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Overview

Analyzing how we use observation in our Sling-based apps shows a number of recurring patterns, described on this page.

Cached Content

Scenario

In-memory data structures, or "compiled" versions of some content, are created when content changes.

Typical uses

Configurations, CSS/javascript processing, Sling installer, etc.

Trigger

Fine-grained detection of changes in a tree of content, based on paths, path regexps, node types or any other meaningful property.

Action

Clear an internal cache that is rebuilt the next time someone needs it.

Frequency

Content changes are usually not very frequent, for the above typical uses.

Performance requirements

Some latency between content changes and processing is usually not a problem.

Potential issues

N requests coming just after clearing the cache should cause just one cache rebuild, not N.

Code using this pattern for configurations might rather take advantage of OSGi configurations managed by the Sling installer.

Security considerations

Loading content in memory with an admin session will make it available to all users. Loading just the paths of the corresponding content items, and letting users retrieve the content themselves, avoids this problem.

Content Export, Replication to Remote Systems

Scenario

Content is exported as a file or pushed to a remote system when it changes.

This is similar to the Cached Content pattern.

Content Ingestion

Scenario

Files that are dropped into the repository are parsed or processed, resulting in content changes and/or workflow events.

Typical uses

Ingesting digital assets, parsing incoming email or other structured files.

Trigger

A file appears in a watched folder.

Action

Process the file, create the corresponding content, move the file to a "processed" or "rejected" folder.

Frequency

Depends on the application.

Performance requirements

Some latency is usually not a problem, but some applications need to process large number of files quickly.

Potential issues

Processing partially saved files too early can be a problem, depending on how files are added to the repository.

Security considerations

Ingestion folders must be properly secured, and incoming content quarantined unless it can be proven safe.

Content Tree Replication

Scenario

Two or more content trees are kept in sync, usually with customizable mappings and transformations.

Typical uses

Management of federations of websites, which have some common parts and some specific parts.

Trigger

Fine-grained detection of changes in a tree of content, based on paths, path regexps, node types or any other meaningful property.

Action

Replicate the source tree to the target tree(s), optionally applying customizable content transformations.

Frequency

Source events might be quite frequent depending on authoring activity.

Performance requirements

Tree transformations might be costly and usually need to run as background jobs.

Potential issues

An explosion in the number of application-level and repository-level operations is possible depending on the shape of the content tree federation and on the frequency of source content changes.

Aggregation of changes

Scenario

Collect a number of change events over time and/or for a content subtree, and provide an aggregated view.

Typical uses

Detect and act on changes to digital assets, without reacting to each and every small change.

Trigger

Fine-grained detection of changes in a tree of content, based on paths, path regexps, node types or any other meaningful property.

Action

Store and aggregate events and deliver the results on demand.

Frequency

Might be quite frequent on a busy content tree.

Performance requirements

Aggregating events efficiently, as well as storing them until they're not needed anymore, can impact performance.

Potential issues

Might create a noticeable load on the eventing/observation system, if listening to many detailed events.

Security considerations

Careless aggregation might expose privileged data.

Consistency Checks and Fixes

Scenario

Watch specific content subtrees for specific changes (nodes moved etc.) and react to them to avoid inconsistencies in the content.

Typical uses

Adapt paths that point to other pieces of content when content moves around.

Trigger

Fine-grained detection of changes in a tree of content, based on paths, path regexps, node types or any other meaningful property.

Action

Modify content to keep it consistent

Frequency

Usually not very frequent as that's mostly meant to handle edge cases.

Performance requirements

A time window during which content can be seen as inconsistent is often unavoidable, keeping that window small is useful.

Potential issues

Content must not be modified by JCR listeners, that should happen asynchronously if using JCR observation.

Workflow/Job Trigger

Scenario

Trigger workflows and jobs when content changes.

Typical uses

The unix print queue system is a good example, with folders named incoming, printing, done, rejected that store print job definitions.

Trigger

Detection of new content items in the incoming folder(s).

Action

Execute the corresponding tasks and move the job definition nodes according to the results.

Frequency

Depends on the application.

Performance requirements

Depends on the application.

Potential issues

Locking must be used if several job processors are competing for incoming jobs.

Distributed processing of those jobs introduces cluster management requirements.

Message Queue

Scenario

Implement a simple message queue backed by a content repository subtree.

This is very similar to the Workflow/Job trigger use case: messages are exchanged between producers and consumers, in the workflow/job trigger case the consumers are job processors.

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4 Comments

  1. My impression is that we don't need this (wink)

    The idea of Sling's conversion of JCR observation events to OSGi EventAdmin events is exactly this level of abstraction. By registering an OSGi EventHandler service an declaring what kinds of events you are interested in you can exactly have this filtering.

    1. If by "this" you mean the tentative API, then agreed, I'll remove it.

      The interesting thing is that a number of the patterns mentioned here might not need observation at all, a form of slow polling would work for most of them and be more scalable.

      As you say, the Sling JcrResourceListener makes it easy to listen to specific events, but to support that it has to blindly relay a lot of JCR events to the OSGi event queue, which might also hinder scalability. I'll run some tests with Oak to measure what the actual impact is.

      1. Just a minor detail on impact: OSGi events are dispatched fully asynchronous and thus don't block event processing in the repository.

  2. Note that many of the patterns here aren't really fundamentally "eventing" problems, they just appear that so far the only way of implementing them is through observation. Thus instead of trying to come up with a single eventing API or mechanism to cover all these patterns it would be better to look at the patterns separately and look for broader solutions that could be used instead of just eventing.