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From "Joseph Lynch (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-14297) Optional startup delay for peers should wait for count rather than percentage
Date Fri, 24 Aug 2018 00:47:00 GMT


Joseph Lynch commented on CASSANDRA-14297:

I updated the patch to fix the merge conflicts, and reduced it two just two options to make
life easier (the default is tuned to wait for all but 2 local DC nodes and not care about
non local DC == local_quorum).

This is ready for review.

> Optional startup delay for peers should wait for count rather than percentage
> -----------------------------------------------------------------------------
>                 Key: CASSANDRA-14297
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Lifecycle
>            Reporter: Joseph Lynch
>            Assignee: Joseph Lynch
>            Priority: Minor
>              Labels: 4.0-feature-freeze-review-requested, PatchAvailable
> As I commented in CASSANDRA-13993, the current wait for functionality is a great step
in the right direction, but I don't think that the current setting (70% of nodes in the cluster)
is the right configuration option. First I think this because 70% will not protect against
errors as if you wait for 70% of the cluster you could still very easily have {{UnavailableException}}
or {{ReadTimeoutException}} exceptions. This is because if you have even two nodes down in
different racks in a Cassandra cluster these exceptions are possible (or with the default
{{num_tokens}} setting of 256 it is basically guaranteed). Second I think this option is not
easy for operators to set, the only setting I could think of that would "just work" is 100%.
> I proposed in that ticket instead of having `block_for_peers_percentage` defaulting to
70%, we instead have `block_for_peers` as a count of nodes that are allowed to be down before
the starting node makes itself available as a coordinator. Of course, we would still have
the timeout to limit startup time and deal with really extreme situations (whole datacenters
down etc).
> I started working on a patch for this change [on github|],
and am happy to finish it up with unit tests and such if someone can review/commit it (maybe
> I think the short version of my proposal is we replace:
> {noformat}
> block_for_peers_percentage: <percentage needed up, defaults to 70%>
> {noformat}
> with either
> {noformat}
> block_for_peers: <number that can be down, defaults to 1>
> {noformat}
> or, if we want to do even better imo and enable advanced operators to finely tune this
behavior (while still having good defaults that work for almost everyone):
> {noformat}
> block_for_peers_local_dc:  <number that can be down, defaults to 1>
> block_for_peers_each_dc: <number that can be down, defaults to sys.maxint>
> block_for_peers_all_dcs: <number that can be down, defaults to sys.maxint>
> {noformat}
> For example if an operator knows that they must be available at {{LOCAL_QUORUM}} they
would set {{block_for_peers_local_dc=1}}, if they use {{EACH_QUOURM}} they would set {{block_for_peers_local_dc=1}},
if they use {{QUORUM}} (RF=3, dcs=2) they would set {{block_for_peers_all_dcs=2}}. Naturally
everything would of course have a timeout to prevent startup taking too long.

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