Sync Logic

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. 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.

Only modify the default sync logic as needed, and always exercise caution.

Target

The member to sync from must be a valid source for data in the set. To sync from a member, the member must:

• Have data. It cannot be an arbiter, in startup or recovering mode, and must be able to answer data queries.
• Be accessible.
• Be a member of the same set in the replica set configuration.
• Build indexes with the members[n].buildIndexes setting.
• A different member of the set, to prevent syncing from itself.

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.

Persistence

replSetSyncFrom provide a temporary override of default behavior. mongod will revert to the default sync behavior in the following situations:

• The mongod instance restarts.
• The connection between the mongod and the sync target closes.
• If the sync target falls more than 30 seconds behind another member of the replica set.

For more information the use of replSetSyncFrom, see Configure a Secondary’s Sync Target.