Transactions事务

Create Collections and Indexes In a Transaction在事务中创建集合和索引

Starting in MongoDB 4.4, you can perform the following operations inside of a multi-document transaction as long as the transaction is not a cross-shard write transaction:从MongoDB 4.4开始，只要事务不是跨分片写事务，就可以在多文档事务内部执行以下操作：

• Create collections.创建集合。
• Create indexes on new empty collections created earlier in the same transaction.对先前在同一事务中创建的新的空集合创建索引。

In MongoDB 4.2 and earlier, operations that affect the database catalog, such as creating or dropping a collection or an index, are disallowed in transactions.在MongoDB 4.2及更早版本中，事务中不允许执行影响数据库目录的操作，例如创建或删除集合或索引。

When creating a collection inside a transaction:在事务中创建集合时：

• You can implicitly create a collection, such as with:您可以隐式创建集合，例如使用：
  • an insert operation against a non-existing collection, or针对不存在的集合的插入操作，或者
  • an update/findAndModify operation with upsert: true against a non-existing collection.针对不存在的集合执行upsert:true的update/findAndModify操作。
• You can explicitly create a collection using the create command or its helper db.createCollection().可以使用create命令或其助手db.createCollection()显式创建集合。

When creating an index inside a transaction [1], the index to create must be on either:在事务中创建索引时[1]，要创建的索引必须位于以下任一位置：

• a non-existing collection. 不存在的集合。The collection is created as part of the operation.集合是作为操作的一部分创建的。
• a new empty collection created earlier in the same transaction.在同一事务的早期创建的新的空集合。

Count Operation计数操作

To perform a count operation within a transaction, use the $count aggregation stage or the $group (with a $sum expression) aggregation stage.要在事务中执行计数操作，请使用$count聚合阶段或$group（带有$sum表达式）聚合阶段。

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 $sum expression to perform a count. The 4.0 drivers have deprecated the count() API.与4.0功能兼容的MongoDB驱动程序提供了一个集合级API countDocuments(filter, options)作为一个助手方法，使用带有$sum表达式的$group来执行计数。4.0驱动程序已弃用count()API。

Starting in MongoDB 4.0.3, mongosh provides the db.collection.countDocuments() helper method that uses the $group with a $sum expression to perform a count.从MongoDB 4.0.3开始，mongosh提供了db.collection.countDocuments()辅助方法，该方法使用带有$sum表达式的$group来执行计数。

Distinct Operation剔除重复项操作

To perform a distinct operation within a transaction:要在事务中执行剔除重复项操作：

• For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.对于未排序的集合，可以使用db.collection.distinct()方法/distinct命令以及带有$group阶段的聚合管道。
• For sharded collections, you cannot use the db.collection.distinct() method or the distinct command.对于分片集合，不能使用db.collection.distinct()方法或distinct命令。

  To find the distinct values for a sharded collection, use the aggregation pipeline with the $group stage instead. 要查找分片集合的不同值，请使用带有$group阶段的聚合管道。For example:例如：

  • Instead of db.coll.distinct("x"), use不使用db.coll.distinct("x")，而是使用

    db.coll.aggregate([
       { $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
       { $project: { _id: 0 } }
    ])

  • Instead of db.coll.distinct("x", { status: "A" }), use:不使用db.coll.distinct("x", { status: "A" })，而是使用：

    db.coll.aggregate([
       { $match: { status: "A" } },
       { $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
       { $project: { _id: 0 } }
    ])

  The pipeline returns a cursor to a document:管道将游标返回到文档：

  { "distinctValues" : [ 2, 3, 1 ] }

  Iterate the cursor to access the results document.迭代游标以访问结果文档。

Informational Operations信息操作

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.事务中允许使用信息命令，如hello、buildInfo、connectionStatus（及其辅助方法）；但是，它们不能是事务中的第一个操作。

Restricted Operations受限操作

The following operations are not allowed in transactions:事务记录中不允许执行以下操作：

• Operations that affect the database catalog, such as creating or dropping a collection or an index when using MongoDB 4.2 or lower. 影响数据库目录的操作，例如在使用MongoDB 4.2或更低版本时创建或删除集合或索引。Starting in MongoDB 4.4, you can create collections and indexes in transactions unless the transaction is a cross-shard write transaction. 从MongoDB 4.4开始，您可以在事务中创建集合和索引，除非事务是跨分片写入事务。For details, see Create Collections and Indexes In a Transaction.有关详细信息，请参阅在事务中创建集合和索引。
• Creating new collections in cross-shard write transactions. 在跨分片写入事务中创建新集合。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.例如，如果您在一个分片中写入一个现有集合，并在另一个分片中隐式创建一个集合，则MongoDB无法在同一事务中执行这两个操作。
• 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".当使用“local”以外的读取关注级别时，显式创建集合，例如db.createCollection()方法，以及索引，例如db.collection.createIndexes()和db.collection.createIndex()方法。
• The listCollections and listIndexes commands and their helper methods.listCollections和listIndexes命令及其辅助方法。
• Other non-CRUD and non-informational operations, such as createUser, getParameter, count, etc. and their helpers.其他非CRUD和非信息性操作，如createUser、getParameter、count等及其助手。

Transactions and Read Preference事务和读取首选项

Operations in a transaction use the transaction-level read preference.事务中的操作使用事务级别的读取首选项。

Using the drivers, you can set the transaction-level read preference at the transaction start:使用驱动程序，您可以在事务开始时设置事务级别的读取首选项：

