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From "Hoss Man (JIRA)" <>
Subject [jira] [Commented] (SOLR-13490) waitForState/registerCollectionStateWatcher can see stale liveNodes data due to (Zk) Watcher race condition
Date Fri, 14 Jun 2019 21:56:00 GMT


Hoss Man commented on SOLR-13490:

Gah ... it jus occured to me i shouldn't be adding a {{DocCollectionPredicate}} interface
-- we should just be using a {{java.util.function.Predicate<DocCollection>}}.

I'll work on simplify the patch in this way...

> waitForState/registerCollectionStateWatcher can see stale liveNodes data due to (Zk)
Watcher race condition
> -----------------------------------------------------------------------------------------------------------
>                 Key: SOLR-13490
>                 URL:
>             Project: Solr
>          Issue Type: Bug
>            Reporter: Hoss Man
>            Assignee: Hoss Man
>            Priority: Major
>         Attachments: SOLR-13490.patch, SOLR-13490.patch, SOLR-13490.patch
> I was investigating some failures in {{TestCloudSearcherWarming.testRepFactor1LeaderStartup}}
which lead me to the hunch that {{waitForState}} wasn't ensuring that the predicates registered
would always be called if/when a node was shutdown.
> Digging into it a bit more, I found that the root cause seems to be the way the {{CollectionStateWatcher}}
/ {{CollectionStatePredicate}} APIs pass in *both* the {{DocCollection}}, and the "current"
{{liveNodes}} - but are only _triggered_ by the {{StateWatcher}} on the {{state.json}} (which
is used to rebuild the {{DocCollection}}) - when the {{CollectionStateWatcher}} / {{CollectionStatePredicate}}
are called, they get the "fresh" {{DocCollection}} but they get the _cached_ {{ZkStateReader.liveNodes}}
> Meanwhile, the {{LiveNodeWatcher}} only calls {{refreshLiveNodes()}} only updates {{ZkStateReader.liveNodes}}
and triggers any {{LiveNodesListener}} - it does *NOT* invoke any {{CollectionStateWatcher}}
that may have replicas hosted on any of changed nodes.
> Since there is no garunteed order that Watchers will be triggered, this means there is
a race condition where the following can happen...
>  * client1 has a ZkStateReader with cached {{liveNodes=[N1, N2, N3]}}
>  * client1 registers a {{CollectionStateWatcher}} "watcherZ" that cares if "replicaX"
of collectionA is on a "down" node
>  * client2 causes shutdown of node N1 which is hosting replicaX
>  * client1's zkStateReader gets a WatchedEvent for state.json of collectionA
>  ** DocCollection for collectionA is rebuilt
>  ** watcherZ is fired w/cached {{liveNodes=[N1, N2, N3]}} and the new DocCollection
>  *** watcherZ sees that replicaX is on N1, but thinks N1 is live
>  *** watcherZ says "everything ok, not the event i was waiting for" and doesn't take
any action
>  * client1's zkStateReader gets a WatchedEvent for LIVE_NODES_ZKNODE
>  ** zkStateReader.liveNodes is rebuilt
> no point in this sequence (or after this) will watcherZ be notified fired with
the updated liveNodes (unless/until another {{state.json}} change is made for collectionA.
> ----
> While this is definitely be problematic in _tests_ that deal with node lifecyle and use
things like {{SolrCloudTestCase.waitForState(..., SolrCloudTestCase.clusterShape(...))}} to
check for the expected shards/replicas, a cursory search of how/where {{ZkStateReader.waitForState(...)}}
and {{ZkStateReader.registerCollectionStateWatcher(...)}} are used in solr-core suggests that
could also lead to bad behavior in situations like reacting to shard leader loss, waiting
for all leaders of SYSTEM_COLL to come online for upgrade, running PrepRecoveryOp, etc...
(anywhere that liveNodes is used by the watcher/predicate)

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