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.在不同的数据库服务器上有多个数据副本的情况下，复制提供了一定程度的容错能力，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，由于客户端可以将读取操作发送到不同的服务器，因此复制可以提供更大的读取容量。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拷贝可以提高分布式应用程序的数据位置和可用性。You can also maintain additional copies for dedicated purposes, such as disaster recovery, reporting, or backup.您还可以为专用目的维护额外的拷贝，例如灾难恢复、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. 副本集包含几个数据承载节点和可选的一个仲裁器节点。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，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，它能够通过{ w: "majority" }写入关注来确认写入；尽管在某些情况下，另一个mongod实例可能会暂时相信自己也是主要的。[1] The primary records all changes to its data sets in its operation log, i.e. oplog. 主服务器将对其数据集的所有更改记录在其操作日志（即oplog）中。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Primary。

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. secondary复制primary的操作日志，并将操作应用于其数据集，以便辅助设备的数据集反映主设备的数据集中。If the primary is unavailable, an eligible secondary will hold an election to elect itself the new primary. 如果初选不可用，符合条件的中学将举行选举，选举自己为新的初选。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Secondary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器，而在选举期间，primary可能会下台并成为secondary，secondary可能成为primary。

Asynchronous Replication异步复制

Secondaries replicate the primary's oplog and apply the operations to their data sets asynchronously. 辅助对象复制主对象的操作日志，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.通过让辅助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. 当主要成员在超过配置的electionTimeoutMillis时间段（默认情况下为10秒）的时间内未与集合的其他成员通信时，符合条件的次要成员会要求进行选举以提名自己为新的主要成员。The cluster attempts to complete the election of a new primary and resume normal operations.集群试图完成新初选的选举并恢复正常运行。

The replica set cannot process write operations until the election completes successfully. 在选举成功完成之前，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primary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. 假设默认的副本配置设置，群集选择新主服务器之前的中间时间通常不应超过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、调用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举完成所需的时间，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。These factors are dependent on your particular cluster architecture.这些因素取决于您的特定集群体系结构。

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的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，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. 您的应用程序连接逻辑应该包括对自动故障切换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，并一次自动重试某些写入操作，从而提供对自动故障切换和选择的额外内置处理：

Compatible drivers enable retryable writes by default兼容的驱动程序默认情况下启用可重试写入

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版本开始，MongoDB为预热可选举辅助成员的缓存提供镜像读取，其中包含最近访问的数据。Pre-warming the cache of a secondary can help restore performance more quickly after an election.预先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副本集选择
• 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primary[1]读取；但是，客户端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 to secondaries means that reads from secondaries may return data that does not reflect the state of the data on the primary.到辅助设备的异步复制意味着从辅助设备读取的数据可能会返回不反映主设备上数据状态的数据。

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.有关从副本集中读取的信息，请参阅读取首选项。

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

db.serverStatus( { mirroredReads: 1 } )

Transactions事务

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. 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。

Until a transaction commits, the data changes made in the transaction are not visible outside the transaction.在事务提交之前，在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. 然而，当一个事务写入多个分片时，并不是所有的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，并且写1在分片a上可见，但写2在分片B上还不可见，则外部读取时关注"local"可以读取写1的结果，而不会看到写2。

Change Streams更改流

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，而不会带来跟踪操作日志的复杂性和风险。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集合上的所有数据更改。

Additional Features其他功能

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的需要。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，您可以使用多个数据中心成员部署副本集，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.副本集还支持专用成员执行报告、灾难恢复或备份功能。

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、隐藏副本集成员和延迟副本集成员。