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  1. Cassandra
  2. CASSANDRA-4071

Topology changes can lead to bad counters (at RF=1)



    • Bug
    • Status: Resolved
    • Normal
    • Resolution: Duplicate
    • None
    • None
    • Normal


      A counter is broken into shards (partitions), each shard being 'owned' by a given replica (meaning that only this replica will increment that shard). For a given node A, the resolution of 2 shards (having the same owner) follows the following rules:

      • if the shards are owned by A, then sum the values (in the original patch, 'owned by A' was based on the machine IP address, in the current code, it's based on the shard having a delta flag but the principle is the same)
      • otherwise, keep the maximum value (based on the shards clocks)

      During topology changes (boostrap/move/decommission), we transfer data from A to B, but the shards owned by A are not owned by B (and we cannot make them owned by B because during those operations (boostrap, ...) a given shard would be owned by A and B which would break counters). But this means that B won't interpret the streamed shards correctly.

      Concretely, if A receives a number of counter increments that end up in different sstables (the shards should thus be summed) and then those increments are streamed to B as part of boostrap, B will not sum the increments but use the clocks to keep the maximum value.

      I've pushed a test that show the breakeage at https://github.com/riptano/cassandra-dtest/commits/counters_test (the test needs CASSANDRA-4070 to work correctly).

      Note that in practice, replication will hide this (even though B will have the bad value after the boostrap, read or read/repair from the other replica will repair it). This is a problem for RF=1 however.

      Another problem is that during repair, a node won't correctly repair other nodes on it's own shards (unless everything is fully compacted).




            Unassigned Unassigned
            slebresne Sylvain Lebresne
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