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https://issues.apache.org/jira/browse/SOLR-9706?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15696067#comment-15696067
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Erick Erickson commented on SOLR-9706:
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Jeremy:
I haven't seen this in a straight SolrCloud setup. IIUC in regular cloud mode
we wouldn't _get_ to index fetching unless the replica were marked as something
other than "active", in which case it wouldn't get requests. It's only when
using replication outside of the normal SolrCloud usage that you can be in a
state where the index is being replicated _and_ the replica receives queries.
Thanks for taking the time to update this with your research BTW, that'll help
a lot!
> fetchIndex blocks incoming queries when issued on a replica in SolrCloud
> ------------------------------------------------------------------------
>
> Key: SOLR-9706
> URL: https://issues.apache.org/jira/browse/SOLR-9706
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Affects Versions: 6.3, trunk
> Reporter: Erick Erickson
>
> This is something of an edge case, but it's perfectly possible to issue a
> fetchIndex command through the core admin API to a replica in SolrCloud.
> While the fetch is going on, incoming queries are blocked. Then when the
> fetch completes, all the queued-up queries execute.
> In the normal case, this is probably the proper behavior as a fetchIndex
> during "normal" SolrCloud operation indicates that the replica's index is too
> far out of date and _shouldn't_ serve queries, this is a special case.
> Why would one want to do this? Well, in _extremely_ high indexing throughput
> situations, the additional time taken for the leader forwarding the query on
> to a follower is too high. So there is an indexing cluster and a search
> cluster and an external process that issues a fetchIndex to each replica in
> the search cluster periodiclally.
> What do people think about an "expert" option for fetchIndex that would cause
> a replica to behave like the old master/slave days and continue serving
> queries while the fetchindex was going on? Or another solution?
> FWIW, here's the stack traces where the blocking is going on (6.3 about).
> This is not hard to reproduce if you introduce an artificial delay in the
> fetch command then submit a fetchIndex and try to query.
> Blocked query thread(s)
> DefaultSolrCoreState.loci(159)
> DefaultSolrCoreState.getIndexWriter (104)
> SolrCore.openNewSearcher(1781)
> SolrCore.getSearcher(1931)
> SolrCore.getSearchers(1677)
> SolrCore.getSearcher(1577)
> SolrQueryRequestBase.getSearcher(115)
> QueryComponent.process(308).
> The stack trace that releases this is
> DefaultSolrCoreState.createMainIndexWriter(240)
> DefaultSolrCoreState.changeWriter(203)
> DefaultSolrCoreState.openIndexWriter(228) // LOCK RELEASED 2 lines later
> IndexFetcher.fetchLatestIndex(493) (approx, I have debugging code in there.
> It's in the "finally" clause anyway.)
> IndexFetcher.fetchLatestIndex(251).
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