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https://issues.apache.org/jira/browse/SOLR-15232?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17316328#comment-17316328
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Andrzej Bialecki commented on SOLR-15232:
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Ad 1.
Solr autoscaling is not aware of how the pods are managed. When you add a new
pod and want to populate it this always requires additional actions,
orchestrated by an outside agent.
In Solr 8x you could automate part of it (adding replicas to new empty nodes)
by using {{nodeAdded}} triggers but this is gone in 9x.
Ad 2.
AFAIK there's no specific mechanism in k8s that would allow you to "customize"
a particular pod instance on startup, i.e. to specify during pod creation that
it should host a specific replica. Furthermore, external agents need to first
check that the pod is up before proceeding, which complicates their design.
This PR fills this void because you don't need any external agent to
orchestrate the creation of replicas on new pods - you can just pass system
properties to tell the pod to automatically add replicas that you want to host
there, as soon as the Solr CoreContainer is up - and shut it down if it fails
to initialize. This provides an easier way to auto-scale by using k8s
autoscaler without modifications.
Ad 3.
This is again a multiple step process that requires an external agent to
coordinate it. I'm not saying it can't be done (obviously), but the way I
propose it could simplify the process.
> Add replica(s) as a part of node startup
> ----------------------------------------
>
> Key: SOLR-15232
> URL: https://issues.apache.org/jira/browse/SOLR-15232
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
> Priority: Major
> Fix For: main (9.0)
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> In containerized environments it would make sense to be able to initialize a
> new node (pod) and designate it immediately to hold newly created replica(s)
> of specified collection/shard(s) once it's up and running.
> Currently this is not easy to do, it requires the intervention of an external
> agent that additionally has to first check if the node is up, all of which
> makes the process needlessly complicated.
> This functionality could be as simple as adding a command-line switch to
> {{bin/solr start}}, which would cause it to invoke appropriate ADDREPLICA
> commands once it verifies the node is up.
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