Replica Set Data Synchronization复制集数据同步

Logical Initial Sync Process逻辑初始同步过程

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

1. Clones all databases except the local database. 克隆除local数据库之外的所有数据库。To clone, the mongod scans every collection in each source database and inserts all data into its own copies of these collections.为了克隆，mongod扫描每个源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生成所有集合索引。
3. Pulls newly added oplog records during the data copy. 在数据复制期间提取新添加的oplog记录。Ensure that the target member has enough disk space in the local database to temporarily store these oplog records for the duration of this data copy stage.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. 将所有更改应用于数据集。Using the oplog from the source, the mongod updates its data set to reflect the current state of the replica set.mongod使用源中的oplog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，成员从STARTUP2转换到SECONDARY。

To perform an initial sync, see Resync a Member of a Replica Set.要执行初始同步，请参阅重新同步副本集的成员。

File Copy Based Initial Sync基于初始同步的文件拷贝

File copy based initial sync基于初始同步的文件拷贝 runs the initial sync process by copying and moving files on the file system.通过复制和移动文件系统上的文件来运行初始同步过程。

File copy based initial sync can be faster than logical initial sync.基于文件拷贝的初始同步可以比逻辑初始同步更快。

It is only available on Enterprise Server.它仅在Enterprise Server上可用。

Fault Tolerance容错

If a secondary performing initial sync encounters a non-transient(i.e. persistent) network error during the sync process, the secondary restarts the initial sync process from the beginning.如果执行初始同步的辅助设备在同步过程中遇到非瞬时（即持续）网络错误，则辅助设备将从一开始重新启动初始同步过程。

Starting in MongoDB 4.4, a secondary performing initial sync can attempt to resume the sync process if interrupted by a transient (i.e. temporary) network error, collection drop, or collection rename. 从MongoDB 4.4开始，执行初始同步的二级服务器可以尝试在被暂时（即临时）网络错误、集合删除或集合重命名中断时恢复同步过程。The sync source must also run MongoDB 4.4 to support resumable initial sync. 同步源还必须运行MongoDB 4.4以支持可恢复的初始同步。If the sync source runs MongoDB 4.2 or earlier, the secondary must restart the initial sync process as if it encountered a non-transient network error.如果同步源运行MongoDB 4.2或更早版本，则辅助服务器必须重新启动初始同步进程，就像遇到非瞬态网络错误一样。

By default, the secondary tries to resume initial sync for 24 hours. 默认情况下，辅助服务器会尝试在24小时内恢复初始同步。MongoDB 4.4 adds the initialSyncTransientErrorRetryPeriodSeconds server parameter for controlling the amount of time the secondary attempts to resume initial sync. MongoDB 4.4添加了initialSyncTransientErrorRetryPeriodSeconds服务器参数，用于控制辅助服务器尝试恢复初始同步的时间量。If the secondary cannot successfully resume the initial sync process during the configured time period, it selects a new healthy source from the replica set and restarts the initial synchronization process from the beginning.如果辅助服务器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，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.在返回致命错误之前，辅助服务器最多尝试重新启动初始同步10次。

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 (new in 4.4):初始同步源选择取决于mongod启动参数initialSyncSourceReadPreference的值（4.4中新增）：

• For initialSyncSourceReadPreference set to primary (default if chaining is disabled), select the primary as the sync source. 如果initialSyncSourceReadPreference设置为primary（如果禁用了chaining，则为默认值），请选择primary作为同步源。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 sync source. 对于initialSyncSourceReadPreference设置为primaryPreferred（投票副本集成员的默认设置），请尝试选择primary作为同步源。If the primary is unavailable or unreachable, perform sync source selection from the remaining replica set members.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选择。
• For all other supported read modes, perform sync source selection from the replica set members.对于所有其他受支持的读取模式，请从副本集成员中执行同步源选择。

Members performing initial sync source selection make two passes through the list of all replica set members:执行初始同步源选择的成员在所有复制集成员列表中进行两次遍历：

