solr/elasticsearch 搜索请求是由分片还是副本处理？

Question

I have designed solr/elasticsearch for searching, I have a particular question. suppose I have 10K search request/seconds. so where will be my search on Shards or replica. I know replica is backup of shards.

if it happens on shards then how/why and if its on replica then how/why ?

Answer 1

Primary Shard is the original copy of data, while the replica shard is a copy of your original data.

While Indexing always happens on the original copy ie primary shards and then copied to replica shards, but the search can happen on any of the copy irrespective of original or copy of data.

Hence replicas are not only created for fault-tolerance where if you lose one copy, it can recover from copy of it, But also to improve the search performance where if one shard is overloaded (primary or replica) then search happens on the least loaded copy ie another replica.

Please refer to Adaptive replica selection in ES on how/why replicas improve the search latency.

Feel free to let me know if you need more information.

EDIT based on OP comment :

From ES 7 adaptive replica selection is by default on, so it would send to a least loaded replica but even if all shards are underutilized still it wouldn't send all search requests to primary shards to avoid overloading it. Also before ARS(adaptive replica selection), ES used to send these search requests on round-robin fashion to avoid overloading one shard.