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Transactions

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  • Transactions API
  • Transactions and Atomicity
  • Transactions and Operations
  • Transactions and Sessions
  • Read Concern/Write Concern/Read Preference
  • General Information
  • Additional Transactions Topics

In MongoDB, an operation on a single document is atomic. Because you can use embedded documents and arrays to capture relationships between data in a single document structure instead of normalizing across multiple documents and collections, this single-document atomicity obviates the need for multi-document transactions for many practical use cases.

For situations that require atomicity of reads and writes to multiple documents (in a single or multiple collections), MongoDB supports multi-document transactions. With distributed transactions, transactions can be used across multiple operations, collections, databases, documents, and shards.


Use the Select your language drop-down menu in the upper-right to set the language of the following example.


This example highlights the key components of the transactions API.

Tip

Note

Distributed Transactions and Multi-Document Transactions

Starting in MongoDB 4.2, the two terms are synonymous. Distributed transactions refer to multi-document transactions on sharded clusters and replica sets. Multi-document transactions (whether on sharded clusters or replica sets) are also known as distributed transactions starting in MongoDB 4.2.

For situations that require atomicity of reads and writes to multiple documents (in a single or multiple collections), MongoDB supports multi-document transactions:

  • In version 4.0, MongoDB supports multi-document transactions on replica sets.

  • In version 4.2, MongoDB introduces distributed transactions, which adds support for multi-document transactions on sharded clusters and incorporates the existing support for multi-document transactions on replica sets.

    To use transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.

Multi-document transactions are atomic (i.e. provide an "all-or-nothing" proposition):

  • When a transaction commits, all data changes made in the transaction are saved and visible outside the transaction. That is, a transaction will not commit some of its changes while rolling back others.

    Until a transaction commits, the data changes made in the transaction are not visible outside the transaction.

    However, when a transaction writes to multiple shards, not all outside read operations need to wait for the result of the committed transaction to be visible across the shards. For example, if a transaction is committed and write 1 is visible on shard A but write 2 is not yet visible on shard B, an outside read at read concern "local" can read the results of write 1 without seeing write 2.

  • When a transaction aborts, all data changes made in the transaction are discarded without ever becoming visible. For example, if any operation in the transaction fails, the transaction aborts and all data changes made in the transaction are discarded without ever becoming visible.

Important

In most cases, multi-document transaction incurs a greater performance cost over single document writes, and the availability of multi-document transactions should not be a replacement for effective schema design. For many scenarios, the denormalized data model (embedded documents and arrays) will continue to be optimal for your data and use cases. That is, for many scenarios, modeling your data appropriately will minimize the need for multi-document transactions.

For additional transactions usage considerations (such as runtime limit and oplog size limit), see also Production Considerations.

Distributed transactions can be used across multiple operations, collections, databases, documents, and, starting in MongoDB 4.2, shards.

For transactions:

  • You can specify read/write (CRUD) operations on existing collections. For a list of CRUD operations, see CRUD Operations.

  • Starting in MongoDB 4.4, you can create collections and indexes in transactions. For details, see Create Collections and Indexes In a Transaction

  • The collections used in a transaction can be in different databases.

    Note

    You cannot create new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.

  • You cannot write to capped collections. (Starting in MongoDB 4.2)

  • You cannot use read concern "snapshot" when reading from a capped collection. (Starting in MongoDB 5.0)

  • You cannot read/write to collections in the config, admin, or local databases.

  • You cannot write to system.* collections.

  • You cannot return the supported operation's query plan (i.e. explain).

  • For cursors created outside of a transaction, you cannot call getMore inside the transaction.

  • For cursors created in a transaction, you cannot call getMore outside the transaction.

For a list of operations not supported in transactions, see Restricted Operations.

Tip

When creating or dropping a collection immediately before starting a transaction, if the collection is accessed within the transaction, issue the create or drop operation with write concern "majority" to ensure that the transaction can acquire the required locks.

Starting in MongoDB 4.4, you can perform the following operations inside of a multi-document transaction as long as the transaction is not a cross-shard write transaction:

  • Create collections.

  • Create indexes on new empty collections created earlier in the same transaction.

