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  1. Cassandra
  2. CASSANDRA-15601

Ensure repaired data tracking reads a consistent amount of data across replicas

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      When generating a digest for repaired data tracking, the amount of repaired data that needs to be read may depend on the unrepaired data on the replica. As this may vary between replicas, digest mismatches can be reported even though the repaired data may actually be in sync.

      For example, two replicas, A & B and a table like

      CREATE TABLE t  (pk int, ck int, PRIMARY KEY (pk, ck)) WITH CLUSTERING ORDER BY ck DESC; 
      
      Unrepaired
      ===========
      Instance A
      (0, 5)
      
      Instance B
      (0, 6)
      (0, 5)
      
      
      Repaired (Both A & B)
      =========
      (0, 4)
      (0, 3)
      (0, 2)
      (0, 1)
      (0, 0)
      
      SELECT * FROM tbl WHERE pk = 0 LIMIT 3;
      

      Instance A would read (0, 5) from the unrepaired set and (0, 4) (0, 3) from the repaired set.
      Instance B would read (0, 6) (0, 5) from its unrepaired set and just (0, 4) from repaired data.

      Unrepaired row/range/partition tombstones shadowing repaired data and present on some replicas but not others will have the opposite effect, with more repaired data being read in comparison.

      To fix this, when repaired data tracking is in effect each replica needs to overread during a full data read. Replicas should read up to LIMIT (i.e. the DataLimit of the ReadCommand) from the repaired set, regardless of how much is read from the unrepaired data. At the point where that amount of repaired data has been read, replica should stop updating the digest. So if unrepaired tombstones cause more than LIMIT repaired data to be read, the digest is only calculated over the first LIMIT-worth of repaired data.

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            • Assignee:
              samt Sam Tunnicliffe
              Reporter:
              samt Sam Tunnicliffe
              Authors:
              Sam Tunnicliffe
              Reviewers:
              Aleksey Yeschenko
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              • Created:
                Updated:
                Resolved: