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Transactions and Operations

Transactions and Operations¶

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Operations Supported in Multi-Document Transactions
Restricted 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 write to capped collections. (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 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: `$collStats` `$currentOp` `$indexStats` `$listLocalSessions` `$listSessions` `$merge` `$out` `$planCacheStats`
`db.collection.countDocuments()`		Excluding the following query operator expressions: `$where` `$near` `$nearSphere` The method uses the `$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`
`db.collection.deleteMany()` `db.collection.deleteOne()` `db.collection.remove()`	`delete`
`db.collection.findOneAndDelete()` `db.collection.findOneAndReplace()` `db.collection.findOneAndUpdate()`	`findAndModify`	For `upsert`, only when run against an existing collection.
`db.collection.insertMany()` `db.collection.insertOne()` `db.collection.insert()`	`insert`	Only when run against an existing collection.
`db.collection.save()`		If an insert, only when run against an existing collection.
`db.collection.updateOne()` `db.collection.updateMany()` `db.collection.replaceOne()` `db.collection.update()`	`update`	For `upsert`, only when run against an existing collection.
`db.collection.bulkWrite()` Various Bulk Operation Methods		For insert operations, only when run against an existing collection. For `upsert`, only when run against an existing collection.

Updates to Shard Key Values

Starting in MongoDB 4.2, 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 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
  copy
  db.coll.aggregate([ { $group: { _id: null, distinctValues: { $addToSet: "$x" } } }, { $project: { _id: 0 } } ])
- Instead of db.coll.distinct("x", { status: "A" }), use:
  copy
  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 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:

Operations that affect the database catalog, such as creating or dropping a collection or an index. For example, a 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.