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From "Shalin Shekhar Mangar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-6491) Add preferredLeader as a ROLE and a collections API command to respect this role
Date Tue, 16 Sep 2014 04:53:34 GMT

    [ https://issues.apache.org/jira/browse/SOLR-6491?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14134954#comment-14134954
] 

Shalin Shekhar Mangar commented on SOLR-6491:
---------------------------------------------

bq. Since updates do put some extra load on the leader, this can create an odd load distribution.

What's the extra load? Every replica writes to the transaction log and the index. The leader
has an extra thread or two to write updates to replicas but that is it. I don't see why that
should be expensive or create undue load. We've had bugs in the past such as SOLR-6136 but
if there are others then we should find those and fix them before we go through with this
feature.

> Add preferredLeader as a ROLE and a collections API command to respect this role
> --------------------------------------------------------------------------------
>
>                 Key: SOLR-6491
>                 URL: https://issues.apache.org/jira/browse/SOLR-6491
>             Project: Solr
>          Issue Type: Improvement
>    Affects Versions: 4.11, 5.0
>            Reporter: Erick Erickson
>            Assignee: Erick Erickson
>
> Leaders can currently get out of balance due to the sequence of how nodes are brought
up in a cluster. For very good reasons shard leadership cannot be permanently assigned.
> However, it seems reasonable that a sys admin could optionally specify that a particular
node be the _preferred_ leader for a particular collection/shard. During leader election,
preference would be given to any node so marked when electing any leader.
> So the proposal here is to add another role for preferredLeader to the collections API,
something like
> ADDROLE?role=preferredLeader&collection=collection_name&shard=shardId
> Second, it would be good to have a new collections API call like ELECTPREFERREDLEADERS?collection=collection_name
> (I really hate that name so far, but you see the idea). That command would (asynchronously?)
make an attempt to transfer leadership for each shard in a collection to the leader labeled
as the preferred leader by the new ADDROLE role.
> I'm going to start working on this, any suggestions welcome!
> This will subsume several other JIRAs, I'll link them momentarily.



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