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https://issues.apache.org/jira/browse/SOLR-14749?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17177739#comment-17177739
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Andrzej Bialecki commented on SOLR-14749:
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{quote}If a plugin wishes to listen to a ZK node change, we should provide a
simple API to listen to that event.
{quote}
Autoscaling plugins should not need any knowledge of how Solr uses ZK for
distributed state mgmgt. This API should provide a clean separation from any ZK
internals and from how Solr implements stuff like live node tracking, or leader
election, or JSON files and their formats - so that the plugins can focus on
implementing their functionality and not on incidental aspects of ZK data.
We know very well that dealing with ZK is not straightforward under the best of
circumstances. Creating a new API that requires this knowledge is harmful in
long-term, even if it's easier short-term.
> 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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