Sharding

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Shardingis a method for distributing data across multiple machines. MongoDB uses sharding to support deployments with very large data sets and high throughput operations.

Database systems with large data sets or high throughput applications can challenge the capacity of a single server. For example, high query rates can exhaust the CPU capacity of the server. Working set sizes larger than the system’s RAM stress the I/O capacity of disk drives.

There are two methods for addressing system growth: vertical and horizontal scaling.

_Vertical Scaling_involves increasing the capacity of a single server, such as using a more powerful CPU, adding more RAM, or increasing the amount of storage space. Limitations in available technology may restrict a single machine from being sufficiently powerful for a given workload. Additionally, Cloud-based providers have hard ceilings based on available hardware configurations. As a result, there is a practical maximum for vertical scaling.

_Horizontal Scaling_involves dividing the system dataset and load over multiple servers, adding additional servers to increase capacity as required. While the overall speed or capacity of a single machine may not be high, each machine handles a subset of the overall workload, potentially providing better efficiency than a single high-speed high-capacity server. Expanding the capacity of the deployment only requires adding additional servers as needed, which can be a lower overall cost than high-end hardware for a single machine. The trade off is increased complexity in infrastructure and maintenance for the deployment.

MongoDB supports_horizontal scaling_throughsharding.

Sharded Cluster

A MongoDBsharded clusterconsists of the following components:

  • shard
    : Each shard contains a subset of the sharded data. Each shard can be deployed as a
    replica set
    .
  • mongos
    : The
    mongos
    acts as a query router, providing an interface between client applications and the sharded cluster.
  • config servers
    : Config servers store metadata and configuration settings for the cluster. As of MongoDB 3.4, config servers must be deployed as a replica set (CSRS).

The following graphic describes the interaction of components within a sharded cluster:

Sharding - 图1

MongoDB shards data at thecollectionlevel, distributing the collection data across the shards in the cluster.

Shard Keys

To distribute the documents in a collection, MongoDBpartitionsthe collection using theshard key. Theshard keyconsists of an immutable field or fields that exist in every document in the target collection.

You choose the shard key when sharding a collection. The choice of shard key cannot be changed after sharding. A sharded collection can have only_one_shard key. SeeShard Key Specification.

To shard a non-empty collection, the collection must have anindexthat starts with the shard key. For empty collections, MongoDB creates the index if the collection does not already have an appropriate index for the specified shard key. SeeShard Key Indexes.

The choice of shard key affects the performance, efficiency, and scalability of a sharded cluster. A cluster with the best possible hardware and infrastructure can be bottlenecked by the choice of shard key. The choice of shard key and its backing index can also affect thesharding strategythat your cluster can use.

See theshard keydocumentation for more information.

Chunks

MongoDB partitions sharded data intochunks. Each chunk has an inclusive lower and exclusive upper range based on theshard key.

MongoDB migrateschunksacross theshardsin thesharded clusterusing thesharded cluster balancer. The balancer attempts to achieve an even balance of chunks across all shards in the cluster.

SeeData Partitioning with Chunksfor more information.

Advantages of Sharding

Reads / Writes

MongoDB distributes the read and write workload across theshardsin thesharded cluster, allowing each shard to process a subset of cluster operations. Both read and write workloads can be scaled horizontally across the cluster by adding more shards.

For queries that include the shard key or the prefix of acompoundshard key,mongoscan target the query at a specific shard or set of shards. Thesetargeted operationsare generally more efficient thanbroadcastingto every shard in the cluster.

Storage Capacity

Shardingdistributes data across theshardsin the cluster, allowing each shard to contain a subset of the total cluster data. As the data set grows, additional shards increase the storage capacity of the cluster.

High Availability

Asharded clustercan continue to perform partial read / write operations even if one or more shards are unavailable. While the subset of data on the unavailable shards cannot be accessed during the downtime, reads or writes directed at the available shards can still succeed.

Starting in MongoDB 3.2, you can deployconfig serversasreplica sets. A sharded cluster with a Config Server Replica Set (CSRS) can continue to process reads and writes as long as a majority of the replica set is available. In version 3.4, MongoDB removes support for SCCC config servers. To upgrade your config servers from SCCC to CSRS, seeUpgrade Config Servers to Replica Set.

