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From "Ariel Weisberg (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-13442) Support a means of strongly consistent highly available replication with storage requirements approximating RF=2
Date Wed, 12 Apr 2017 18:35:41 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Ariel Weisberg updated CASSANDRA-13442:
---------------------------------------
    Description: 
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.

  was:
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.


> 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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