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From "Erick Erickson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-6517) CollectionsAPI call REBALANCELEADERS
Date Wed, 29 Oct 2014 18:20:34 GMT

    [ https://issues.apache.org/jira/browse/SOLR-6517?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14188733#comment-14188733
] 

Erick Erickson commented on SOLR-6517:
--------------------------------------

First, there aren't any guarantees here, it tries its best. For instance, the node may be
down etc...

Barring that though, it's pretty straightforward. The meat of the processing is in
collecitonsHandler.handleBalanceLeaders.

get the DocCollection from the cluster state.
for each slice {
  for each replica {
     if (the replica is active and NOT the leader and has the preferredLeader property set)
queue it up to become the leader
  }
}

There's some bookkeeping here to respect the various parameters about how many to reassign
at once and how long to wait (maxAtOnce and maxWaitSeconds) as well as construct a pretty
response giving all the info it can. This latter is one benefit of the heavy lifting being
in collectionsHandler rather than over in the Overseer.


> CollectionsAPI call REBALANCELEADERS
> ------------------------------------
>
>                 Key: SOLR-6517
>                 URL: https://issues.apache.org/jira/browse/SOLR-6517
>             Project: Solr
>          Issue Type: New Feature
>    Affects Versions: 5.0, Trunk
>            Reporter: Erick Erickson
>            Assignee: Erick Erickson
>             Fix For: 5.0, Trunk
>
>         Attachments: SOLR-6517.patch, SOLR-6517.patch, SOLR-6517.patch
>
>
> Perhaps the final piece of SOLR-6491. Once the preferred leadership roles are assigned,
there has to be a command "make it so Mr. Solr". This is something of a placeholder to collect
ideas. One wouldn't want to flood the system with hundreds of re-assignments at once. Should
this be synchronous or asnych? Should it make the best attempt but not worry about perfection?
Should it???
> a collection=name parameter would be required and it would re-elect all the leaders that
were on the 'wrong' node
> I'm thinking an optionally allowing one to specify a shard in the case where you wanted
to make a very specific change. Note that there's no need to specify a particular replica,
since there should be only a single preferredLeader per slice.
> This command would do nothing to any slice that did not have a replica with a preferredLeader
role. Likewise it would do nothing if the slice in question already had the leader role assigned
to the node with the preferredLeader role.



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