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From "Joseph Lynch (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-14297) Optional startup delay for peers should wait for count rather than percentage
Date Wed, 07 Mar 2018 22:53:00 GMT
Joseph Lynch created CASSANDRA-14297:
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             Summary: Optional startup delay for peers should wait for count rather than percentage
                 Key: CASSANDRA-14297
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14297
             Project: Cassandra
          Issue Type: Bug
          Components: Lifecycle
            Reporter: Joseph Lynch


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|https://github.com/jasobrown/cassandra/compare/13993...jolynch:13993],
and am happy to finish it up with unit tests and such if someone can review/commit it (maybe
[~aweisberg]?).

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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