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https://issues.apache.org/jira/browse/SOLR-8973?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15241505#comment-15241505
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Shalin Shekhar Mangar commented on SOLR-8973:
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bq. Won't the watch-setup fail when collection is not available on ZK?
This is not possible because when a collection create API is invoked, the
Overseer message handler will first create the collection in ZK, wait upto 30
seconds to see the collection in ZK and only then proceeds to create the cores.
So the collection always exists in ZK when the core create is called and in
such a case the watch-setup will always succeed.
> TX-frenzy on Zookeeper when collection is put to use
> ----------------------------------------------------
>
> Key: SOLR-8973
> URL: https://issues.apache.org/jira/browse/SOLR-8973
> Project: Solr
> Issue Type: Bug
> Components: SolrCloud
> Affects Versions: 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, master, 5.6
> Reporter: Janmejay Singh
> Assignee: Shalin Shekhar Mangar
> Labels: collections, patch-available, solrcloud, zookeeper
> Attachments: SOLR-8973.patch
>
>
> This is to do with a distributed data-race. Core-creation happens at a time
> when collection is not yet visible to the node. In this case a fallback
> code-path is used which de-references collection-state lazily (on demand) as
> opposed to setting a watch and keeping it cached locally.
> Due to this, as requests towards the core mount, it generates ZK fetch for
> collection proportionately. On a large solr-cloud cluster, this generates
> several Gbps of TX traffic on ZK nodes. This affects indexing
> throughput(which floors) in addition to running ZK node out of network
> bandwidth.
> On smaller solr-cloud clusters its hard to run into, because probability of
> this race materializing reduces.
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