VReplication resharding cut-over: managing conflicting updates during final synchronization

Consistency Model During Cut-over

Vitess resolves conflicts during the final synchronization phase of a resharding event by treating the source shard as the absolute source of truth. Because VReplication is designed as a unidirectional stream of binlog events, the destination shard is effectively a replica of the source during the migration process. There is no "resolution" of divergent states because, by design, the destination is not permitted to accept independent writes that would create a conflict.

Mechanisms Preventing Data Loss

To ensure zero data loss and strict consistency during the transition, Vitess employs a coordinated sequence of locks and checkpoints:

• Source Write-Lock: Before the final cut-over, Vitess initiates a write-lock on the source shard. This stops all new mutations, ensuring the binlog position becomes static.
• Drain Phase: The VReplication engine continues to pull and apply all remaining binlog events from the source to the destination. The system does not proceed to the traffic switch until the destination has fully caught up to the binlog position recorded at the moment of the lock.
• Sequential Application: Since binlog events are applied in the exact order they occurred on the source, the destination state is guaranteed to be a mirror of the source state at the time of the lock.

Assumptions and Constraints

This behavior assumes the destination shard is kept in a read-only state relative to application traffic throughout the resharding process. If manual writes were performed directly on the destination shard outside of the VReplication stream, the binlog application could fail or result in non-deterministic data states, as Vitess does not implement a multi-master conflict resolution strategy (like CRDTs) for resharding.

Verification and Safety

To verify the synchronization state before initiating the final traffic switch, use the vtctlClient to monitor the VReplication lag. A lag of zero indicates the destination is ready for cut-over.

# Example check for VReplication stream status
vtctlClient GetVReplicationStreamStatus <keyspace> <stream_id>

Missing Diagnostic Detail: To provide a more tailored recommendation, please specify if any custom VReplication filters or IgnoreColumns are configured for this stream, as these can alter which data is synchronized during the final sync.