[jira] [Created] (SOLR-9835) Create another replication mode for SolrCloud

["Cao Manh Dat (JIRA)" <ji...@apache.org>]

Cao Manh Dat created SOLR-9835:
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             Summary: Create another replication mode for SolrCloud
                 Key: SOLR-9835
                 URL: https://issues.apache.org/jira/browse/SOLR-9835
             Project: Solr
          Issue Type: Bug
      Security Level: Public (Default Security Level. Issues are Public)
            Reporter: Cao Manh Dat


The current replication mechanism of SolrCloud is called state machine, which replicas start in same initial state and for each input, the input is distributed across replicas so all replicas will end up with same next state. 

But this type of replication have some drawbacks
- The commit (which costly) have to run on all replicas
- Slow recovery, because if replica miss more than N updates on its down time, the replica have to download entire index from its leader.

So we create create another replication mode for SolrCloud called state transfer, which acts like master/slave replication. In basically
- Leader distribute the update to other replicas, but the leader only apply the update to IW, other replicas just store the update to UpdateLog (act like replication).
- Replicas frequently polling the latest segments from leader.

Pros:
- Lightweight for indexing, because only leader are running the commit, updates.
- Very fast recovery, replicas just have to download the missing segments.



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