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From "Christine Poerschke (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-13257) Enable replica routing affinity for better cache usage
Date Fri, 02 Aug 2019 18:45:00 GMT

    [ https://issues.apache.org/jira/browse/SOLR-13257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16899135#comment-16899135
] 

Christine Poerschke commented on SOLR-13257:
--------------------------------------------

Thanks [~tomasflobbe] for the ping! I left some comments on the pull requests but have no
concerns, it's a very elegant code solution indeed, thank you [~mgibney] for developing and
contributing it.

> 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 50m
>  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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