Drupal Hooks, Modules
Drupal offers a system of callback hooks
at various parts of the core code. The hooks are placed in key
locations within the core code, such as when adding a user. When
the core code hits a hook, it will call out to all implementers
of a given hook, finding implementers by looking for method names
that adhere to a naming scheme (replace the word 'hook' in the
hook's name with name of your module). These implementing methods
are placed in "modules" that are just PHP files that
also follow a particular file-naming scheme (*.module) and are
placed in particular directories (sites/all/modules/*).
Workflow-ng
A module workflow-ng,
for example, implements several hooks that fire when users and
posts are created, updated, deleted, etc. Workflow-ng offers a
GUI configuration screen that can configure Drupal to do such
actions as sending an email somewhere when a new user is registered.
Workflow-ng even creates its own hooks for callback, and employs
its own kind of module system using a ".inc" filename
suffix for implementers of callback functions.
The actions in Workflow-ng include setting the author, changing
status (e.g., to published), scheduling an event, etc. There is
no action in Workflow-ng to execute arbitrary PHP code, but there
is a request for it.
One way to get events from Drupal would be to implement that
'arbitrary PHP code' feature, then use it to send events into
an ActiveMQ
(JMS) message
broker. A similar event broker is MantaRay.
Another alternative would be to add an action to Workflow-ng
that sends a message to some pre-configured message broker for
each event of interest.
Writing module for events
Cutting out the workflow-ng association, one could write
a module that directly sends events to any pre-configured
message broker. This module
development would show a configuration screen like the workflow-ng,
but the only action would be to turn on the flow of messages.
Some configuration would available for specifying the URL for
the message broker, and perhaps configuration would be added for
constructing the individual messages payload. However, it is likely
that the any available information about the event would always
be sent in its entirety in a message payload.
Using the Stomp protocol to JMS
See notes
on getting the Stomp protocol to work to connect php and JMS.
That page's comments can be summarized:
- PHP needs the socket extension (not default on windows)
- Stomp implementation has some deprecations (turn on PHP error
display or check logging)
The msgqueue Module for Drupal
The msgqueue module is a sample
of how this all would work in practice.
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