• If the transaction-level read preference is unset, the transaction uses the session-level read preference.如果事务级别的读取首选项未设置，则事务将使用会话级别的读取首选项。
• If transaction-level and the session-level read preference are unset, the transaction uses the client-level read preference. 如果未设置事务级别和会话级别的读取首选项，则事务将使用客户端级别的读取首选项。By default, the client-level read preference is primary.默认情况下，客户端级别的读取首选项是primary。

Multi-document transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.包含读取操作的多文档事务必须使用读取首选项primary。给定事务中的所有操作都必须路由到同一成员。

Transactions and Read Concern事务和读取关注

Operations in a transaction use the transaction-level read concern. 事务中的操作使用事务级读取关注。That is, any read concern set at the collection and database level is ignored inside the transaction.也就是说，在集合和数据库级别设置的任何读取关注在事务内部都会被忽略。

You can set the transaction-level read concern at the transaction start.您可以在事务开始时设置事务级别的读取关注。

• If the transaction-level read concern is unset, the transaction-level read concern defaults to the session-level read concern.如果事务级读取关注未设置，则事务级读取关心点默认为会话级读取关注。
• If transaction-level and the session-level read concern are unset, the transaction-level read concern defaults to the client-level read concern. 如果未设置事务级和会话级读取关注，则事务级读取关注默认为客户端级读取关注。By default, client-level read concern is "local" for reads against the primary. 默认情况下，客户端级别的读取关注是针对主服务器的读取的"local"。See also:另请参阅：
  • Transactions and Read Preference事务和读取首选项
  • Default MongoDB Read Concerns/Write Concerns默认MongoDB读取关注/写入关注

Transactions support the following read concern levels:事务支持以下读取关注级别：

Transactions and Write Concern事务和写入关注

Transactions use the transaction-level write concern to commit the write operations. 事务使用事务级别的写入关注来提交写操作。Write operations inside transactions must be issued without explicit write concern specification and use the default write concern. 事务内部的写入操作必须在没有明确的写入关系规范的情况下发出，并使用默认的写入关系。At commit time, the writes are then commited using the transaction-level write concern.在提交时，然后使用事务级写入关注提交写操作。

You can set the transaction-level write concern at the transaction start:您可以在事务开始时设置事务级别的写入关注：

• If the transaction-level write concern is unset, the transaction-level write concern defaults to the session-level write concern for the commit.如果未设置事务级写入关注，则事务级写事件默认为提交的会话级写事件。
• If the transaction-level write concern and the session-level write concern are unset, the transaction-level write concern defaults to the client-level write concern of:如果事务级写入关注和会话级写入关注未设置，则事务级写事项默认为客户端级写入关注：
  • w: "majority" in MongoDB 5.0 and later, with differences for deployments containing arbiters. MongoDB 5.0及更高版本中的w: "majority"，与包含仲裁器的部署不同。See Implicit Default Write Concern.请参阅隐式默认写入关注。
  • w: 1 in MongoDB 4.4 and earlier.在MongoDB 4.4及更早版本中。

Transactions support all write concern w values, including:事务支持所有写入关注w值，包括：

Production Considerations生产注意事项

For various production considerations with using transactions, see Production Considerations. 有关使用事务的各种生产注意事项，请参阅生产注意事项。In addition, for sharded clusters, see also Production Considerations (Sharded Clusters).此外，对于分片集群，请参阅生产注意事项（分片集群）。

Arbiters仲裁器

Transactions whose write operations span multiple shards will error and abort if any transaction operation reads from or writes to a shard that contains an arbiter.如果任何事务操作从包含仲裁器的分片中读取或写入，则写入操作跨多个分片的事务将出错并中止。

Shard Configuration Restriction分片配置限制

You cannot run transactions on a sharded cluster that has a shard with writeConcernMajorityJournalDefault set to false (such as a shard with a voting member that uses the in-memory storage engine).您不能在具有writeConcernMajorityJournalDefault设置为false的分片（例如具有使用内存存储引擎的投票成员的分片）的分片集群上运行事务。

Diagnostics诊断

MongoDB provides various transactions metrics:MongoDB提供了各种事务度量：

Feature Compatibility Version (fCV)功能兼容性版本（fCV）

To use transactions, the featureCompatibilityVersion for all members of the deployment must be at least:若要使用事务，部署的所有成员的featureCompatibilityVersion必须至少为：

To check the fCV for a member, connect to the member and run the following command:若要检查成员的fCV，请连接到该成员并运行以下命令：

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

For more information, see the setFeatureCompatibilityVersion reference page.有关更多信息，请参阅setFeatureCompatibilityVersion参考页。

Storage Engines存储引擎

Starting in MongoDB 4.2, multi-document transactions are supported on replica sets and sharded clusters where:从MongoDB 4.2开始，副本集和分片集群支持多文档事务，其中：

• the primary uses the WiredTiger storage engine, and主存储使用WiredTiger存储引擎，并且
• the secondary members use either the WiredTiger storage engine or the in-memory storage engines.次要成员使用WiredTiger存储引擎或内存中存储引擎。

In MongoDB 4.0, only replica sets using the WiredTiger storage engine support transactions.在MongoDB 4.0中，只有使用WiredTiger存储引擎的副本集才支持事务。

Limit Critical Section Wait Time限制关键部分等待时间

Starting in MongoDB 5.2 (and 5.0.4):从MongoDB 5.2（和5.0.4）开始：

• When a query accesses a shard, a chunk migration or DDL operation may hold the critical section for the collection.当查询访问分片时，区块迁移或DDL操作可能会保留集合的关键部分。
• To limit the time a shard waits for a critical section within a transaction, use the metadataRefreshInTransactionMaxWaitBehindCritSecMS parameter.要限制分片等待事务中关键部分的时间，请使用metadataRefreshInTransactionMaxWaitBehindCritSecMS参数。