In MongoDB 4.2 and earlier, operations that affect the database catalog, such as creating or dropping a collection or an index, are disallowed in transactions.

When creating a collection inside a transaction:

When creating an index inside a transaction [1], the index to create must be on either:

  • a non-existing collection. The collection is created as part of the operation.

  • a new empty collection created earlier in the same transaction.

[1] You can also run db.collection.createIndex() and db.collection.createIndexes() on existing indexes to check for existence. These operations return successfully without creating the index.
  • You cannot create new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.

  • For explicit creation of a collection or an index inside a transaction, the transaction read concern level must be "local". Explicit creation is through:

    Command
    Method
    create

To perform a count operation within a transaction, use the $count aggregation stage or the $group (with a $sum expression) aggregation stage.

MongoDB drivers compatible with the 4.0 features provide a collection-level API countDocuments(filter, options) as a helper method that uses the $group with a $sum expression to perform a count. The 4.0 drivers have deprecated the count() API.

Starting in MongoDB 4.0.3, mongosh provides the db.collection.countDocuments() helper method that uses the $group with a $sum expression to perform a count.

To perform a distinct operation within a transaction:

  • For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.

  • For sharded collections, you cannot use the db.collection.distinct() method or the distinct command.

    To find the distinct values for a sharded collection, use the aggregation pipeline with the $group stage instead. For example:

    • Instead of db.coll.distinct("x"), use

      db.coll.aggregate([
      { $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
      { $project: { _id: 0 } }
      ])
    • Instead of db.coll.distinct("x", { status: "A" }), use:

      db.coll.aggregate([
      { $match: { status: "A" } },
      { $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
      { $project: { _id: 0 } }
      ])

    The pipeline returns a cursor to a document:

    { "distinctValues" : [ 2, 3, 1 ] }

    Iterate the cursor to access the results document.

Informational commands, such as hello, buildInfo, connectionStatus (and their helper methods) are allowed in transactions; however, they cannot be the first operation in the transaction.

Changed in version 4.4.

The following operations are not allowed in transactions:

  • Transactions are associated with a session; i.e. you start a transaction for a session.

  • At any given time, you can have at most one open transaction for a session.

  • When using the drivers, each operation in the transaction must be associated with the session. Refer to your driver specific documentation for details.

  • If a session ends and it has an open transaction, the transaction aborts.

Operations in a transaction use the transaction-level read preference.

Using the drivers, you can set the transaction-level read preference at the transaction start:

  • If the transaction-level read preference is unset, the transaction uses the session-level read preference.

  • If transaction-level and the session-level read preference are unset, the transaction uses the client-level read preference. By default, the client-level read preference is primary.

Multi-document transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.

Operations in a transaction use the transaction-level read concern. That is, any read concern set at the collection and database level is ignored inside the transaction.

You can set the transaction-level read concern at the transaction start.

  • If the transaction-level read concern is unset, the transaction-level read concern defaults to the session-level read concern.

  • If transaction-level and the session-level read concern are unset, the transaction-level read concern defaults to the client-level read concern. By default, client-level read concern is "local" for reads against the primary. See also:

Transactions support the following read concern levels:

  • Read concern "local" returns the most recent data available from the node but can be rolled back.

  • For transactions on sharded cluster, "local" read concern cannot guarantee that the data is from the same snapshot view across the shards. If snapshot isolation is required, use "snapshot" read concern.

  • Starting in MongoDB 4.4, you can create collections and indexes inside a transaction. If explicitly creating a collection or an index, the transaction must use read concern "local". Implicit creation of a collection can use any of the read concerns available for transactions.

  • Read concern "majority" returns data that has been acknowledged by a majority of the replica set members (i.e. data cannot be rolled back) if the transaction commits with write concern "majority".

  • If the transaction does not use write concern "majority" for the commit, the "majority" read concern provides no guarantees that read operations read majority-committed data.

  • For transactions on sharded cluster, "majority" read concern cannot guarantee that the data is from the same snapshot view across the shards. If snapshot isolation is required, use "snapshot" read concern.

  • Read concern "snapshot" returns data from a snapshot of majority committed data if the transaction commits with write concern "majority".

