Replication复制

Redundancy and Data Availability冗余和数据可用性

Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication provides a level of fault tolerance against the loss of a single database server.复制提供了冗余并提高了数据可用性。由于不同数据库服务器上有多个数据副本，复制提供了对单个数据库服务器丢失的容错级别。

In some cases, replication can provide increased read capacity as clients can send read operations to different servers. Maintaining copies of data in different data centers can increase data locality and availability for distributed applications. 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。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.您还可以为特定目的维护其他副本，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. 副本集是一组维护相同数据集的mongod实例。A replica set contains several data bearing nodes and optionally one arbiter node. Of the data bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.副本集包含多个数据承载节点和可选的一个仲裁器节点。在数据承载节点中，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. [1] The primary records all changes to its data sets in its operation log, i.e. oplog.一个副本集只能有一个主副本能够确认具有{ w: "majority" }写入关注的写入；尽管在某些情况下，另一个蒙古人可能会暂时认为自己也是主要的。[1]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.有关主节点操作的详细信息，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. 次级服务器复制初级服务器的oplog，并将操作应用于其数据集，使次级服务器的数据集反映初级服务器的数据集中。If the primary is unavailable, an eligible secondary will hold an election to elect itself the new primary. For more information on secondary members, see Replica Set Secondary Members.如果初选不可用，符合条件的中学将举行选举，选出自己的新初选。有关次要成员的详细信息，请参阅副本集次要成员。

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（例如您有一个主实例和一个辅助实例，但成本限制禁止添加另一个辅助），您可以选择将mongod实例添加到副本集中作为仲裁器。An arbiter participates in elections but does not hold data (i.e. does not provide data redundancy). 仲裁器参与选举，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.仲裁器将始终是仲裁者，而primary可能会下台并成为secondary，而secondary可能会在选举期间成为primary。

Asynchronous Replication异步复制

Secondaries replicate the primary's oplog and apply the operations to their data sets asynchronously. 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.次要人员复制主要人员的oplog，并将操作异步应用于他们的数据集。通过使次级数据集反映初级数据集，副本集可以在一个或多个成员发生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，请参阅副本集操作日志和副本集数据同步。

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. The cluster attempts to complete the election of a new primary and resume normal operations.当一个初选与该组其他成员的通信时间超过配置的electionTimeoutMillis（选举超时毫秒）期（默认为10秒）时，符合条件的二级初选会要求选举提名自己为新的初选。该小组试图完成新初选的选举并恢复正常运作。

The replica set cannot process write operations until the election completes successfully. The replica set can continue to serve read queries if such queries are configured to run on secondaries while the primary is offline.在选举成功完成之前，副本集无法处理写入操作。如果将读取查询配置为在主服务器脱机时在secondary上运行，则副本集可以继续为读取查询提供服务。

The median time before a cluster elects a new primary should not typically exceed 12 seconds, assuming default replica configuration settings. 假设默认副本配置设置，集群选择新主服务器之前的中位时间通常不应超过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以及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. 您可以通过修改设置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. These factors are dependent on your particular cluster architecture.网络延迟等因素可能会延长完成副本集选举所需的时间，这反过来又会影响集群在没有主服务器的情况下运行的时间。这些因素取决于特定集群架构。

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到主故障。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，由于临时网络延迟等因素，即使主网络在其他方面是健康的，集群也可能更频繁地调用选举。This can result in increased rollbacks for w : 1 write operations.这可能会导致w:1写入操作的回滚增加。

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对自动故障转移和后续选择的容忍度。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:MongoDB驱动程序可以检测到主写入操作的丢失，并自动重试某些写入操作一次，提供额外的内置自动故障转移和选择处理：

Compatible drivers enable retryable writes by default默认情况下，兼容的驱动程序允许可重试的写入

MongoDB provides mirrored reads to pre-warm electable secondary members' cache with the most recently accessed data. Pre-warming the cache of a secondary can help restore performance more quickly after an election.MongoDB为预热可选择的辅助成员缓存提供镜像读取，其中包含最近访问的数据。预热辅助缓存可以帮助在选举后更快地恢复性能。

To learn more about MongoDB's failover process, see:要了解有关MongoDB故障转移过程的更多信息，请参阅：

• Replica Set Elections副本集选举
• Retryable Writes可重试写入
• Rollbacks During Replica Set Failover副本集故障转移期间的回滚

Read Operations读取操作

Read Preference读取首选项

By default, clients read from the primary [1]; however, clients can specify a read preference to send read operations to secondaries.默认情况下，客户端从主服务器读取[1]；但是，客户端可以指定读取首选项，以将读取操作发送给次级服务器。

Asynchronous replication to secondaries means that reads from secondaries may return data that does not reflect the state of the data on the primary.异步复制到次服务器意味着从次服务器读取的数据可能不会反映主服务器上的数据状态。

Distributed transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.包含读取操作的分布式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。

For information on reading from replica sets, see Read Preference.有关从副本集读取的信息，请参阅读取首选项。

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

db.serverStatus( { mirroredReads: 1 } )

Transactions事务

Multi-document transactions are available for replica sets.多文档事务可用于副本集。

Distributed transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.包含读取操作的分布式事务必须使用读取首选项主。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不可见。

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. 然而，当一个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事务的结果在分片之间可见。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，如果一个事务已提交，并且写1在分片a上可见，但写2在分片B上尚不可见，则外部读取关注"local"可以读取写1的结果，而看不到写2。

Change Streams更改流

Change streams are available for replica sets and sharded clusters. Change streams allow applications to access real-time data changes without the complexity and risk of tailing the oplog. Applications can use change streams to subscribe to all data changes on a collection or collections.更改流可用于副本集和分片集群。变更流允许应用程序访问实时数据变更，而不会带来跟踪oplog的复杂性和风险。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. 副本集提供了许多选项来支持应用程序需求。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一个包含成员的副本集，或者通过调整某些成员的members[n].priority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0副本集成员、隐藏副本集成员和延迟副本集成员。