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https://issues.apache.org/jira/browse/SOLR-6513?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14155271#comment-14155271
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Erick Erickson commented on SOLR-6513:
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By assigning the property, the cluster will tend toward the desired
configuration without _any_ "reassign leader now" command simply because each
node with the property will put itself at the head of the list for leader
election. So having an auto-assignment that does this is an easy way to set up
a cluster that stays balanced "well enough most of the time". Then the
"reassign leaders now" command can be used when the system gets out of whack.
It's _really_ tedious to assign, say, 100+ shards manually ;).
Additionally, (see the discussion at SOLR-6491) consider a heterogeneous
network of machines of varying capacities hosting many collections. Building a
tool to auto-rebalance them all is a R&D project ;). Absent that, having a
preferred leader property that can be assigned allows the operators to
incorporate their knowledge of the hardware, loads (i.e. one collection may be
much hotter than others) etc.
Given the need for manual assignment, using the same mechanism for optionally
assigning the property automatically is actually somewhat simpler overall, at
least to me.
> Add a collectionsAPI call BALANCESLICEUNIQUE
> --------------------------------------------
>
> Key: SOLR-6513
> URL: https://issues.apache.org/jira/browse/SOLR-6513
> Project: Solr
> Issue Type: Improvement
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Attachments: SOLR-6513.patch
>
>
> Another sub-task for SOLR-6491. The ability to assign a property on a
> node-by-node basis is nice, but tedious to get right for a sysadmin,
> especially if there are, say, 100s of nodes hosting a system. This JIRA would
> essentially provide an automatic mechanism for assigning a property. This
> particular command simply changes the cluster state, it doesn't do anything
> like re-assign functions.
> My idea for this version is fairly limited. You'd have to specify a
> collection and there would be no attempt to, say, evenly distribute the
> preferred leader role/property for this collection by looking at _other_
> collections. Or by looking at underlying hardware capabilities. Or....
> It would be a pretty simple round-robin assignment. About the only
> intelligence built in would be to change as few roles/properties as possible.
> Let's say that the correct number of nodes for this role turned out to be 3.
> Any node currently having 3 properties for this collection would NOT be
> changed. Any node having 2 properties would have one added that would be
> taken from some node with > 3 properties like this.
> This probably needs an optional parameter, something like
> "includeInactiveNodes=true|false"
> Since this is an arbitrary property, one must specify sliceUnique=true. So
> for the "preferredLeader" functionality, one would specify something like:
> action=BALANCESLICEUNIQUE&property=preferredLeader&proprety.value=true.
> There are checks in this code that require the preferredLeader to have a t/f
> value and require that sliceUnique bet true. That said, this can be called on
> an arbitrary property that has only one such property per slice.
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