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https://issues.apache.org/jira/browse/SOLR-3939?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13480162#comment-13480162
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Mark Miller commented on SOLR-3939:
-----------------------------------

I mean, in the general case, don't just let someone be the leader if they are 
active. Make them sync and only let them be the leader if they are successful.

However, if they have no recent versions, let them be the leader even if the 
sync fails. This lets us keep consistency in almost all cases. True, without 
the sync, there will still not be data loss, but I'd still like to try and 
force consistency if possible as well.
                
> Solr Cloud recovery and leader election when unloading leader core
> ------------------------------------------------------------------
>
>                 Key: SOLR-3939
>                 URL: https://issues.apache.org/jira/browse/SOLR-3939
>             Project: Solr
>          Issue Type: Bug
>          Components: SolrCloud
>    Affects Versions: 4.0-BETA, 4.0
>            Reporter: Joel Bernstein
>            Assignee: Mark Miller
>            Priority: Critical
>              Labels: 4.0.1_Candidate
>             Fix For: 4.1, 5.0
>
>         Attachments: cloud2.log, cloud.log, SOLR-3939.patch, SOLR-3939.patch
>
>
> When a leader core is unloaded using the core admin api, the followers in the 
> shard go into recovery but do not come out. Leader election doesn't take 
> place and the shard goes down.
> This effects the ability to move a micro-shard from one Solr instance to 
> another Solr instance.
> The problem does not occur 100% of the time but a large % of the time. 
> To setup a test, startup Solr Cloud with a single shard. Add cores to that 
> shard as replicas using core admin. Then unload the leader core using core 
> admin. 

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