Replication复制

Redundancy and Data Availability冗余和数据可用性

Replication provides redundancy and increases data availability. 复制提供了冗余并提高了数据可用性。With multiple copies of data on different database servers, replication provides a level of fault tolerance against the loss of a single database server.由于不同数据库服务器上有多个数据副本，复制提供了一定程度的容错能力，可以防止丢失单个数据库服务器。

In some cases, replication can provide increased read capacity as clients can send read operations to different servers. 在某些情况下，复制可以提供更大的读取容量，因为客户端可以向不同的服务器发送读取操作。Maintaining copies of data in different data centers can increase data locality and availability for distributed applications. 在不同的数据中心维护数据副本可以提高分布式应用程序的数据局部性和可用性。You can also maintain additional copies for dedicated purposes, such as disaster recovery, reporting, or backup.您还可以出于专用目的维护其他副本，如灾难恢复、报告或备份。

Replication in MongoDBMongoDB中的复制

A replica set is a group of mongod instances that maintain the same data set. 副本集是维护同一数据集的一组mongod实例。A replica set contains several data bearing nodes and optionally one arbiter node. 副本集包含多个数据承载节点和一个仲裁节点（可选）。Of the data bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.在数据承载节点中，一个且只有一个成员被视为主节点，而其他节点被视为次节点。

The primary node receives all write operations. 主节点接收所有写入操作。A replica set can have only one primary capable of confirming writes with { w: "majority" } write concern; although in some circumstances, another mongod instance may transiently believe itself to also be primary. 一个副本集只能有一个主副本，能够用{ w: "majority" }写问题确认写操作；尽管在某些情况下，另一个mongod实例可能会暂时认为自己也是主要的。[1] The primary records all changes to its data sets in its operation log, i.e. oplog. 主服务器在其操作日志（即oplog）中记录对其数据集的所有更改。For more information on primary node operation, see Replica Set Primary.有关主节点操作的更多信息，请参阅副本集主节点。

The secondaries replicate the primary's oplog and apply the operations to their data sets such that the secondaries' data sets reflect the primary's data set. 二级数据库复制主数据库的oplog，并将操作应用于其数据集，以便二级数据库的数据集反映主数据库的数据集。If the primary is unavailable, an eligible secondary will hold an election to elect itself the new primary. 如果无法进行初选，符合条件的中学将举行选举，选出新的初选。For more information on secondary members, see Replica Set Secondary Members.有关辅助成员的详细信息，请参阅副本集辅助成员。

In some circumstances (such as you have a primary and a secondary but cost constraints prohibit adding another secondary), you may choose to add a mongod instance to a replica set as an arbiter. 在某些情况下（例如您有一个主副本和一个辅助副本，但成本限制禁止添加另一个辅助副本），您可以选择将mongod实例作为仲裁器添加到副本集中。An arbiter participates in elections but does not hold data (i.e. does not provide data redundancy). 仲裁人参与选举但不保存数据（即不提供数据冗余）。For more information on arbiters, see Replica Set Arbiter.有关仲裁器的更多信息，请参阅副本集仲裁器。

An arbiter will always be an arbiter whereas a primary may step down and become a secondary and a secondary may become the primary during an election.仲裁器永远是仲裁器，而primary可能会下台，成为secondary人，secondary可能会在选举中成为primary。

Asynchronous Replication异步复制

Secondaries replicate the primary's oplog and apply the operations to their data sets asynchronously. 辅助设备复制主设备的oplog，并将操作异步应用于其数据集。By having the secondaries' data sets reflect the primary's data set, the replica set can continue to function despite the failure of one or more members.通过让二级数据集反映主数据集，副本集可以在一个或多个成员出现故障的情况下继续运行。

For more information on replication mechanics, see Replica Set Oplog and Replica Set Data Synchronization.有关复制机制的更多信息，请参阅副本集Oplog和副本集数据同步。

Automatic Failover自动故障切换

When a primary does not communicate with the other members of the set for more than the configured electionTimeoutMillis period (10 seconds by default), an eligible secondary calls for an election to nominate itself as the new primary. 当主要成员与集合中的其他成员的通信时间超过配置的electionTimeoutMillis期间（默认为10秒）时，符合条件的次要成员将调用选举以指定自己为新的主要成员。The cluster attempts to complete the election of a new primary and resume normal operations.该小组试图完成新的初选并恢复正常运作。

The replica set cannot process write operations until the election completes successfully. 在选择成功完成之前，副本集无法处理写入操作。The replica set can continue to serve read queries if such queries are configured to run on secondaries while the primary is offline.如果将读取查询配置为在主设备脱机时在辅助设备上运行，则副本集可以继续提供读取查询。

The median time before a cluster elects a new primary should not typically exceed 12 seconds, assuming default replica configuration settings. 假定默认的副本配置设置，集群选择新主服务器之前的平均时间通常不应超过12秒。This includes time required to mark the primary as unavailable and call and complete an election. 这包括将初选标记为不可用、呼叫并完成选举所需的时间。You can tune this time period by modifying the settings.electionTimeoutMillis replication configuration option. 您可以通过修改settings.electionTimeoutMillis复制配置选项来调整此时间段。Factors such as network latency may extend the time required for replica set elections to complete, which in turn affects the amount of time your cluster may operate without a primary. 网络延迟等因素可能会延长副本集选择完成所需的时间，这反过来会影响群集在没有主服务器的情况下运行的时间。These factors are dependent on your particular cluster architecture.这些因素取决于特定的集群体系结构。

