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https://issues.apache.org/jira/browse/SOLR-10265?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15931560#comment-15931560
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Mark Miller commented on SOLR-10265:
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The main reason a zk queue was used instead of in memory was that there was a
grand idea that eventually you could create the collection by simply putting
the command on the queue and call it successful, even if things crashed. The
Overseer would have the create captured and work towards its parameters over
time.
That is still ideal vs the current system. It's never been done though, and
could be accomplished without the zk queue largely.
> Overseer can become the bottleneck in very large clusters
> ---------------------------------------------------------
>
> Key: SOLR-10265
> URL: https://issues.apache.org/jira/browse/SOLR-10265
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Varun Thacker
>
> Let's say we have a large cluster. Some numbers:
> - To ingest the data at the volume we want to I need roughly a 600 shard
> collection.
> - Index into the collection for 1 hour and then create a new collection
> - For a 30 days retention window with these numbers we would end up wth
> ~400k cores in the cluster
> - Just a rolling restart of this cluster can take hours because the overseer
> queue gets backed up. If a few nodes looses connectivity to ZooKeeper then
> also we can end up with lots of messages in the Overseer queue
> With some tests here are the two high level problems we have identified:
> 1> How fast can the overseer process operations:
> The rate at which the overseer processes events is too slow at this scale.
> I ran {{OverseerTest#testPerformance}} which creates 10 collections ( 1 shard
> 1 replica ) and generates 20k state change events. The test took 119 seconds
> to run on my machine which means ~170 events a second. Let's say a server can
> process 5x of my machine so 1k events a second.
> Total events generated by a 400k replica cluster = 400k * 4 ( state changes
> till replica become active ) = 1.6M / 1k events a second will be 1600 minutes.
> Second observation was that the rate at which the overseer can process events
> slows down when the number of items in the queue gets larger
> I ran the same {{OverseerTest#testPerformance}} but changed the number of
> events generated to 2000 instead. The test took only 5 seconds to run. So it
> was a lot faster than the test run which generated 20k events
> 2> State changes overwhelming ZK:
> For every state change Solr is writing out a big state.json to zookeeper.
> This can lead to the zookeeper transaction logs going out of control even
> with auto purging etc set .
> I haven't debugged why the transaction logs ran into terabytes without taking
> into snapshots but this was my assumption based on the other problems we
> observed
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