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https://issues.apache.org/jira/browse/CASSANDRA-13225?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15870593#comment-15870593
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Sergio Bossa commented on CASSANDRA-13225:
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bq. Does the back-pressure strategy set at SLOW and a consistency level of
Quorum mean that while we will only wait for 3 replicas out of 5 for the write
to succeed we will overall be limiting the number of write requests to all
replicas based on the slowest responses we are getting from the cluster?
Yes. Specifically, the rate limiting is computed based on the ratio between
sent messages and received responses.
bq. Does this then mean that a large set of individual writes will be rate
limited by the slowest write responses and we will in effect be waiting for all
replicas even though each individual write is only waiting for Quorum replicas?
Kinda. Keep in mind we're still not actually *waiting* for all replicas, but
just rate limiting (so pausing) based on the feedback loop produced by messages
sent and responses received.
> Best Consistency Level
> ----------------------
>
> Key: CASSANDRA-13225
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13225
> Project: Cassandra
> Issue Type: New Feature
> Reporter: Connor Warrington
> Priority: Minor
>
> When writing data into a cluster there are a few consistency levels to choose
> from. When choosing the consistency level to write with you are making a
> tradeoff between consistency and failover availability. If you choose
> consistency level ALL then all replicas have to be up and when a write
> succeeds all replicas received the write. If you choose consistency level
> QUORUM then a quorum number of replicas have to be up and when a write
> succeeds at quorum number of replicas received the write. The tradeoff comes
> in when there are more then quorum nodes available for the write. We would
> like a write to succeed only when all replicas that are up have received the
> write. Hence the suggestion of best as a consistency level. This would be
> available for the existing consistency levels. The main idea behind this
> feature request is that we are okay with a replica going down (fault
> tolerance) but when the cluster is in a good state we don't mind waiting for
> all nodes to get the write. This would enable the writer to operate at speed
> of the slowest node instead of potentially getting into a state where that
> slow node gets even further behind. This would also enable back pressure to
> be better propagated through the system as the slowest node likely has back
> pressure which is trying to tell the client about but if we don't wait for
> that node the writer loses that information.
> Example scenarios:
> If we have replication factor of 3:
> ALL consistency means 3 replicas have to be up and 3 replicas have to
> successfully get the write.
> QUORUM consistency means 2 replicas have to be up and 2 replicas have to
> successfully get the write.
> BEST_QUORUM consistency means 2 replicas have be up and all up replicas have
> to successfully get the write.
> If 3 replicas are up with replication factor of 3:
> ALL would succeed as all 3 replicas are up and would return success when all
> 3 replicas get the write
> QUORUM would succeed as all 3 replicas are up and would return success when 2
> replicas get the write
> BEST_QUORUM would succeed as all 3 replicas are up and would return success
> when all 3 replicas get the write
> If 2 replicas are up with replication factor of 3:
> ALL would fail as only 2 replicas are up
> QUORUM would succeed as 2 replicas are up and would return success when 2
> replicas get the write
> BEST_QUORUM would succeed as 2 replicas are up and would return success when
> 2 replicas get the write
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