Transactions and Operations

For transactions:

• You can specify read/write (CRUD) operations on existingcollections. The collections can be in different databases. For alist of CRUD operations, see CRUD Operations.
• You cannot write to cappedcollections. (Starting in MongoDB 4.2)
• 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 callgetMore inside the transaction.
• For cursors created in a transaction, you cannot callgetMore outside the transaction.
• Starting in MongoDB 4.2, you cannot specify killCursors asthe first operation in a transaction.

Operations that affect the database catalog, such as creating ordropping a collection or an index, are not allowed in multi-documenttransactions. For example, a multi-document transaction cannot includean insert operation that would result in the creation of a newcollection. See Restricted Operations.

Operations Supported in Multi-Document Transactions

CRUD Operations

The following read/write operations are allowed in transactions:

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 compatible with the 4.0 features provide acollection-level API countDocuments(filter, options) as a helpermethod that uses the $group with a $sum expressionto perform a count. The 4.0 drivers have deprecated the count() API.

Starting in MongoDB 4.0.3, the mongo shell provides thedb.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 thedb.collection.distinct() method/thedistinct command as well as the aggregation pipelinewith the $group stage.

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

To find the distinct values for a sharded collection, use theaggregation 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 Operations

Informational commands, such as isMaster,buildInfo, connectionStatus (and theirhelper methods) are allowed in transactions; however, they cannot bethe first operation in the transaction.

Restricted Operations

The following operations are not allowed in transactions:

• Operations that affect the database catalog, such as creating ordropping a collection or an index. For example, atransaction cannot include an insert operation that would resultin the creation of a new collection.

The listCollections and listIndexescommands and their helper methods are also excluded.

• Non-CRUD and non-informational operations, such ascreateUser, getParameter,count, etc. and their helpers.

See also

Pending DDL Operations and Transactions