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https://issues.apache.org/jira/browse/SOLR-11985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16518480#comment-16518480
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Jerry Bao commented on SOLR-11985:
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[~noble.paul] What would happen if I had 5 replicas and 3 zones for a shard? Is
it possible to make a rule that balances the replicas on a shard as 2 on
us-east-1a, 2 on us-east-1b, and 1 on us-east-1c?
> Allow percentage in replica attribute in policy
> -----------------------------------------------
>
> Key: SOLR-11985
> URL: https://issues.apache.org/jira/browse/SOLR-11985
> Project: Solr
> Issue Type: Sub-task
> Security Level: Public(Default Security Level. Issues are Public)
> Components: AutoScaling, SolrCloud
> Reporter: Shalin Shekhar Mangar
> Assignee: Noble Paul
> Priority: Major
> Fix For: master (8.0), 7.5
>
> Attachments: SOLR-11985.patch, SOLR-11985.patch
>
>
> Today we can only specify an absolute number in the 'replica' attribute in
> the policy rules. It'd be useful to write a percentage value to make certain
> use-cases easier. For example:
> {code:java}
> // Keep a third of the the replicas of each shard in east region
> {"replica" : "<34%", "shard" : "#EACH", "sysprop:region": "east"}
> // Keep two thirds of the the replicas of each shard in west region
> {"replica" : "<67%", "shard" : "#EACH", "sysprop:region": "west"}
> {code}
> Today the above must be represented by different rules for each collection if
> they have different replication factors. Also if the replication factor
> changes later, the absolute value has to be changed in tandem. So expressing
> a percentage removes both of these restrictions.
> This feature means that the value of the attribute {{"replica"}} is only
> available just in time. We call such values {{"computed values"}} . The
> computed value for this attribute depends on other attributes as well.
> Take the following 2 rules
> {code:java}
> //example 1
> {"replica" : "<34%", "shard" : "#EACH", "sysprop:region": "east"}
> //example 2
> {"replica" : "<34%", "sysprop:region": "east"}
> {code}
> assume we have collection {{"A"}} with 2 shards and {{replicationFactor=3}}
> *example 1* would mean that the value of replica is computed as
> {{3 * 34 / 100 = 1.02}}
> Which means *_for each shard_* keep less than 1.02 replica in east
> availability zone
>
> *example 2* would mean that the value of replica is computed as
> {{3 * 2 * 34 / 100 = 2.04}}
>
> which means _*for each collection*_ keep less than 2.04 replicas on east
> availability zone
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