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Peter Schuller commented on CASSANDRA-2338:
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I think this highlights even better than CASSANDRA-2013 that there is at least
interest (if not need) to be able to specify with finer granularity what your
requirements are. One can probably come up with a list of 15 cases and suddenly
ConsistencyLevel is a huge beast. A few seemingly reasonable/realistic
situations I can think of:
* Your situation, except there are 3 DC:s, 2 of which are production and one
analysis. Now you want LOCAL_QUORUM_PLUS_ONE_IN_THIS_OTHER_PARTICULAR_REMOTE_DC
* You may want the reverse of requiring additional replicas; e.g., "reads shall
never try to go beyond the local DC; I prefer failures/timeouts)"
* Maybe you one one replicate per DC, but no QUOROM in each.
It would be a bigger API change, and a more invasive code change. The simplest
way to do this I can think of, is to allow the passing of a
"ReplicaSelectionPolicy" (for lack of a better word right now) which allows you
to specify, globally and on a per-dc level (assuming dc:s are named):
(1) How many replicates are *allowed* to be consulted.
(2) How many replicas are *required* to be consulted.
(1) would be of interest for reads, and (2) for both reads and writes.
Is this complete overkill?
> C* needs a LOCAL_QUORUM_PLUS_ONE_REMOTE consistency level
> ---------------------------------------------------------
>
> Key: CASSANDRA-2338
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2338
> Project: Cassandra
> Issue Type: New Feature
> Reporter: Matthew F. Dennis
> Priority: Minor
>
> for cases where people want to run C* across multiple DCs for disaster
> recovery et cetera where normal operations only happen in the first DC (e.g.
> no writes/reads happen in the remove DC under normal operation) neither
> LOCAL_QUORUM or EACH_QUORUM really suffices.
> Consider the case with RF of DC1:3 DC2:2
> LOCAL_QUORUM doesn't provide any guarantee that data is in the remote DC.
> EACH_QUORUM requires that both nodes in the remote DC are up.
> It would be useful in some situations to be able to specify a LOCAL_QUORUM
> (for local consistency) + "at least one remote" for durability/disaster
> proofing.
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