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https://issues.apache.org/jira/browse/SOLR-13257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16899041#comment-16899041
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Tomás Fernández Löbbe commented on SOLR-13257:
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Sorry [~mgibney], I've been quite busy these days. Code looks good, I'll merge
soon unless there are any concerns ([~cpoerschke]?)
> Enable replica routing affinity for better cache usage
> ------------------------------------------------------
>
> Key: SOLR-13257
> URL: https://issues.apache.org/jira/browse/SOLR-13257
> Project: Solr
> Issue Type: New Feature
> Components: SolrCloud
> Reporter: Michael Gibney
> Assignee: Tomás Fernández Löbbe
> Priority: Minor
> Attachments: AffinityShardHandlerFactory.java, SOLR-13257.patch,
> SOLR-13257.patch
>
> Time Spent: 2h 20m
> Remaining Estimate: 0h
>
> For each shard in a distributed request, Solr currently routes each request
> randomly via
> [ShufflingReplicaListTransformer|https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/handler/component/ShufflingReplicaListTransformer.java]
> to a particular replica. In setups with replication factor >1, this normally
> results in a situation where subsequent requests (which one would hope/expect
> to leverage cached results from previous related requests) end up getting
> routed to a replica that hasn't seen any related requests.
> The problem can be replicated by issuing a relatively expensive query (maybe
> containing common terms?). The first request initializes the
> {{queryResultCache}} on the consulted replicas. If replication factor >1 and
> there are a sufficient number of shards, subsequent requests will likely be
> routed to at least one replica that _hasn't_ seen the query before. The
> replicas with uninitialized caches become a bottleneck, and from the client's
> perspective, many subsequent requests appear not to benefit from caching at
> all.
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