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Resync a Member of a Replica Set

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A replica set member becomes "stale" when its replication process falls so far behind that the primary overwrites oplog entries the member has not yet replicated. The member cannot catch up and becomes "stale." When this occurs, you must completely resynchronize the member by removing its data and performing an initial sync.

This tutorial addresses both resyncing a stale member and creating a new member using seed data from another member, both of which can be used to restore a replica set member. When syncing a member, choose a time when the system has the bandwidth to move a large amount of data. Schedule the synchronization during a time of low usage or during a maintenance window.

MongoDB provides two options for performing an initial sync:

Note

To prevent changing the write quorum, never rotate more than one replica set member at a time.

Warning

During initial sync, mongod removes the contents of the dbPath directory.

This procedure relies on MongoDB's regular process for Replica Set Syncing. This stores the current data on the member. For an overview of MongoDB initial sync process, see the Replica Set Syncing section.

Initial sync operations can impact the other members of the set and create additional traffic to the primary. The syncing member requires another member of the set that is accessible and up to date.

If the instance has no data, you can follow the Add Members to a Replica Set or Replace a Replica Set Member procedure to add a new member to a replica set.

You can also force a mongod that is already a member of the set to perform an initial sync by restarting the instance without the contents of the dbPath directory:

  1. Stop the member's mongod instance. To ensure a clean shutdown, use the db.shutdownServer() method from mongosh or on Linux systems, the mongod --shutdown option.
  2. (Optional) Make a backup of all data and sub-directories from the member's dbPath directory. If a full backup is not required, consider backing up just the diagnostic.data directory to preserve potentially-useful troubleshooting data in the event of an issue. See Full Time Diagnostic Data Capture for more information.
  3. Delete all data and sub-directories from the member's dbPath directory.
  4. Start the mongod process, specifying Replication Options with a new members[n]._id.

At this point, the mongod performs an initial sync. The length of the initial sync process depends on the size of the database and the network latency between members of the replica set.

This approach "seeds" a new or stale member using the data files from an existing member of the replica set. The data files must be sufficiently recent to allow the new member to catch up with the oplog. Otherwise the member would need to perform an initial sync.

You can capture the data files as either a snapshot or a direct copy. However, in most cases you cannot copy data files from a running mongod instance to another because the data files will change during the file copy operation.

Important

If copying data files, ensure that your copy includes the content of the local database.

You cannot use a mongodump backup for the data files: only a snapshot backup. For approaches to capturing a consistent snapshot of a running mongod instance, see the MongoDB Backup Methods documentation.

After you have copied the data files from the "seed" source, start the mongod instance with a new members[n]._id and allow it to apply all operations from the oplog until it reflects the current state of the replica set. To see the current status of the replica set, use rs.printSecondaryReplicationInfo() or rs.status().

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