rs.syncFrom()

Sync Logic同步逻辑

If an initial sync operation is in progress when you run replSetSyncFrom, replSetSyncFrom stops the in-progress initial sync and restarts the sync process with the new target. 如果在运行replSetSyncFrom时正在进行初始同步操作，replSetSyncFrom将停止正在进行的初始同步，并重新启动与新目标的同步进程。In previous versions, if you run replSetSyncFrom during initial sync, MongoDB produces no error messages, but the sync target will not change until after the initial sync operation.在以前的版本中，如果在初始同步期间运行replSetSyncFrom，MongoDB不会生成错误消息，但同步目标在初始同步操作之后才会更改。

Only modify the default sync logic as needed, and always exercise caution.仅根据需要修改默认同步逻辑，并始终保持谨慎。

Target目标

The member to sync from must be a valid source for data in the set. 要从中同步的成员必须是集中数据的有效源。To sync from a member, the member must:要从成员同步，该成员必须：

• Have data. 有数据。It cannot be an arbiter, in startup or recovering mode, and must be able to answer data queries.它不能是处于启动或恢复模式的仲裁器，必须能够回答数据查询。
• Be accessible.易于接近。
• Be a member of the same set in the replica set configuration.在副本集配置中成为同一集的成员。
• Build indexes with the members[n].buildIndexes setting.使用members[n].buildIndexes设置构建索引。
• A different member of the set, to prevent syncing from itself.集合的不同成员，以防止与自身同步。

If you attempt to replicate from a member that is more than 10 seconds behind the current member, mongod will log a warning but will still replicate from the lagging member. 如果您尝试从比当前成员落后10秒以上的成员进行复制，mongod将记录一条警告，但仍将从落后成员进行复制。See also Replication Lag and Flow Control.另请参见复制滞后和流控制。

Persistence坚持不懈

replSetSyncFrom provide a temporary override of default behavior. 提供默认行为的临时覆盖。mongod will revert to the default sync behavior in the following situations:在以下情况下将恢复为默认同步行为：

• The mongod instance restarts.mongod实例重新启动。
• The connection between the mongod and the sync target closes.mongod和sync目标之间的连接关闭。
• If the sync target falls more than 30 seconds behind another member of the replica set.如果同步目标落后副本集的另一个成员30秒以上。