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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