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https://issues.apache.org/jira/browse/SOLR-9706?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15625330#comment-15625330
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Shalin Shekhar Mangar commented on SOLR-9706:
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Interesting. Does this affect non-cloud master/slave setups as well?
> 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 far out of date and _shouldn't_ serve queries, this is a special case.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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