[wildfly-dev] Service assumptions and the web profile

[Jeff Mesnil]

On 3 Jun 2014, at 22:01, Brian Stansberry <brian.stansberry at redhat.com> wrote:

>> One obvious question with this scheme is: what happens when Y is added,
>> after the fact?  The answer to this question depends on X.  X must, in
>> the same transaction, detect the addition of Y and decide what to do.
>> Several actions are possible, depending on how X works and how its
>> dependency on Y works.  Options include automatically removing and
>> rebuilding services with the new dependency, retroactively modifying X
>> in some way so as to update it without stopping it, or simply ignoring
>> the addition of Y, using a "reload-required" or similar state to
>> indicate to the user that the running system is inconsistent with the
>> stored model.
>> 
>> I know we don't really have any APIs to directly facilitate these
>> concepts, but this is a part of what the management SPI redesign is all
>> about.  In the new model, one will be able to express optional
>> dependencies at a resource level rather than a service level.
> 
> Jeff Mesnil -- I'm curious how useful the internal notification stuff 
> you've added in the existing code will be for this use case. Prior to 
> that there was nothing at all that X could count on to become aware of 
> the later addition Y.

Notifications would work a the resource level.
Notifications for added/removed resources are emitted[1] at the end of the step that adds/removes a resource in the MODEL stage.

So a resource X could register a notification listener on the resource Y's path address.
When the Y resource is added, the listener will be notified and X would be notified of the addition of Y and act accordingly.

If I understand the example correctly, when a resource X is added, it could check whether the resource Y is already there and use it if that’s the case.
Otherwise, it would register a notification listener and postpone this execution until the resource Y is added (with no guarantee it ever will).

[1] https://github.com/jmesnil/wildfly/commits/WFLY-266_WFLY-3159_notification_support_and_jmx#diff-889ac0c285b120937da9477f6d61ab1dR689
-- 
Jeff Mesnil
JBoss, a division of Red Hat
http://jmesnil.net/