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https://issues.apache.org/jira/browse/SOLR-13405?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16818312#comment-16818312
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Yonik Seeley commented on SOLR-13405:
-------------------------------------

0 replica support thoughts:
The idea of bringing up another replica if 1 replica seems down can naturally 
be extended to include 0 replica support.  The idea can be recast as requesting 
a new replica on demand if all existing replicas (including 0) seem down to a 
client.  One area where this is a little different is the indexing side... 
there would need to be code in the indexing paths that recognize 0 replicas 
configured and bring one up on demand.  After a certain period of inactivity, 
we'd want to return to 0 replicas.  This could probably be split off into a 
different JIRA.


> Support 1 or 0 replicas per shard
> ---------------------------------
>
>                 Key: SOLR-13405
>                 URL: https://issues.apache.org/jira/browse/SOLR-13405
>             Project: Solr
>          Issue Type: New Feature
>      Security Level: Public(Default Security Level. Issues are Public) 
>            Reporter: Yonik Seeley
>            Priority: Major
>
> When multiple replicas per shard are not needed for data durability (because 
> of shared storage support on HDFS or S3, etc), other cluster configurations 
> suddenly make sense like allowing 1 or even 0 replicas per shard (primarily 
> to lower costs.)
> One big issue with a single replica per shard is that zookeeper (and thus the 
> overseer) waits for a session timeout before marking the node as down.  
> Instead of queries having to wait this long (~30 sec), if a SolrJ query 
> client detects that a node died, it can ask the overseer to quickly bring up 
> another replica.



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