client_session – Logical sessions for sequential operations

Logical sessions for ordering sequential operations.

Requires MongoDB 3.6.

New in version 3.6.

Causally Consistent Reads

with client.start_session(causal_consistency=True) as session:
    collection = client.db.collection
    collection.update_one({'_id': 1}, {'$set': {'x': 10}}, session=session)
    secondary_c = collection.with_options(
        read_preference=ReadPreference.SECONDARY)
    # A secondary read waits for replication of the write.
    secondary_c.find_one({'_id': 1}, session=session)

If causal_consistency is True (the default), read operations that use the session are causally after previous read and write operations. Using a causally consistent session, an application can read its own writes and is guaranteed monotonic reads, even when reading from replica set secondaries.

See also

The MongoDB documentation on

causal-consistency

Transactions

MongoDB 4.0 adds support for transactions on replica set primaries. A transaction is associated with a ClientSession. To start a transaction on a session, use ClientSession.start_transaction() in a with-statement. Then, execute an operation within the transaction by passing the session to the operation:

orders = client.db.orders
inventory = client.db.inventory
with client.start_session() as session:
    with session.start_transaction():
        orders.insert_one({"sku": "abc123", "qty": 100}, session=session)
        inventory.update_one({"sku": "abc123", "qty": {"$gte": 100}},
                             {"$inc": {"qty": -100}}, session=session)

Upon normal completion of with session.start_transaction() block, the transaction automatically calls ClientSession.commit_transaction(). If the block exits with an exception, the transaction automatically calls ClientSession.abort_transaction().

In general, multi-document transactions only support read/write (CRUD) operations on existing collections. However, MongoDB 4.4 adds support for creating collections and indexes with some limitations, including an insert operation that would result in the creation of a new collection. For a complete description of all the supported and unsupported operations see the MongoDB server’s documentation for transactions.

A session may only have a single active transaction at a time, multiple transactions on the same session can be executed in sequence.

New in version 3.7.

Sharded Transactions

PyMongo 3.9 adds support for transactions on sharded clusters running MongoDB 4.2. Sharded transactions have the same API as replica set transactions. When running a transaction against a sharded cluster, the session is pinned to the mongos server selected for the first operation in the transaction. All subsequent operations that are part of the same transaction are routed to the same mongos server. When the transaction is completed, by running either commitTransaction or abortTransaction, the session is unpinned.

New in version 3.9.

See also

The MongoDB documentation on

transactions

Classes

class pymongo.client_session.ClientSession(client, server_session, options, authset, implicit)

A session for ordering sequential operations.

ClientSession instances are not thread-safe or fork-safe. They can only be used by one thread or process at a time. A single ClientSession cannot be used to run multiple operations concurrently.

Should not be initialized directly by application developers - to create a ClientSession, call start_session().

• abort_transaction()

  Abort a multi-statement transaction.

  New in version 3.7.

• advance_cluster_time(cluster_time)

  Update the cluster time for this session.

  Parameters:	cluster_time: The cluster_time from another ClientSession instance.

• advance_operation_time(operation_time)

  Update the operation time for this session.

  Parameters:	operation_time: The operation_time from another ClientSession instance.

• client

  The MongoClient this session was created from.

• cluster_time

  The cluster time returned by the last operation executed in this session.

• commit_transaction()

  Commit a multi-statement transaction.

  New in version 3.7.

• end_session()

  Finish this session. If a transaction has started, abort it.

  It is an error to use the session after the session has ended.

• has_ended

  True if this session is finished.

• in_transaction

  True if this session has an active multi-statement transaction.

  New in version 3.10.

• operation_time

  The operation time returned by the last operation executed in this session.

• options

  The SessionOptions this session was created with.

• session_id

  A BSON document, the opaque server session identifier.

• start_transaction(read_concern=None, write_concern=None, read_preference=None, max_commit_time_ms=None)

  Start a multi-statement transaction.

  Takes the same arguments as TransactionOptions.

  Changed in version 3.9: Added the max_commit_time_ms option.

  New in version 3.7.

• with_transaction(callback, read_concern=None, write_concern=None, read_preference=None, max_commit_time_ms=None)

  Execute a callback in a transaction.

  This method starts a transaction on this session, executes callback once, and then commits the transaction. For example:

  def callback(session):
      orders = session.client.db.orders
      inventory = session.client.db.inventory
      orders.insert_one({"sku": "abc123", "qty": 100}, session=session)
      inventory.update_one({"sku": "abc123", "qty": {"$gte": 100}},
                           {"$inc": {"qty": -100}}, session=session)
  with client.start_session() as session:
      session.with_transaction(callback)

  To pass arbitrary arguments to the callback, wrap your callable with a lambda like this:

  def callback(session, custom_arg, custom_kwarg=None):
      # Transaction operations...
  with client.start_session() as session:
      session.with_transaction(
          lambda s: callback(s, "custom_arg", custom_kwarg=1))

  In the event of an exception, with_transaction may retry the commit or the entire transaction, therefore callback may be invoked multiple times by a single call to with_transaction. Developers should be mindful of this possiblity when writing a callback that modifies application state or has any other side-effects. Note that even when the callback is invoked multiple times, with_transaction ensures that the transaction will be committed at-most-once on the server.

  The callback should not attempt to start new transactions, but should simply run operations meant to be contained within a transaction. The callback should also not commit the transaction; this is handled automatically by with_transaction. If the callback does commit or abort the transaction without error, however, with_transaction will return without taking further action.

  ClientSession instances are not thread-safe or fork-safe. Consequently, the callback must not attempt to execute multiple operations concurrently.

  When callback raises an exception, with_transaction automatically aborts the current transaction. When callback or commit_transaction() raises an exception that includes the "TransientTransactionError" error label, with_transaction starts a new transaction and re-executes the callback.

  When commit_transaction() raises an exception with the "UnknownTransactionCommitResult" error label, with_transaction retries the commit until the result of the transaction is known.

  This method will cease retrying after 120 seconds has elapsed. This timeout is not configurable and any exception raised by the callback or by ClientSession.commit_transaction() after the timeout is reached will be re-raised. Applications that desire a different timeout duration should not use this method.

  Parameters:

  callback: The callable callback to run inside a transaction. The callable must accept a single argument, this session. Note, under certain error conditions the callback may be run multiple times. read_concern (optional): The ReadConcern to use for this transaction. write_concern (optional): The WriteConcern to use for this transaction. read_preference (optional): The read preference to use for this transaction. If None (the default) the read_preference of this Database is used. See read_preferences for options.

  Returns:

  The return value of the callback.

  New in version 3.9.

class pymongo.client_session.SessionOptions(causal_consistency=True, default_transaction_options=None)

Options for a new ClientSession.

• causal_consistency

  Whether causal consistency is configured.

• default_transaction_options

  The default TransactionOptions to use for transactions started on this session.

  New in version 3.7.

class pymongo.client_session.TransactionOptions(read_concern=None, write_concern=None, read_preference=None, max_commit_time_ms=None)

Options for ClientSession.start_transaction().

Changed in version 3.9: Added the max_commit_time_ms option.

New in version 3.7.

• max_commit_time_ms

  The maxTimeMS to use when running a commitTransaction command.

  New in version 3.9.

• read_concern

  This transaction’s ReadConcern.

• read_preference

  This transaction’s ReadPreference.

• write_concern

  This transaction’s WriteConcern.