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From "Joel Knighton (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13700) Heartbeats can cause gossip information to go permanently missing on certain nodes
Date Thu, 20 Jul 2017 16:26:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16094911#comment-16094911
] 

Joel Knighton commented on CASSANDRA-13700:
-------------------------------------------

Thanks, Jason! In this case, I agree the first option is safer for this issue. Something like
the second likely makes sense eventually, at least as part of a larger audit of correctness
issues in gossip. I believe your volatile suggestion is correct.

I don't have a lot of helpful information to reproduce this; it reproduces in larger clusters,
particularly with higher latency levels. We can see the effects locally with a few well-timed
sleeps in MessagingService, but that isn't terribly representative.

Branches pushed here:
||branch||
|[13700-2.1|https://github.com/jkni/cassandra/tree/13700-2.1]||
|[13700-2.2|https://github.com/jkni/cassandra/tree/13700-2.2]||
|[13700-3.0|https://github.com/jkni/cassandra/tree/13700-3.0]||
|[13700-3.11|https://github.com/jkni/cassandra/tree/13700-3.11]||
|[13700-trunk|https://github.com/jkni/cassandra/tree/13700-trunk]||

There's a somewhat conceptually similar issue when we bump the gossip generation in the middle
of constructing a reply - I believe that's the cause in [CASSANDRA-11825], which presents
similar problems. I'm choosing to address them separately because they're indeed distinct
problems and 11825 requires an additional trigger (enabling and disabling gossip during runtime).

> Heartbeats can cause gossip information to go permanently missing on certain nodes
> ----------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-13700
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13700
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Distributed Metadata
>            Reporter: Joel Knighton
>            Assignee: Joel Knighton
>            Priority: Critical
>
> In {{Gossiper.getStateForVersionBiggerThan}}, we add the {{HeartBeatState}} from the
corresponding {{EndpointState}} to the {{EndpointState}} to send. When we're getting state
for ourselves, this means that we add a reference to the local {{HeartBeatState}}. Then, once
we've built a message (in either the Syn or Ack handler), we send it through the {{MessagingService}}.
In the case that the {{MessagingService}} is sufficiently slow, the {{GossipTask}} may run
before serialization of the Syn or Ack. This means that when the {{GossipTask}} acquires the
gossip {{taskLock}}, it may increment the {{HeartBeatState}} version of the local node as
stored in the endpoint state map. Then, when we finally serialize the Syn or Ack, we'll follow
the reference to the {{HeartBeatState}} and serialize it with a higher version than we saw
when constructing the Ack or Ack2.
> Consider the case where we see {{HeartBeatState}} with version 4 when constructing an
Ack and send it through the {{MessagingService}}. Then, we add some piece of state with version
5 to our local {{EndpointState}}. If {{GossipTask}} runs and increases the {{HeartBeatState}}
version to 6 before the {{MessageOut}} containing the Ack is serialized, the node receiving
the Ack will believe it is current to version 6, despite the fact that it has never received
a message containing the {{ApplicationState}} tagged with version 5.
> I've reproduced in this in several versions; so far, I believe this is possible in all
versions.



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