[jira] [Updated] (SLING-10077) Mode to raise events only locally

["Timothee Maret (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SLING-10077?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Timothee Maret updated SLING-10077:
-----------------------------------
    Description: 
Currently the {{o/a/s/d/a/p/queued}} and {{o/a/s/d/a/p/distributed}} events are raised as local events in each author instance of a cluster. Handling those events in each author instance makes sense for use cases that require maintaining a distributed data structure like a cache. However, some use cases like content invalidation, would need to handle the events only once.

Currently, event handlers can support the later category of use cases by deduplicating the events using the Apache Discovery API. This works but it adds complexity in the event handlers and in some edge case, some events may be lost.

To solve both use cases better, we should move from raising local events in each author instance to raising the events only on the author leader instance and [distribute the events|https://sling.apache.org/documentation/bundles/apache-sling-eventing-and-job-handling.html#distributed-events] across the cluster. Use cases that need to de-duplicate will be able to do that by filtering the events with 

{code}
if (org.apache.sling.event.EventUtil.isLocal(event)) {
	// Process the event on the instance that sent it and thus once per cluster
}
{code}

The PackageDistributedNotifier would be run only on the cluster leader instance. The leader instance can be detected using the [Discovery API|https://sling.apache.org/documentation/bundles/discovery-api-and-impl.html#topology-changes]. We'd need to introduce a new component that listen to topology change and in case it runs on the leader, register the PackageDistributedNotifier service using

{code}
org.osgi.framework.BundleContext.registerService(PackageDistributedNotifier.class.getName(), packageDistributedNotifier, props)
{code}

  was:
Currently the `o/a/s/d/a/p/queued` and `o/a/s/d/a/p/distributed` events are raised as local events in each instance of a cluster. Raising the events in the cluster make sense for use cases that require maintaining a distributed data structure like a cache.

Some use case trigger logic on the cluster (not individual instances) and would benefit from raising a single event.

As an improvement, we could add a mode to distribute or not distribute all journal distribution events. Distribution would be enabled by default.


> Mode to raise events only locally
> ---------------------------------
>
>                 Key: SLING-10077
>                 URL: https://issues.apache.org/jira/browse/SLING-10077
>             Project: Sling
>          Issue Type: Improvement
>    Affects Versions: Content Distribution Journal Core 0.1.18
>            Reporter: Timothee Maret
>            Assignee: Timothee Maret
>            Priority: Major
>
> Currently the {{o/a/s/d/a/p/queued}} and {{o/a/s/d/a/p/distributed}} events are raised as local events in each author instance of a cluster. Handling those events in each author instance makes sense for use cases that require maintaining a distributed data structure like a cache. However, some use cases like content invalidation, would need to handle the events only once.
> Currently, event handlers can support the later category of use cases by deduplicating the events using the Apache Discovery API. This works but it adds complexity in the event handlers and in some edge case, some events may be lost.
> To solve both use cases better, we should move from raising local events in each author instance to raising the events only on the author leader instance and [distribute the events|https://sling.apache.org/documentation/bundles/apache-sling-eventing-and-job-handling.html#distributed-events] across the cluster. Use cases that need to de-duplicate will be able to do that by filtering the events with 
> {code}
> if (org.apache.sling.event.EventUtil.isLocal(event)) {
> 	// Process the event on the instance that sent it and thus once per cluster
> }
> {code}
> The PackageDistributedNotifier would be run only on the cluster leader instance. The leader instance can be detected using the [Discovery API|https://sling.apache.org/documentation/bundles/discovery-api-and-impl.html#topology-changes]. We'd need to introduce a new component that listen to topology change and in case it runs on the leader, register the PackageDistributedNotifier service using
> {code}
> org.osgi.framework.BundleContext.registerService(PackageDistributedNotifier.class.getName(), packageDistributedNotifier, props)
> {code}



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