Replica Set Data Synchronization副本集数据同步

Logical Initial Sync Process逻辑初始同步过程

When you perform a logical initial sync, MongoDB:当您执行逻辑初始同步时，MongoDB：

1. Clones all databases except the local database. To clone, the mongod scans every collection in each source member database and inserts all data into its own copies of these collections.克隆除local数据库之外的所有数据库。为了克隆，mongod扫描每个源成员数据库中的每个集合，并将所有数据插入到这些集合的自己的副本中。
2. Builds all collection indexes as the documents are copied for each collection.在为每个集合复制文档时构建所有集合索引。
3. Pulls newly added oplog records during the data copy. Ensure that the destination member has enough disk space in the local database to temporarily store these oplog records for the duration of this data copy stage.在数据复制过程中提取新添加的oplog记录。确保目标成员在local数据库中有足够的磁盘空间，以便在此数据复制阶段期间临时存储这些oplog记录。
4. Applies all changes to the data set. Using the oplog from the source member, the mongod updates its data set to reflect the current state of the replica set.将所有更改应用于数据集。使用源成员的oplog，mongod更新其数据集以反映副本集的当前状态。

When the initial sync finishes, the member transitions from STARTUP2 to SECONDARY.当初始同步完成时，成员将从STARTUP2转换为SECONDARY。

To perform an initial sync, see Resync a Member of a Self-Managed Replica Set.要执行初始同步，请参阅重新同步自我管理副本集的成员。

File Copy Based Initial Sync基于文件复制的初始同步

Available in MongoDB Enterprise only.仅在MongoDB Enterprise中可用。

File copy based initial sync runs the initial sync process by copying and moving files on the file system. This sync method can be faster than logical initial sync.基于文件复制的初始同步通过复制和移动文件系统上的文件来运行初始同步过程。此同步方法可能比逻辑初始同步更快。

Initial Sync on NVMe ClustersNVMe群集上的初始同步

You must perform an initial sync on clusters that use the local Non-Volatile Memory Express (NVMe) SSD storage option, including if you're using Atlas auto-scaling. 您必须在使用本地非易失性存储器快速（NVMe）SSD存储选项的群集上执行初始同步，包括使用Atlas自动扩展的群集。Atlas NVMe clusters auto-scale to the next higher tier when 90% of the storage space is full. An initial sync takes longer to complete compared to subsequent syncs, and reduces the performance of the primary from which the data is read.当90%的存储空间已满时，Atlas NVMe集群会自动扩展到下一个更高的层。与后续同步相比，初始同步需要更长的时间才能完成，并降低了从primary中读取数据的性能。

Fault Tolerance容错

If a destination member performing initial sync encounters a persistent network error during the sync process, the destination member restarts the initial sync process from the beginning.如果执行初始同步的目标成员在同步过程中遇到持续的网络错误，目标成员将从头开始重新启动初始同步过程。

A destination member performing initial sync can attempt to resume the sync process if interrupted by a temporary network error, collection drop, or collection rename.如果因临时网络错误、集合删除或集合重命名而中断，执行初始同步的目标成员可以尝试恢复同步过程。

By default, the destination member tries to resume initial sync for 24 hours. 默认情况下，目标成员会尝试在24小时内恢复初始同步。You can use the initialSyncTransientErrorRetryPeriodSeconds server parameter to control the amount of time the destination member attempts to resume initial sync. 您可以使用initialSyncTransientErrorRetryPeriodSeconds服务器参数来控制目标成员尝试恢复初始同步的时间量。If the destination member cannot successfully resume the initial sync process during the configured time period, it selects a new healthy source member from the replica set and restarts the initial synchronization process from the beginning.如果目标成员在配置的时间段内无法成功恢复初始同步过程，它将从副本集中选择一个新的健康源成员，并从头开始重新启动初始同步过程。

The secondary attempts to restart the initial sync up to 10 times before returning a fatal error.次要尝试重新启动初始同步最多10次，然后返回致命错误。

Initial Sync Source Selection初始同步源选择

Initial sync source selection depends on the value of the mongod startup parameter initialSyncSourceReadPreference:初始同步源选择取决于mongod启动参数initialSyncSourceReadPreference的值：

• For initialSyncSourceReadPreference set to primary (default if chainingAllowed is disabled), select the primary as the source member. 对于初始initialSyncSourceReadPreference置为主（如果禁用chainingAllowed，则默认设置），请选择primary作为源成员。If the primary is unavailable or unreachable, log an error and periodically check for primary availability.如果主服务器不可用或无法访问，请记录错误并定期检查主服务器的可用性。
• For initialSyncSourceReadPreference set to primaryPreferred (default for voting replica set members), attempt to select the primary as the source member. If the primary is unavailable or unreachable, perform sync source member selection from the remaining replica set members.对于initialSyncSourceReadPreference设置为primaryPreferred（投票副本集成员的默认设置），尝试选择主成员作为源成员。如果主副本不可用或无法访问，请从其余副本集成员中执行同步源成员选择。
• For all other supported read modes, perform sync source member selection from the destination members.对于所有其他支持的读取模式，请从目标成员中执行同步源成员选择。

Members performing initial source member selection make two passes through the list of all replica set members:执行初始源成员选择的成员对所有副本集成员的列表进行两次遍历：

