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See also https://issues.apache.org/jira/browse/SLING-3148 for the timeline/history of this.

For now we have a basic prototype at http://svn.apache.org/repos/asf/sling/whiteboard/bdelacretaz/feature-flags/ that allows for checking if a named feature flag is active or not. This is the easy part.

What sets a feature flag

As described by Roy Fielding on the Sling dev list (http://markmail.org/message/rueoiuacmft5fdet)

Likewise, you should expect to have both a global setting for each flag and a per-user mask, and the person running the flip switch control panel needs to be able to choose from

a) off for all users
b) off for unassigned users
c) proportional assignment to on (X out of N users, max M)
d) logical (custom code) assignment to on
e) on for unassigned users
f) on for all users

Feature Flags use cases

Work in Progress

This is still experimental, work in progress, not final etc...just rough ideas for now.

Show/Hide Resources

Make some Resources completely invisible in specific requests, based on feature flags.

All descendant Resources of a hidden Resource are hidden as well.

Need to be careful about caching.

Alternate between resources based on feature flags

Resource A is visible only if SomeFeature is not active.
Resource B is visible only if SomeFeature is active.
A and B are never visible simultaneously.
Having to create multiple related features for that would be a pain.

This is basic A/B testing and also applies to soft launching a new feature (B) that replaces an existing one (A).

The constraints of the show/hide resources use case above also apply here.

Alter Resource rendering

Can be achieved transparently by hiding rendering scripts and servlets.

Need to be careful about caching.

Might need to happen on the client as well as on the server, depending on the desired caching behavior.

Use variants of OSGi services

A call to sling.getService("foo") in a rendering script, for example, can return different variants of the "foo" service depending on which feature flags are active.

The usage value needs to be weighed against implementation difficulty...

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  1. Re. Show/Hide Resources: This is currently implemented. Yet the descendant Resources ... are hidden as well part is only true if enumerating down the tree using listChildren. Directly accessing a resource below a hidden resource is still possible.

    Re Alternate between resources: This is currently implemented: a resource can be tagged with a flag name is only visible if the flag is set. When prefixing the flag name with a dash (-), the resource is only visible if the flag is disabled.

    Re Alter Resource rendering: Since scripts are resources, they can be hidden with flags and thus this should be possible.

    Re Use variants of OSGi Services: I don't think we should do that at all, because it causes different behaviour depending on how you access the service. IMHO these methods should really just be wrappers around the service access methods in the OSGi BundleContext.

    Preliminary documentation is available in the documentation staging area on the Feature Flags page.