[jira] [Created] (SOLR-13257) enable replica routing affinity for better cache usage

["Michael Gibney (JIRA)" <ji...@apache.org>]

Michael Gibney created SOLR-13257:
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             Summary: enable replica routing affinity for better cache usage
                 Key: SOLR-13257
                 URL: https://issues.apache.org/jira/browse/SOLR-13257
             Project: Solr
          Issue Type: Improvement
      Security Level: Public (Default Security Level. Issues are Public)
          Components: SolrCloud
    Affects Versions: 7.4, master (9.0)
            Reporter: Michael Gibney


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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