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From "Kurt Greaves (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13442) Support a means of strongly consistent highly available replication with tunable storage requirements
Date Mon, 09 Oct 2017 03:59:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16196481#comment-16196481
] 

Kurt Greaves commented on CASSANDRA-13442:
------------------------------------------

It seems to me that the main constraint to running dense nodes is that streaming currently
takes a long time. Considering this seems to be mostly about reducing storage costs so write
bound workloads can run "dense" nodes, and storage is meant to be cheap, it seems to me a
less complex alternative would just be to remove the barriers to having large amounts of physical
storage per node.

While this idea does seem interesting, it seems very complex and you are still trading off
replicas for additional storage. Seems that the primary use case would be multiple datacenters
with transient replicas, which granted would be nice, but to some degree if you're happy storing
transient replicas in each DC well you're probably able to just store less replicas in each
datacenter anyway, at least if we had more flexible consistency levels.

> Support a means of strongly consistent highly available replication with tunable storage
requirements
> -----------------------------------------------------------------------------------------------------
>
>                 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.
> Configuration for something like this in NTS might be something similar to { DC1="3-1",
DC2="3-2" } where the first value is the replication factor used for consistency and the second
values is the number of transient replicas. If you specify { DC1=3, DC2=3 } then the number
of transient replicas defaults to 0 and you get the same behavior you have today.



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