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

["T Jake Luciani (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15972014#comment-15972014 ] 

T Jake Luciani edited comment on CASSANDRA-13442 at 4/18/17 2:25 AM:
---------------------------------------------------------------------

bq. This ticket is not about changing consistency levels and doesn't require applications to change their usage of consistency levels to benefit.

7168 added a new CL as a way to opt-in to this new feature. Once its fully vetted it would be trivial to make it automatically use it when appropriate.

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

This idea seem risky to me. At the least it should be opt-in.  From a operators perspective you would need to consider how to handle bootstrapping or replacing a node. Also, how to handle backup and restores, etc.

The topology of the cluster would also have a new dimension that the drivers would need to consider.  Since for CL.ONE queries you would need to only use one of the replicas with all the data on it.


was (Author: tjake):
.bq This ticket is not about changing consistency levels and doesn't require applications to change their usage of consistency levels to benefit.

7168 added a new CL as a way to opt-in to this new feature. Once its fully vetted it would be trivial to make it automatically use it when appropriate.

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

This idea seem risky to me. At the least it should be opt-in.  From a operators perspective you would need to consider how to handle bootstrapping or replacing a node. Also, how to handle backup and restores, etc.

The topology of the cluster would also have a new dimension that the drivers would need to consider.  Since for CL.ONE queries you would need to only use one of the replicas with all the data on it.

> 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.



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