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https://issues.apache.org/jira/browse/SOLR-13257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16917248#comment-16917248
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ASF subversion and git services commented on SOLR-13257:
--------------------------------------------------------

Commit 0c9ec35f88fc07aa6ecb0fef2deea705aaee39c2 in lucene-solr's branch 
refs/heads/master from Anshum Gupta
[ https://gitbox.apache.org/repos/asf?p=lucene-solr.git;h=0c9ec35 ]

SOLR-13257: Cleanup code and make the AffinityReplicaTransformer constructors 
private (#848)

SOLR-13257: Cleanup code and make the constructors private as the constructor 
is supposed to be called via the static getInstance method.

> 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: Anshum Gupta
>            Priority: Minor
>             Fix For: master (9.0), 8.3
>
>         Attachments: AffinityShardHandlerFactory.java, SOLR-13257.patch, 
> SOLR-13257.patch
>
>          Time Spent: 4h 40m
>  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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