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https://issues.apache.org/jira/browse/SOLR-13439?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16840616#comment-16840616
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Erick Erickson commented on SOLR-13439:
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That code has been extensively re-worked, [[email protected]] would have
a much better idea of its current state.
> Make collection properties easier and safer to use in code
> ----------------------------------------------------------
>
> Key: SOLR-13439
> URL: https://issues.apache.org/jira/browse/SOLR-13439
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SolrCloud
> Affects Versions: master (9.0)
> Reporter: Gus Heck
> Assignee: Gus Heck
> Priority: Major
> Attachments: SOLR-13439.patch, SOLR-13439.patch
>
>
> (breaking this out from SOLR-13420, please read there for further background)
> Before this patch the api is quite confusing (IMHO):
> # any code that wanted to know what the properties for a collection are
> could call zkStateReader.getCollectionProperties(collection) but this was a
> dangerous and trappy API because that was a query to zookeeper every time. If
> a naive user auto-completed that in their IDE without investigating, heavy
> use of zookeeper would ensue.
> # To "do it right" for any code that might get called on a per-doc or per
> request basis one had to cause caching by registering a watcher. At which
> point the getCollectionProperties(collection) magically becomes safe to use,
> but the watcher pattern probably looks famillar induces a user who hasn't
> read the solr code closely to create their own cache and update it when their
> watcher is notified. If the caching side effect of watches isn't understood
> this will lead to many in-memory copies of collection properties maintained
> in user code.
> # This also creates a task to be scheduled on a thread (PropsNotification)
> and induces an extra thread-scheduling lag before the changes can be observed
> by user code.
> # The code that cares about collection properties needs to have a lifecycle
> tied to either a collection or something other object with an even more
> ephemeral life cycle such as an URP. The user now also has to remember to
> ensure the watch is unregistered, or there is a leak.
> After this patch
> # Calls to getCollectionProperties(collection) are always safe to use in any
> code anywhere. Caching and cleanup are automatic.
> # Code that really actually wants to know if a collection property changes
> so it can wake up and do something (autoscaling?) still has the option of
> registering a watcher that will asynchronously send them a notification.
> # Updates can be observed sooner via getCollectionProperties with no need to
> wait for a thread to run. (vs a cache held in user code)
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