  • If the transaction does not use write concern "majority" for the commit, the "snapshot" read concern provides no guarantee that read operations used a snapshot of majority-committed data.

  • For transactions on sharded clusters, the "snapshot" view of the data is synchronized across shards.

Transactions use the transaction-level write concern to commit the write operations. Write operations inside transactions must be issued without explicit write concern specification and use the default write concern. At commit time, the writes are then commited using the transaction-level write concern.

Tip

Do not explicitly set the write concern for the individual write operations inside a transaction. Setting write concerns for the individual write operations inside a transaction results in an error.

You can set the transaction-level write concern at the transaction start:

  • If the transaction-level write concern is unset, the transaction-level write concern defaults to the session-level write concern for the commit.

  • If the transaction-level write concern and the session-level write concern are unset, the transaction-level write concern defaults to the client-level write concern of:

Transactions support all write concern w values, including:

  • Write concern w: 1 returns acknowledgement after the commit has been applied to the primary.

    Important

    When you commit with w: 1, your transaction can be rolled back if there is a failover.

  • When you commit with w: 1 write concern, transaction-level "majority" read concern provides no guarantees that read operations in the transaction read majority-committed data.

  • When you commit with w: 1 write concern, transaction-level "snapshot" read concern provides no guarantee that read operations in the transaction used a snapshot of majority-committed data.

  • Write concern w: "majority" returns acknowledgement after the commit has been applied to a majority (M) of voting members; i.e. the commit has been applied to the primary and (M-1) voting secondaries.

  • When you commit with w: "majority" write concern, transaction-level "majority" read concern guarantees that operations have read majority-committed data. For transactions on sharded clusters, this view of the majority-committed data is not synchronized across shards.

  • When you commit with w: "majority" write concern, transaction-level "snapshot" read concern guarantees that operations have from a synchronized snapshot of majority-committed data.

Note

Regardless of the write concern specified for the transaction, the commit operation for a sharded cluster transaction includes some parts that use {w: "majority", j: true} write concern.

Starting in MongoDB 5.0, the new server parameter coordinateCommitReturnImmediatelyAfterPersistingDecision controls when transaction commit decisions are returned to the client. In previous versions of MongoDB, the shard transaction coordinator waited for all members to acknowledge a multi-document transaction commit before returning the commit decision to the client.

If you specify a "majority" write concern for a multi-document transaction and the transaction fails to replicate to the calculated majority of replica set members, then the transaction may not immediately roll back on replica set members. The replica set will be eventually consistent. A transaction is always applied or rolled back on all replica set members.

For various production considerations with using transactions, see Production Considerations. In addition, or sharded clusters, see also Production Considerations (Sharded Clusters).

Transactions whose write operations span multiple shards will error and abort if any transaction operation reads from or writes to a shard that contains an arbiter.

You cannot run transactions on a sharded cluster that has a shard with writeConcernMajorityJournalDefault set to false (such as a shard with a voting member that uses the in-memory storage engine).

Note

Regardless of the write concern specified for the transaction, the commit operation for a sharded cluster transaction includes some parts that use {w: "majority", j: true} write concern.

MongoDB provides various transactions metrics:

Source
Returns
Returns transactions metrics.
$currentOp aggregation pipeline

Returns:

Returns:

mongod and mongos log messages
Includes information on slow transactions (i.e. transactions that exceed the operationProfiling.slowOpThresholdMs threshold) under the TXN log component.

To use transactions, the featureCompatibilityVersion for all members of the deployment must be at least:

Deployment
Minimum featureCompatibilityVersion
Replica Set
4.0
Sharded Cluster
4.2

To check the fCV for a member, connect to the member and run the following command:

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

For more information, see the setFeatureCompatibilityVersion reference page.

Starting in MongoDB 4.2, multi-document transactions are supported on replica sets and sharded clusters where:

  • the primary uses the WiredTiger storage engine, and

  • the secondary members use either the WiredTiger storage engine or the in-memory storage engines.

In MongoDB 4.0, only replica sets using the WiredTiger storage engine supported transactions.

Note

You cannot run transactions on a sharded cluster that has a shard with writeConcernMajorityJournalDefault set to false, such as a shard with a voting member that uses the in-memory storage engine.

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