In production environments, individual shards should be deployed asreplica sets, providing increased redundancy and availability.

Considerations Before Sharding

Sharded cluster infrastructure requirements and complexity require careful planning, execution, and maintenance.

Careful consideration in choosing the shard key is necessary for ensuring cluster performance and efficiency. You cannot change the shard key after sharding, nor can you unshard a sharded collection. SeeChoosing a Shard Key.

Sharding has certainoperational requirements and restrictions. SeeOperational Restrictions in Sharded Clustersfor more information.

If queries do_not_include the shard key or the prefix of acompoundshard key,mongosperforms abroadcast operation, querying_all_shards in the sharded cluster. These scatter/gather queries can be long running operations.

NOTE

If you have an active support contract with MongoDB, consider contacting your account representative for assistance with sharded cluster planning and deployment.

Sharded and Non-Sharded Collections

A database can have a mixture of sharded and unsharded collections. Sharded collections arepartitionedand distributed across theshardsin the cluster. Unsharded collections are stored on aprimary shard. Each database has its own primary shard.

Sharding - 图2

Connecting to a Sharded Cluster

You must connect to amongosrouter to interact with any collection in thesharded cluster. This includes sharded_and_unsharded collections. Clients should_never_connect to a single shard in order to perform read or write operations.

Sharding - 图3

You can connect to amongosthe same way you connect to amongod, such as via themongoshell or a MongoDBdriver.

Sharding Strategy

MongoDB supports two sharding strategies for distributing data acrosssharded clusters.

Hashed Sharding

Hashed Sharding involves computing a hash of the shard key field’s value. Eachchunkis then assigned a range based on the hashed shard key values.

TIP

MongoDB automatically computes the hashes when resolving queries using hashed indexes. Applications donotneed to compute hashes.

Sharding - 图4

While a range of shard keys may be “close”, their hashed values are unlikely to be on the samechunk. Data distribution based on hashed values facilitates more even data distribution, especially in data sets where the shard key changesmonotonically.

However, hashed distribution means that ranged-based queries on the shard key are less likely to target a single shard, resulting in more cluster widebroadcast operations

SeeHashed Shardingfor more information.

Ranged Sharding

Ranged sharding involves dividing data into ranges based on the shard key values. Eachchunkis then assigned a range based on the shard key values.

Sharding - 图5

A range of shard keys whose values are “close” are more likely to reside on the samechunk. This allows fortargeted operationsas amongoscan route the operations to only the shards that contain the required data.

The efficiency of ranged sharding depends on the shard key chosen. Poorly considered shard keys can result in uneven distribution of data, which can negate some benefits of sharding or can cause performance bottlenecks. Seeshard key selectionfor ranged sharding.

SeeRanged Shardingfor more information.

Zones in Sharded Clusters

In sharded clusters, you can createzonesof sharded data based on theshard key. You can associate each zone with one or more shards in the cluster. A shard can associate with any number of non-conflicting zones. In a balanced cluster, MongoDB migrateschunkscovered by a zone only to those shards associated with the zone.

Each zone covers one or more ranges ofshard keyvalues. Each range a zone covers is always inclusive of its lower boundary and exclusive of its upper boundary.

Sharding - 图6

You must use fields contained in theshard keywhen defining a new range for a zone to cover. If using acompoundshard key, the range must include the prefix of the shard key. Seeshard keys in zonesfor more information.

When choosing a shard key, carefully consider the possibility of using zone sharding in the future, as you cannot change the shard key after sharding the collection.

Most commonly, zones serve to improve the locality of data for sharded clusters that span multiple data centers.

Seezonesfor more information.

Collations in Sharding

Use theshardCollectioncommand with thecollation:{locale:"simple"}option to shard a collection which has adefault collation. Successful sharding requires that:

  • The collection must have an index whose prefix is the shard key
  • The index must have the collation
    {
    locale:
    "simple"
    }

When creating new collections with a collation, ensure these conditions are met prior to sharding the collection.

NOTE

Queries on the sharded collection continue to use the default collation configured for the collection. To use the shard key index’ssimplecollation, specify{locale:"simple"}in the query’scollation document.

SeeshardCollectionfor more information about sharding and collation.

Additional Resources