{
  "id": "syncer",
  "title": "Syncer process",
  "url": "https://redis.io/docs/latest/operate/rs/7.8/databases/active-active/syncer/",
  "summary": "Detailed information about the syncer process and its role in distributed databases.",
  "content": "\n## Syncer process\n\nEach node in a cluster containing an instance of an Active-Active database hosts a process called the syncer.\nThe syncer process:\n\n1. Connects to the proxy on another participating cluster\n1. Reads data from that database instance\n1. Writes the data to the local cluster's primary(master) shard\n\nSome replication capabilities are also included in [Redis Open Source]().\n\nThe primary (also known as master) shard at the top of the primary-replica tree creates a replication ID.\nThis replication ID is identical for all replicas in that tree.\nWhen a new primary is appointed, the replication ID changes, but a partial sync from the previous ID is still possible.\n\n\nIn a partial sync, the backlog of operations since the offset are transferred as raw operations.\nIn a full sync, the data from the primary is transferred to the replica as an RDB file which is followed by a partial sync. \n\nPartial synchronization requires a backlog large enough to store the data operations until connection is restored. See [replication backlog]() for more info on changing the replication backlog size.\n\n### Syncer in Active-Active replication\n\nIn the case of an Active-Active database:\n\n- Multiple past replication IDs and offsets are stored to allow for multiple syncs \n- The [Active-Active replication backlog]() is also sent to the replica during a full sync. \n\n\nFull sync triggers heavy data transfers between geo-replicated instances of an Active-Active database. \n\n\nAn Active-Active database uses partial synchronization in the following situations:\n\n- Failover of primary shard to replica shard\n- Restart or crash of replica shard that requires sync from primary\n- Migrate replica shard to another node\n- Migrate primary shard to another node as a replica using failover and replica migration\n- Migrate primary shard and preserve roles using failover, replica migration, and second failover to return shard to primary\n\n\nSynchronization of data from the primary shard to the replica shard is always a full synchronization.\n\n",
  "tags": ["docs","operate","rs"],
  "last_updated": "2026-04-01T08:10:08-05:00"
}