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https://issues.apache.org/jira/browse/CASSANDRA-21010?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ariel Weisberg updated CASSANDRA-21010:
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Summary: CEP-46: Witness enable/disable implementation, testing,
documentation (was: Witness enable/disable implementation, testing,
documentation)
> CEP-46: Witness enable/disable implementation, testing, documentation
> ---------------------------------------------------------------------
>
> Key: CASSANDRA-21010
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21010
> Project: Apache Cassandra
> Issue Type: Sub-task
> Reporter: Ariel Weisberg
> Priority: Normal
>
> The original process for enabling/disabling witnesses was that enabling them
> entailed changing the schema and then running nodetool cleanup. I believe
> this is still the case.
> The original process for disabling witnesses is reduce the number of
> witnesses to 0 and then increase the number of full replicas. With say 6
> replicas total and only 4 full replicas this means f will be 1 in terms of
> availability. In terms of write durability since quorum will be 3/4 you can
> still lose 2 nodes without losing any writes. This may be undesirable, but
> fine. If you ran RF=5 (or had only 3 full replicas) write durability would
> only allow for 1 lost replica which is more problematic. You would also
> immediately lose availability with only 3 full replicas and 3 witnesses when
> you attempted to disable the witnesses.
> The code that documents this is in `AlterKeyspaceStatement`:
> ```
> //This is true right now because the transition from transient -> full
> lacks the pending state
> //necessary for correctness. What would happen if we allowed this is
> that we would attempt
> //to read from a transient replica as if it were a full replica.
> if (oldFull > newFull && oldTrans > 0)
> throw new ConfigurationException("Can't add full replicas if there
> are any transient replicas. You must first remove all transient replicas,
> then change the # of full replicas, then add back the transient replicas");
> ```
> It would be more optimal to have a pending state for witnesses and then add
> them back to the read data placement when repair completes.
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