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From "Ariel Weisberg (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13442) Support a means of strongly consistent highly available replication with storage requirements approximating RF=2
Date Tue, 18 Apr 2017 01:54:41 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15971985#comment-15971985
] 

Ariel Weisberg commented on CASSANDRA-13442:
--------------------------------------------

CASSANDRA-7168 is about adding a consistency level. This ticket is not about changing consistency
levels and doesn't require applications to change their usage of consistency levels to benefit.

7168 also does not have reducing storage requirements as a goal.

They both share the observation that repair creates opportunities to make assumptions about
the data set and what is necessary to provide strong consistency.

> Support a means of strongly consistent highly available 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 (where N
varies with RF) 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.



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