[jira] [Created] (CASSANDRA-13442) Support a means of strongly consistent replication with storage requirements approximating RF=2

["Ariel Weisberg (JIRA)" <ji...@apache.org>]

Ariel Weisberg created CASSANDRA-13442:
------------------------------------------

             Summary: Support a means of strongly consistent replication with storage requirements approximating RF=2
                 Key: CASSANDRA-13442
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13442
             Project: Cassandra
          Issue Type: Improvement
          Components: Compaction, Coordination, Distributed Metadata, Local Write-Read Paths
            Reporter: Ariel Weisberg


Replication factors like RF=2 can't provide strong consistency and availability because if a single node is lost it's impossible to reach a quorum of replicas. Stepping up to RF=3 will allow you to lose a node and still achieve quorum for reads and writes, but requires committing additional storage.

The requirement of a quorum for writes/reads doesn't seem to be something that can be relaxed without additional constraints on queries, but it seems like it should be possible to relax the requirement that 3 full copies of the entire data set are kept. What is actually required is a covering data set for the range and we should be able to achieve a covering data set and high availability without having three full copies. 

After a repair we know that some subset of the data set is fully replicated. At that point we don't have to read from a quorum of nodes for the repaired data. It is sufficient to read from a single node for the repaired data and a quorum of nodes for the unrepaired data.

One way to exploit this would be to have N replicas, say the last N replicas in the preference list, delete all repaired data after a repair completes. Subsequent quorum reads will be able to retrieve the repaired data from any of the two full replicas and the unrepaired data from a quorum read of any replica including the "transient" replicas.



--
This message was sent by Atlassian JIRA
(v6.3.15#6346)