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Transactions

Transactions and Operations

On this page

  • Operations Supported in Multi-Document Transactions
  • CRUD Operations
  • Count Operation
  • Distinct Operation
  • Administration Operations
  • Informational Operations
  • Restricted Operations

For transactions:

  • 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.

  • 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 using explain or similar commands.

  • 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.

Operations Supported in Multi-Document Transactions

CRUD Operations

The following read/write operations are allowed in transactions:

Method
Command
Note

Excluding the following query operator expressions:

The method uses the $match aggregation stage for the query and $group aggregation stage with a $sum expression to perform the count.

Available on unsharded collections.

For sharded collections, use the aggregation pipeline with the $group stage. See Distinct Operation.

If the update or replace operation is run with upsert: true on a non-existing collection, the collection is implicitly created.

For more details, see Administration Operations.

If run on a non-existing collection, the collection is implicitly created.

For more details, see Administration Operations.

If run on a non-existing collection, the collection is implicitly created.

For more details, see Administration Operations.

If run on a non-existing collection, the collection is implicitly created.

For more details, see Administration Operations.

Note

Updates to Shard Key Values

You can update a document's shard key value (unless the shard key field is the immutable _id field) by issuing single-document update / findAndModify operations either in a transaction or as a retryable write. For details, see Change a Document's Shard Key Value.

Count Operation

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

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

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

Distinct Operation

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.

Administration Operations

You can create collections and indexes inside a distributed transaction if the transaction is not a cross-shard write transaction.

Explicit Create Operations

Command
Method
Notes
The index to create must either be on a non-existing collection, in which case, the collection is created as part of the operation, or on a new empty collection created earlier in the same transaction.

Note

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

For more information on creating collections and indexes in a transaction, see Create Collections and Indexes in a Transaction.

Implicit Create Operations

You can also implicitly create a collection through the following write operations against a non-existing collection:

Method Run against Non-Existing Collection
Command Run against Non-Existing Collection
findAndModify with upsert: true
db.collection.updateOne() with upsert: true
db.collection.updateMany() with upsert: true
db.collection.replaceOne() with upsert: true
update with upsert: true
db.collection.bulkWrite() with insert or upsert:true operations
Various Bulk Operation Methods with insert or upsert:true operations

For other CRUD operations allowed in transactions, see CRUD Operations.

For more information on creating collections and indexes in a transaction, see Create Collections and Indexes in a Transaction.

Informational Operations

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.

Restricted Operations

The following operations are not allowed in transactions:

Tip

See also:

Pending DDL Operations and Transactions

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