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. For a list of CRUD operations, see CRUD Operations.
When using feature compatibility version (fcv) "4.4" or greater, 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 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 feature compatibility version (fcv) `"4.4"` or greater, if the update/replace operation is run with `upsert: true` against a non-existing collection, the collection is implicitly created. For fcv `"4.2"` or less, if `upsert: true`, the operation must be run against an existing collection. ~~See also~~参阅 DDL Operations
`db.collection.insertMany()` `db.collection.insertOne()` `db.collection.insert()`	`insert`	For feature compatibility version (fcv) `"4.4"` or greater, when run against a non-existing collection, the collection is implicitly created. For fcv `"4.2"` or less, can only be run against an existing collection. ~~See also~~参阅 DDL Operations
`db.collection.save()`		For feature compatibility version (fcv) `"4.4"` or greater, if an insert against a non-existing collection, the collection is implicitly created. With fcv `"4.2"` or less, if an insert, can only be run against an existing collection. ~~See also~~参阅 DDL Operations
`db.collection.updateOne()` `db.collection.updateMany()` `db.collection.replaceOne()` `db.collection.update()`	`update`	For feature compatibility version (fcv) `"4.4"` or greater, if run with `upsert: true` against a non-existing collection, the collection is implicitly created. For fcv `"4.2"` or less, if `upsert: true`, the operation must be run against an existing collection. ~~See also~~参阅 DDL Operations
`db.collection.bulkWrite()` Various Bulk Operation Methods		For feature compatibility version (fcv) `"4.4"` and greater, if an insert operation or update operation with `upsert: true` is run in a transaction against a non-existing collection, the collection is implicitly created. For fcv `"4.2"` or less, the collection must already exist for insert and `upsert: true` operations. ~~See also~~参阅 DDL Operations

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:~~例如：
- 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.

DDL Operations¶

Starting in MongoDB 4.4 with feature compatibility version (fcv) "4.4", you can create collections and indexes inside a multi-document transaction if the transaction is not a cross-shard write transaction.

Explicit DDL Operations¶

Command	Method	~~Notes~~备注
`create`	`db.createCollection()`	See also the Implicit DDL Operations.
`createIndexes`	`db.collection.createIndex()` `db.collection.createIndexes()`	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 DDL 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
`db.collection.findAndModify()` with `upsert: true` `db.collection.findOneAndReplace()` with `upsert: true` `db.collection.findOneAndUpdate()` with `upsert: true`	`findAndModify` with `upsert: true`
`db.collection.insertMany()` `db.collection.insertOne()` `db.collection.insert()`	`insert`
`db.collection.save()` results in an insert
`db.collection.updateOne()` with `upsert: true` `db.collection.updateMany()` with `upsert: true` `db.collection.replaceOne()` with `upsert: true` `db.collection.update()` 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 isMaster, buildInfo, connectionStatus (and their helper methods) are allowed in transactions; however, they cannot be the first operation in the transaction.

Restricted Operations¶

~~Changed in version 4.4.~~在版本4.4中更改。

The following operations are not allowed in transactions:

Operations that affect the database catalog, such as creating or dropping a collection or an index when using feature compatibility version (fcv) "4.2" or lower. With fcv "4.4" or greater, you can create collections and indexes in transactions unless the transaction is a cross-shard write transaction. For details, see Create Collections and Indexes In a Transaction.
Creating 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.
Explicit creation of collections, e.g. db.createCollection() method, and indexes, e.g. db.collection.createIndexes() and db.collection.createIndex() methods, when using a read concern level other than "local".
The listCollections and listIndexes commands and their helper methods.
Other non-CRUD and non-informational operations, such as createUser, getParameter, count, etc. and their helpers.

~~See also~~参阅

Pending DDL Operations and Transactions