Counter shard merging is not thread safe
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Key: CASSANDRA-3178
URL: https://issues.apache.org/jira/browse/CASSANDRA-3178
Project: Cassandra
Issue Type: Bug
Components: Core
Affects Versions: 0.8.5
Reporter: Sylvain Lebresne
Fix For: 0.8.6
The first part of the counter shard merging process is done during counter
replication. This was done there because it requires that all replica are made
aware of the merging (we could only rely on nodetool repair for that but that
seems much too fragile, it's better as just a safety net). However this part
isn't thread safe as multiple threads can do the merging for the same shard at
the same time (which shouldn't really "corrupt" the counter value per se, but
result in an incorrect context).
Synchronizing that part of the code would be very costly in term of
performance, so instance I propose to move the part of the shard merging done
during replication to compaction. It's a better place anyway. The only downside
is that it means compaction will sometime send mutations to other node as a
side effect, which doesn't feel very clean but is probably not a big deal
either.
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