If the destination member cannot select a source member after two passes, it logs an error and waits 1 second before restarting the selection process. 如果目标成员在两次通过后无法选择源成员，则会记录一个错误并等待1秒，然后再重新启动选择过程。The secondary mongod can restart the initial sync source selection process up to 10 times before exiting with an error.辅助mongod最多可以重新启动初始同步源选择过程10次，然后退出并出现错误。

Oplog Window操作日志窗口

The oplog window must be long enough so that a destination member can fetch any new oplog entries that occur between the start and end of the Logical Initial Sync Process. oplog窗口必须足够长，以便目标成员可以获取逻辑初始同步过程开始和结束之间出现的任何新oplog条目。If the window isn't long enough, there is a risk that some entries may fall off the oplog before the destination member can apply them.如果窗口不够长，那么在目标成员应用之前，一些条目可能会从oplog中掉下来。

It is recommended that you size the oplog for additional time to fetch any new oplog entries. This allows for changes that may occur during initial syncs.建议您调整oplog的大小，以便有更多时间获取任何新的oplog条目。这允许在初始同步期间发生更改。

For more information, see Oplog Size.有关更多信息，请参阅Oplog大小。

Streaming Replication流式复制

Source members send a continuous stream of oplog entries to their destination members. Streaming replication mitigates replication lag in high-load and high-latency networks. It also:源成员向其目标成员发送源源不断的oplog条目。流式复制减轻了高负载和高延迟网络中的复制延迟。它还：

• Reduces staleness for reads from secondaries.减少从中学阅读的过时性。
• Reduces risk of losing write operations with w: 1 due to primary failover.降低了由于主故障转移而丢失w:1写入操作的风险。
• Reduces latency on write operations with w: "majority" and w: >1 (that is, any write concern that requires waiting for replication).通过w: "majority"和w: >1（即需要等待复制的任何写入关注）减少写入操作的延迟。

Use the oplogFetcherUsesExhaust startup parameter to disable streaming replication and using the older replication behavior. 使用oplogFetcherUsesExhaust启动参数禁用流式复制并使用旧的复制行为。Set the oplogFetcherUsesExhaust parameter to false only if there are any resource constraints on the source member or if you wish to limit MongoDB's usage of network bandwidth for replication.仅当源成员存在任何资源限制，或者您希望限制MongoDB对网络带宽的使用以进行复制时，才将oplogFetcherUsesExhaust参数设置为false。

Multithreaded Replication多线程复制

MongoDB applies write operations in batches using multiple threads to improve concurrency. MongoDB groups batches by document ID (WiredTiger) and simultaneously applies each group of operations using a different thread. MongoDB always applies write operations to a given document in their original write order.MongoDB使用多线程批量应用写操作以提高并发性。MongoDB按文档ID（WiredTiger）对批处理进行分组，并使用不同的线程同时应用每组操作。MongoDB总是按照原始写入顺序对给定文档应用写入操作。

Read operations that target secondaries and are configured with a read concern level of "local" or "majority" read from a WiredTiger snapshot of the data if the read takes place on a secondary where replication batches are being applied.如果读取发生在正在应用复制批处理的辅助服务器上，则针对辅助服务器并配置为从数据的WiredTiger快照读取"local"或"majority"读取关注级别的读取操作。

Reading from a snapshot guarantees a consistent view of the data, and allows the read to occur simultaneously with the ongoing replication without the need for a lock. As a result, secondary reads requiring these read concern levels no longer need to wait for replication batches to be applied, and can be handled as they are received.从快照读取保证了数据视图的一致性，并允许读取与正在进行的复制同时进行，而不需要锁。因此，需要这些读取关注级别的辅助读取不再需要等待应用复制批，并且可以在收到复制批时进行处理。

Flow Control流量控制

Administrators can limit the rate at which the primary applies its writes with the goal of keeping the majority committed lag under a configurable maximum value flowControlTargetLagSeconds.管理员可以限制主应用其写入的速率，目的是将大多数提交的延迟保持在可配置的最大值flowControlTargetLagSeconds以下。

By default, flow control is enabled.默认情况下，启用流控制。

For more information, see Flow Control.有关详细信息，请参阅流量控制。

Replication Sync Source Selection复制同步源选择

Replication source member selection depends on the replica set chaining setting:复制源成员的选择取决于副本集chaining设置：

• With chaining enabled (default), perform source member selection from the destination members.启用链接（默认）后，从目标成员中执行源成员选择。
• With chaining disabled, select the primary as the source member. If the primary is unavailable or unreachable, log an error and periodically check for primary availability.禁用链接后，选择primary作为源成员。如果primary不可用或无法访问，请记录错误并定期检查主服务器的可用性。

Members performing replication source member selection make two passes through the list of all replica set members:执行复制源成员选择的成员对所有副本集成员的列表进行两次遍历：

If the member cannot select a sync source after two passes, it logs an error and waits 1 second before restarting the selection process.如果成员在两次通过后无法选择同步源，则会记录一个错误并等待1秒，然后再重新启动选择过程。

The number of times a source member can be changed per hour is configurable by setting the maxNumSyncSourceChangesPerHour parameter.通过设置maxNumSyncSourceChangesPerHour参数，可以配置源成员每小时可以更改的次数。