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https://issues.apache.org/jira/browse/SOLR-14749?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17192974#comment-17192974
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Ishan Chattopadhyaya commented on SOLR-14749:
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I'd prefer if this were to be completely isolated away from core. Transitive 
dependencies are supported today if all modules are in contrib and their jars 
are in classpath. Anyway, let's refactor it after this is done. I can live with 
this for now. +1 to dealing with triggers totally inside packages and not 
making the concept first class. Thanks for your work on this and patience. 

> 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
>          Components: AutoScaling
>            Reporter: Andrzej Bialecki
>            Assignee: Andrzej Bialecki
>            Priority: Major
>              Labels: clean-api
>          Time Spent: 9h
>  Remaining Estimate: 0h
>
> 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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