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client_session – Logical sessions for sequential operations¶
Logical sessions for ordering sequential operations.
Added in version 3.6.
Causally Consistent Reads¶async with client.start_session(causal_consistency=True) as session:
collection = client.db.collection
await collection.update_one({"_id": 1}, {"$set": {"x": 10}}, session=session)
secondary_c = collection.with_options(read_preference=ReadPreference.SECONDARY)
# A secondary read waits for replication of the write.
await secondary_c.find_one({"_id": 1}, session=session)
If causal_consistency is True (the default), read operations that use the session are causally after previous read and write operations. Using a causally consistent session, an application can read its own writes and is guaranteed monotonic reads, even when reading from replica set secondaries.
Transactions¶Added in version 3.7.
MongoDB 4.0 adds support for transactions on replica set primaries. A transaction is associated with a AsyncClientSession. To start a transaction on a session, use AsyncClientSession.start_transaction() in a with-statement. Then, execute an operation within the transaction by passing the session to the operation:
orders = client.db.orders
inventory = client.db.inventory
async with client.start_session() as session:
async with await session.start_transaction():
await orders.insert_one({"sku": "abc123", "qty": 100}, session=session)
await inventory.update_one(
{"sku": "abc123", "qty": {"$gte": 100}},
{"$inc": {"qty": -100}},
session=session,
)
Upon normal completion of async with await session.start_transaction() block, the transaction automatically calls AsyncClientSession.commit_transaction(). If the block exits with an exception, the transaction automatically calls AsyncClientSession.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.
Sharded Transactions¶Added in version 3.9.
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.
Snapshot Reads¶Added in version 3.12.
MongoDB 5.0 adds support for snapshot reads. Snapshot reads are requested by passing the snapshot option to start_session(). If snapshot is True, all read operations that use this session read data from the same snapshot timestamp. The server chooses the latest majority-committed snapshot timestamp when executing the first read operation using the session. Subsequent reads on this session read from the same snapshot timestamp. Snapshot reads are also supported when reading from replica set secondaries.
# Each read using this session reads data from the same point in time.
async with client.start_session(snapshot=True) as session:
order = await orders.find_one({"sku": "abc123"}, session=session)
inventory = await inventory.find_one({"sku": "abc123"}, session=session)
Snapshot Reads Limitations¶
Snapshot reads sessions are incompatible with causal_consistency=True. Only the following read operations are supported in a snapshot reads session:
distinct() (on unsharded collections)
A session for ordering sequential operations.
AsyncClientSession instances are not thread-safe or fork-safe. They can only be used by one thread or process at a time. A single AsyncClientSession cannot be used to run multiple operations concurrently.
Should not be initialized directly by application developers - to create a AsyncClientSession, call start_session().
client (AsyncMongoClient)
server_session (Any)
options (SessionOptions)
implicit (bool)
Abort a multi-statement transaction.
Added in version 3.7.
None
Update the cluster time for this session.
cluster_time (Mapping[str, Any]) – The cluster_time from another AsyncClientSession instance.
None
Update the operation time for this session.
operation_time (Timestamp) – The operation_time from another AsyncClientSession instance.
None
The AsyncMongoClient this session was created from.
The cluster time returned by the last operation executed in this session.
Commit a multi-statement transaction.
Added in version 3.7.
None
Finish this session. If a transaction has started, abort it.
It is an error to use the session after the session has ended.
None
True if this session is finished.
True if this session has an active multi-statement transaction.
Added in version 3.10.
The operation time returned by the last operation executed in this session.
The SessionOptions this session was created with.
A BSON document, the opaque server session identifier.
Start a multi-statement transaction.
Takes the same arguments as TransactionOptions.
Changed in version 3.9: Added the max_commit_time_ms option.
Added in version 3.7.
read_concern (ReadConcern | None)
write_concern (WriteConcern | None)
read_preference (_ServerMode | None)
max_commit_time_ms (int | 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:
async def callback(session):
orders = session.client.db.orders
inventory = session.client.db.inventory
await orders.insert_one({"sku": "abc123", "qty": 100}, session=session)
await inventory.update_one({"sku": "abc123", "qty": {"$gte": 100}},
{"$inc": {"qty": -100}}, session=session)
async with client.start_session() as session:
await session.with_transaction(callback)
To pass arbitrary arguments to the callback, wrap your callable with a lambda like this:
async def callback(session, custom_arg, custom_kwarg=None):
# Transaction operations...
async with client.start_session() as session:
await 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 possibility 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.
AsyncClientSession 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.
The callback MUST NOT silently handle command errors without allowing such errors to propagate. Command errors may abort the transaction on the server, and an attempt to commit the transaction will be rejected with a NoSuchTransaction error. For more information see the transactions specification.
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 AsyncClientSession.commit_transaction() after the timeout is reached will be re-raised. Applications that desire a different timeout duration should not use this method.
callback (Callable[[AsyncClientSession], Coroutine[Any, Any, _T]]) – 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 (ReadConcern | None) – The ReadConcern to use for this transaction.
write_concern (WriteConcern | None) – The WriteConcern to use for this transaction.
read_preference (_ServerMode | None) – The read preference to use for this transaction. If None (the default) the read_preference of this AsyncDatabase is used. See read_preferences for options.
max_commit_time_ms (int | None)
The return value of the callback.
_T
Added in version 3.9.
Options for a new AsyncClientSession.
causal_consistency (Optional[bool]) – If True, read operations are causally ordered within the session. Defaults to True when the snapshot option is False.
default_transaction_options (Optional[TransactionOptions]) – The default TransactionOptions to use for transactions started on this session.
snapshot (Optional[bool]) – If True, then all reads performed using this session will read from the same snapshot. This option is incompatible with causal_consistency=True. Defaults to False.
Changed in version 3.12: Added the snapshot parameter.
Whether causal consistency is configured.
The default TransactionOptions to use for transactions started on this session.
Added in version 3.7.
Whether snapshot reads are configured.
Added in version 3.12.
Options for AsyncClientSession.start_transaction().
read_concern (Optional[ReadConcern]) – The ReadConcern to use for this transaction. If None (the default) the read_preference of the AsyncMongoClient is used.
write_concern (Optional[WriteConcern]) – The WriteConcern to use for this transaction. If None (the default) the read_preference of the AsyncMongoClient is used.
read_preference (Optional[_ServerMode]) – The read preference to use. If None (the default) the read_preference of this AsyncMongoClient is used. See read_preferences for options. Transactions which read must use PRIMARY.
max_commit_time_ms (Optional[int]) – The maximum amount of time to allow a single commitTransaction command to run. This option is an alias for maxTimeMS option on the commitTransaction command. If None (the default) maxTimeMS is not used.
Changed in version 3.9: Added the max_commit_time_ms option.
Added in version 3.7.
The maxTimeMS to use when running a commitTransaction command.
Added in version 3.9.
This transaction’s ReadConcern.
This transaction’s ReadPreference.
This transaction’s WriteConcern.
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