Lowering the electionTimeoutMillis replication configuration option from the default 10000 (10 seconds) can result in faster detection of primary failure. 将electionTimeoutMillis复制配置选项从默认值10000（10秒）降低可以更快地检测主故障。However, the cluster may call elections more frequently due to factors such as temporary network latency even if the primary is otherwise healthy. 然而，由于诸如临时网络延迟之类的因素，集群可能会更频繁地调用选举，即使主节点在其他方面是健康的。This can result in increased rollbacks for w : 1 write operations.这可能会导致w : 1写操作的回滚增加。

Your application connection logic should include tolerance for automatic failovers and the subsequent elections. 应用程序连接逻辑应包括自动故障切换和后续选择的容差。Starting in MongoDB 3.6, MongoDB drivers can detect the loss of the primary and automatically retry certain write operations a single time, providing additional built-in handling of automatic failovers and elections:从MongoDB 3.6开始，MongoDB驱动程序可以检测到主设备的丢失，并一次性自动重试某些写入操作，从而提供自动故障切换和选择的额外内置处理：

• MongoDB 4.2+ compatible drivers enable retryable writes by default默认情况下，MongoDB 4.2+兼容驱动程序支持可重试写入
• MongoDB 4.0 and 3.6-compatible drivers must explicitly enable retryable writes by including retryWrites=true in the connection string.MongoDB 4.0和3.6兼容的驱动程序必须通过在连接字符串中包含retryWrites=true来显式启用可重试写入。

Starting in version 4.4, MongoDB provides mirrored reads to pre-warm electable secondary members' cache with the most recently accessed data. 从4.4版开始，MongoDB提供镜像读取，以使用最近访问的数据预热可选举辅助成员的缓存。Pre-warming the cache of a secondary can help restore performance more quickly after an election.在选举结束后，预热辅助缓存有助于更快地恢复性能。

To learn more about MongoDB’s failover process, see:要了解有关MongoDB故障切换过程的更多信息，请参阅：

• Replica Set Elections复制集选举
• Retryable Writes可复述的文字
• Rollbacks During Replica Set Failover副本集故障切换期间的回滚

Read Operations读取操作

Read Preference读取首选项

By default, clients read from the primary 默认情况下，客户端从主服务器读取数据[1];however, clients can specify a read preference to send read operations to secondaries.;但是，客户机可以指定一个读取首选项，以便将读取操作发送到辅助设备。

Asynchronous replication异步复制 to secondaries means that reads from secondaries may return data that does not reflect the state of the data on the primary.到secondary意味着从secondary读取的数据可能返回不反映主服务器上数据状态的数据。

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

For information on reading from replica sets, see Read Preference.有关读取副本集的信息，请参阅读取首选项。

db.adminCommand( {
  setParameter: 1,
  mirrorReads: { samplingRate: 0.0 }
} )

db.serverStatus( { mirroredReads: 1 } )

Transactions事务

Starting in MongoDB 4.0, multi-document transactions are available for replica sets.从MongoDB 4.0开始，多文档事务可用于副本集。

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

Until a transaction commits, the data changes made in the transaction are not visible outside the transaction.在事务提交之前，事务中所做的数据更改在事务外部是不可见的。

However, when a transaction writes to multiple shards, not all outside read operations need to wait for the result of the committed transaction to be visible across the shards. 但是，当事务写入多个分片时，并非所有外部读取操作都需要等待提交的事务的结果在分片中可见。For example, if a transaction is committed and write 1 is visible on shard A but write 2 is not yet visible on shard B, an outside read at read concern "local" can read the results of write 1 without seeing write 2.例如，如果事务已提交，且写入1在分片a上可见，但写入2在分片B上尚不可见，则外部读取-读取关注点"local"可以读取写入1的结果，而不查看写入2。

Change Streams变更流

Starting in MongoDB 3.6, change streams are available for replica sets and sharded clusters. 从MongoDB 3.6开始，变更流可用于副本集和分片集群。Change streams allow applications to access real-time data changes without the complexity and risk of tailing the oplog. 更改流允许应用程序访问实时数据更改，而不必承担跟踪oplog的复杂性和风险。Applications can use change streams to subscribe to all data changes on a collection or collections.应用程序可以使用更改流订阅一个或多个集合上的所有数据更改。

Additional Features附加功能

Replica sets provide a number of options to support application needs. 应用程序需要多个副本集来支持。For example, you may deploy a replica set with members in multiple data centers, or control the outcome of elections by adjusting the members[n].priority of some members. 例如，您可以在多个数据中心部署包含成员的副本集，或者通过调整某些成员的成员[n]优先级来控制选举结果。Replica sets also support dedicated members for reporting, disaster recovery, or backup functions.副本集还支持用于报告、灾难恢复或备份功能的专用成员。

See Priority 0 Replica Set Members, Hidden Replica Set Members and Delayed Replica Set Members for more information.有关详细信息，请参阅优先级为0的副本集成员、隐藏的副本集成员和延迟的副本集成员。