[jira] [Created] (SOLR-16438) Shard split should be able to set preferred leaders on other replicas

["Bruno Roustant (Jira)" <ji...@apache.org>]

Bruno Roustant created SOLR-16438:
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             Summary: Shard split should be able to set preferred leaders on other replicas
                 Key: SOLR-16438
                 URL: https://issues.apache.org/jira/browse/SOLR-16438
             Project: Solr
          Issue Type: Improvement
      Security Level: Public (Default Security Level. Issues are Public)
            Reporter: Bruno Roustant


Currently, shard split always create a first replica for each sub-shard on the current host. Then it creates other replicas and their sub-shard is in RECOVERY state. The effect is that the first replica (on the current host) is always the leader, meaning that if the sub-shards are split themselves, their sub-sub-shards leaders are also on the same host.
This can lead to very unbalanced situation where the same host is the leader for a whole set of shards.

A solution to distribute evenly the leaders is to flag some other replicas with the preferredLeader property during the split. Then a rebalance-leaders command can elect the appropriate leaders. If we do that for each split, then all the sub-shards have their leaders correctly balanced.

To go further, we can improve CollectionsHandler#CollectionOperation to support combined operations. That way a CollectionOperation#SPLITSHARD_OP can trigger a split op, then a wait for split completion op, and then a rebalance leaders op.



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