Repairing nodes 

Node repair topics.

Over time, data in a replica can become inconsistent with other replicas due to the distributed nature of the database. Node repair corrects the inconsistencies so that eventually all nodes have the same and most up-to-date data. It is important part of regular maintenance for every Cassandra cluster.

Cassandra provides the following repair processes:
  • Hinted Handoff

    If a node becomes unable to receive a particular write, the write's coordinator node preserves the data to be written as a set of hints. When the node comes back online, the coordinator effects repair by handing off hints so that the node can catch up with the required writes.

  • Read Repair

    During the read path, a query assembles data from several nodes. The coordinator node for this write compares the data from each replica node. If any replica node has outdated data, the coordinator node sends it the most recent version. The scope of this type of repair depends on the keyspace's replication factor. During a write, Cassandra collects only enough replica data to satisfy the replication factor, and only performs read repair on nodes that participate in that write operation.

    But Cassandra can also choose a write at random and perform read repair on all replicas, regardless of the replication factor.

  • Anti-Entropy Repair

    Cassandra provides the nodetool repair tool to ensure data consistency across replicas; it compares the data across all replicas and then updates the data to the most recent version. Use nodetool repair as part of your regular maintenance routine.

    Warning: DataStax recommends stopping repair operations during topology changes; the Repair Service does this automatically. Repairs running during a topology change are likely to error when it involves moving ranges.

You can use Cassandra settings or Cassandra tools to configure each type of repair. For details on when to use each type of repair and how to configure each one, see the pages listed above.