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https://issues.apache.org/jira/browse/SOLR-11985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16520919#comment-16520919
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Noble Paul edited comment on SOLR-11985 at 6/23/18 1:53 AM:
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bq. for the collection with 4 replicas. In the collection with 4 replicas, you
could have 2 replicas on us-east-1a and 2 replicas on us-east-1b. What we
really want is 1 on each before having the 4th replica on another zone...
In reality that is what happens. it starts allotting one at a time and you end
up with 1 on each zone and another one ends up in a random zone.
But the problem is that once you are already in a badly distributed cluster, it
won't show any violations.
Once we are done with SOLR-12511, that ceases to be a problem. your rules will
look like
{code}
{"replica" : "33%", "shard" : "#EACH", "sysprop:region": "us-east-1a"}
{"replica" : "33%", "shard" : "#EACH", "sysprop:region": "us-east-1b"}
{"replica" : "33%", "shard" : "#EACH", "sysprop:region": "us-east-1c"}
{code}
this means the effective policy for a shard with 4 replicas is
{code}
{"replica" : "1.33", "shard" : "#EACH", "sysprop:region": "us-east-1a"}
{"replica" : "1.33", "shard" : "#EACH", "sysprop:region": "us-east-1b"}
{"replica" : "1.33", "shard" : "#EACH", "sysprop:region": "us-east-1c"}
{code}
This means that any zone with 0 replicas is a violation.
was (Author: noble.paul):
bq. for the collection with 4 replicas. In the collection with 4 replicas, you
could have 2 replicas on us-east-1a and 2 replicas on us-east-1b. What we
really want is 1 on each before having the 4th replica on another zone...
In reality that is what happens. it starts allotting one at a time and you end
up with 1 on each zone and another one ends up in a random zone.
Once we are done with SOLR-12511, that ceases to be a problem. your rules will
look like
{code}
{"replica" : "33%", "shard" : "#EACH", "sysprop:region": "us-east-1a"}
{"replica" : "33%", "shard" : "#EACH", "sysprop:region": "us-east-1b"}
{"replica" : "33%", "shard" : "#EACH", "sysprop:region": "us-east-1c"}
{code}
this means the effective policy for a shard with 4 replicas is
{code}
{"replica" : "1.33", "shard" : "#EACH", "sysprop:region": "us-east-1a"}
{"replica" : "1.33", "shard" : "#EACH", "sysprop:region": "us-east-1b"}
{"replica" : "1.33", "shard" : "#EACH", "sysprop:region": "us-east-1c"}
{code}
This means that any zone with 0 replicas is a violation.
> Allow percentage in replica attribute in policy
> -----------------------------------------------
>
> Key: SOLR-11985
> URL: https://issues.apache.org/jira/browse/SOLR-11985
> Project: Solr
> Issue Type: New Feature
> 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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