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https://issues.apache.org/jira/browse/CASSANDRA-8119?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16035453#comment-16035453
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Jay Zhuang commented on CASSANDRA-8119:
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Basically to make sure the system could tolerate any one DC down.
For example, we have a cluster in 3 DCs (AZs in AWS), RF=3 in each DC (9 copies
in total). The requirement is to make sure the system working fine and no data
loss when one DC down.
{{local_quorum}} cannot guarantee no data loss if one DC down. {{each_quorum}}
will cause requests fail after losing one DC. {{quorum}} is what we use right
now, which requires 5 nodes alive and acknowledging the request. vs.
{{quorum_quorum}} will only need 4.
And for {{quorum_one}}, there's no easy workaround but to reduce RF to 1 in
each DC and use quorum. Which might be fine actually, if CL is one then RF=3
might not be necessary.
[~rbfblk] we're very interested in this feature, please let me know if I could
help. Or I could implement a {{quorum_quorum}} CL, it would be much easier to
do.
> More Expressive Consistency Levels
> ----------------------------------
>
> Key: CASSANDRA-8119
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8119
> Project: Cassandra
> Issue Type: New Feature
> Components: CQL
> Reporter: Tyler Hobbs
> Fix For: 4.x
>
>
> For some multi-datacenter environments, the current set of consistency levels
> are too restrictive. For example, the following consistency requirements
> cannot be expressed:
> * LOCAL_QUORUM in two specific DCs
> * LOCAL_QUORUM in the local DC plus LOCAL_QUORUM in at least one other DC
> * LOCAL_QUORUM in the local DC plus N remote replicas in any DC
> I propose that we add a new consistency level: CUSTOM. In the v4 (or v5)
> protocol, this would be accompanied by an additional map argument. A map of
> {DC: CL} or a map of {DC: int} is sufficient to cover the first example. If
> we accept a special keys to represent "any datacenter", the second case can
> be handled. A similar technique could be used for "any other nodes".
> I'm not in love with the special keys, so if anybody has ideas for something
> more elegant, feel free to propose them. The main idea is that we want to be
> flexible enough to cover any reasonable consistency or durability
> requirements.
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