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https://issues.apache.org/jira/browse/SOLR-14749?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17177221#comment-17177221
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Ilan Ginzburg commented on SOLR-14749:
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if there’s an abstraction layer between Solr and Autoscaling, then fixes to
Autoscaling can also be done independent of Solr.
If Autoscaling depends on the internals of Solr (like ZK) then changes to Solr
are more complex and/or break Autoscaling.
A plugin API should focus on the needs of the plugins, not on the current
implementation of Solr. It will be easier to use by plugin developers and not
lock us (Solr) into the current way we do things. Of course it has to reflect
Solr abstractions (node, collection, shard, replica) these likely change slower
than their implementations.
> Provide a clean API for cluster-level event processing
> ------------------------------------------------------
>
> Key: SOLR-14749
> URL: https://issues.apache.org/jira/browse/SOLR-14749
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Components: AutoScaling
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
> Priority: Major
>
> This is a companion issue to SOLR-14613 and it aims at providing a clean,
> strongly typed API for the functionality formerly known as "triggers" - that
> is, a component for generating cluster-level events corresponding to changes
> in the cluster state, and a pluggable API for processing these events.
> The 8x triggers have been removed so this functionality is currently missing
> in 9.0. However, this functionality is crucial for implementing the automatic
> collection repair and re-balancing as the cluster state changes (nodes going
> down / up, becoming overloaded / unused / decommissioned, etc).
> For this reason we need this API and a default implementation of triggers
> that at least can perform automatic collection repair (maintaining the
> desired replication factor in presence of live node changes).
> As before, the actual changes to the collections will be executed using
> existing CollectionAdmin API, which in turn may use the placement plugins
> from SOLR-14613.
> h3. Division of responsibility
> * built-in Solr components (non-pluggable):
> ** cluster state monitoring and event generation,
> ** simple scheduler to periodically generate scheduled events
> * plugins:
> ** automatic collection repair on {{nodeLost}} events (provided by default)
> ** re-balancing of replicas (periodic or on {{nodeAdded}} events)
> ** reporting (eg. requesting additional node provisioning)
> ** scheduled maintenance (eg. removing inactive shards after split)
> h3. Other considerations
> These plugins (unlike the placement plugins) need to execute on one
> designated node in the cluster. Currently the easiest way to implement this
> is to run them on the Overseer leader node.
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