A connector that is stopped for a long period fails to start, and reports the following exception:

  1. Command failed with error 286 (ChangeStreamHistoryLost): 'PlanExecutor error during aggregation :: caused by :: Resume of change stream was not possible, as the resume point may no longer be in the oplog

The preceding exception indicates that the entry that corresponds to the connector’s resume token is no longer present in the oplog. Because the oplog no longer contains the last offset that the connector processed, the connector cannot resume streaming.

You can use either of the following options to recover from the failure:

  • Delete the failed connector, and create a new connector with the same configuration but with a different connector name.

  • Pause the connector and then remove offsets, or change the offset topic.

To help prevent failures related to missing resume tokens, optimize configuration of the oplog.

Kafka Connect process stops gracefully

If Kafka Connect is being run in distributed mode, and a Kafka Connect process is stopped gracefully, then prior to shutdown of that processes Kafka Connect will migrate all of the process’ connector tasks to another Kafka Connect process in that group, and the new connector tasks will pick up exactly where the prior tasks left off. There is a short delay in processing while the connector tasks are stopped gracefully and restarted on the new processes.

If the group contains only one process and that process is stopped gracefully, then Kafka Connect will stop the connector and record the last offset for each replica set. Upon restart, the replica set tasks will continue exactly where they left off.

Kafka Connect process crashes

If the Kafka Connector process stops unexpectedly, then any connector tasks it was running will terminate without recording their most recently-processed offsets. When Kafka Connect is being run in distributed mode, it will restart those connector tasks on other processes. However, the MongoDB connectors will resume from the last offset recorded by the earlier processes, which means that the new replacement tasks may generate some of the same change events that were processed just prior to the crash. The number of duplicate events depends on the offset flush period and the volume of data changes just before the crash.

Because there is a chance that some events may be duplicated during a recovery from failure, consumers should always anticipate some events may be duplicated. Debezium changes are idempotent, so a sequence of events always results in the same state.

Debezium also includes with each change event message the source-specific information about the origin of the event, including the MongoDB event’s unique transaction identifier (h) and timestamp (sec and ord). Consumers can keep track of other of these values to know whether it has already seen a particular event.

Kafka becomes unavailable

As the connector generates change events, the Kafka Connect framework records those events in Kafka using the Kafka producer API. Kafka Connect will also periodically record the latest offset that appears in those change events, at a frequency that you have specified in the Kafka Connect worker configuration. If the Kafka brokers become unavailable, the Kafka Connect worker process running the connectors will simply repeatedly attempt to reconnect to the Kafka brokers. In other words, the connector tasks will simply pause until a connection can be reestablished, at which point the connectors will resume exactly where they left off.

Connector fails after it is stopped for a long interval if snapshot.mode is set to initial

If the connector is gracefully stopped, users might continue to perform operations on replica set members. Changes that occur while the connector is offline continue to be recorded in MongoDB’s oplog. In most cases, after the connector is restarted, it reads the offset value in the oplog to determine the last operation that it streamed for each replica set, and then resumes streaming changes from that point. After the restart, database operations that occurred while the connector was stopped are emitted to Kafka as usual, and after some time, the connector catches up with the database. The amount of time required for the connector to catch up depends on the capabilities and performance of Kafka and the volume of changes that occurred in the database.

However, if the connector remains stopped for a long enough interval, it can occur that MongoDB purges the oplog during the time that the connector is inactive, resulting in the loss of information about the connector’s last position. After the connector restarts, it cannot resume streaming, because the oplog no longer contains the previous offset value that marks the last operation that the connector processed. The connector also cannot perform a snapshot, as it typically would when the snapshot.mode property is set to initial, and no offset value is present. In this case, a mismatch exists, because the oplog does not contain the value of the previous offset, but the offset value is present in the connector’s internal Kafka offsets topic. An error results and the connector fails.

To recover from the failure, delete the failed connector, and create a new connector with the same configuration but with a different connector name. When you start the new connector, it performs a snapshot to ingest the state of database, and then resumes streaming.

MongoDB loses writes

In certain failure situations, MongoDB can lose commits, which results in the MongoDB connector being unable to capture the lost changes. For example, if the primary crashes suddenly after it applies a change and records the change to its oplog, the oplog might become unavailable before secondary nodes can read its contents. As a result, the secondary node that is elected as the new primary node might be missing the most recent changes from its oplog.

At this time, there is no way to prevent this side effect in MongoDB.