[jira] [Commented] (SOLR-9706) fetchIndex blocks incoming queries when issued on a replica in SolrCloud

["Shalin Shekhar Mangar (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-9706?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15625330#comment-15625330 ] 

Shalin Shekhar Mangar commented on SOLR-9706:
---------------------------------------------

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the proper behavior as a fetchIndex during "normal" SolrCloud operation indicates that the replica's index is too far out of date and _shouldn't_ serve queries, this is a special case.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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