If the member cannot select an initial sync source after two passes, it logs an error and waits 1 second before restarting the selection process. 如果成员在两次传递后无法选择初始同步源，则会记录一个错误并等待1秒，然后重新启动选择过程。The secondary mongod can restart the initial sync source selection process up to 10 times before exiting with an error.辅助mongod可以重新启动初始同步源选择过程多达10次，然后退出并出现错误。

Streaming Replication流复制

Starting in MongoDB 4.4, sync from sources send a continuous stream of oplog entries to their syncing secondaries. 从MongoDB 4.4开始，来自源的同步将源源不断的oplog条目发送到它们的同步辅助设备。Streaming replication mitigates replication lag in high-load and high-latency networks. 流式复制减轻了高负载和高延迟网络中的复制延迟。It also:它还：

• Reduces staleness for reads from secondaries.减少从二级数据库读取的陈旧性。
• Reduces risk of losing write operations with w: 1 due to primary failover.减少由于主故障切换而导致w: 1写操作丢失的风险。
• Reduces latency on write operations with w: "majority" and w: >1 (that is, any write concern that requires waiting for replication).使用w: "majority"和w: >1（即任何需要等待复制的写入问题）减少写入操作的延迟。

Prior to MongoDB 4.4, secondaries fetched batches of oplog entries by issuing a request to their sync from source and waiting for a response. 在MongoDB 4.4之前，辅助数据库通过从源发出同步请求并等待响应来获取一批oplog条目。This required a network roundtrip for each batch of oplog entries. 这需要对每批oplog条目进行网络往返。MongoDB 4.4 adds the oplogFetcherUsesExhaust startup parameter for disabling streaming replication and using the older replication behavior. MongoDB 4.4添加了oplogFetcherUsesExhaust启动参数，用于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 sync from source or if you wish to limit MongoDB's usage of network bandwidth for replication.仅当源同步存在任何资源限制或您希望限制MongoDB使用网络带宽进行复制时，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使用多线程批量应用写操作以提高并发性。MongoDB groups batches by document id (WiredTiger) and simultaneously applies each group of operations using a different thread. MongoDB按文档id（WiredTiger）对批进行分组，并使用不同的线程同时应用每组操作。MongoDB always applies write operations to a given document in their original write order.MongoDB始终按照原始写入顺序对给定文档应用写入操作。

Starting in MongoDB 4.0, read operations that target secondaries and are configured with a read concern level of "local" or "majority" will now read from a WiredTiger snapshot of the data if the read takes place on a secondary where replication batches are being applied. 从MongoDB 4.0开始，如果读取发生在应用复制批处理的辅助设备上，则针对辅助设备并配置为"local"或"majority"读取关注级别的读取操作现在将从WiredTiger快照中读取数据。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. 从快照中读取可确保数据的一致视图，并允许读取与正在进行的复制同时进行，而无需锁定。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.因此，需要这些读取关注级别的二次读取不再需要等待应用复制批，并且可以在接收时进行处理。

Flow Control流量控制

Starting in MongoDB 4.2, 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.从MongoDB 4.2开始，管理员可以限制主应用写入的速率，目的是将大多数提交延迟保持在可配置的最大值flowControlTargetLagSeconds之下。

By default, flow control is enabled.默认情况下，流量控制是启用的。

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 sync source selection depends on the replica set chaining setting:复制同步源选择取决于复制集chaining设置：

• With chaining enabled (default), perform sync source selection from the replica set members.启用链接（默认）后，从复制集成员中执行同步源选择。
• With chaining disabled, select the primary as the sync source. 禁用链接后，选择primary作为同步源。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。

Members performing replication sync source selection make two passes through the list of all replica set members:执行复制同步源选择的成员在所有复制集成员列表中进行两次遍历：

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，然后重新启动选择过程。

The number of times a source can be changed per hour is configurable by setting the maxNumSyncSourceChangesPerHour parameter.通过设置maxNumSyncSourceChangesPerHour参数，可以配置每小时更改源的次数。