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Transactions and Operations

For transactions:

  • You can specify read/write (CRUD) operations on existing collections. The collections can be in different databases. For a list of CRUD operations, see CRUD Operations.
  • 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.
  • Starting in MongoDB 4.2, you cannot specify killCursors as the first operation in a transaction.

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

Operations Supported in Multi-Document Transactions

CRUD Operations

The following read/write operations are allowed in transactions:

Method Command Note
db.collection.aggregate() aggregate

Excluding the following stages:

db.collection.countDocuments()  

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.

db.collection.distinct() distinct

Available on unsharded collections.

For sharded collections, use the aggregation pipeline with the $group stage. See Distinct Operation.
db.collection.find() find  
  geoSearch  
delete  
findAndModify For upsert, only when run against an existing collection.
insert Only when run against an existing collection.
db.collection.save()   If an insert, only when run against an existing collection.
update For upsert, only when run against an existing collection.
 
For insert operations, only when run against an existing collection.
For upsert, only when run against an existing collection.

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 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, the mongo shell 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.

Informational Operations

Informational commands, such as isMaster, 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 multi-document transactions:

  • Operations that affect the database catalog, such as creating or dropping a collection or an index. For example, a multi-document transaction cannot include an insert operation that would result in the creation of a new collection.

    The listCollections and listIndexes commands and their helper methods are also excluded.

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