[Zope3-dev] Subscriber Ordering

[Jim Fulton]

Currently, the order that subscription adapters are called in is
undefined [1]_.  Sometimes, we *do* want some control over the order
of subscriber notification.  There are a number of ways to do this:

1. You can publish multiple events.  This has the disadvantage that
    the publisher of the events has to be aware of the desired
    ordering. It doesn't work if the ordering is a policy of the
    subscribers themselves.

2. If a subscriber needs to come after some earlier subscriber, we
    have the earlier subscriber generate an event. The later subscriber
    subscribes to the event generated by the earlier subscriber, rather
    than to the original event.  This requires the collaboration of the
    subscribers to implement the policy.

It would be nice, at times, to provide the ordering policy outside the
subscribers and the notifiers.  Here is an idea.

- We allow subscribers to be named.  There could be multiple
   subscribers of the same type [2]_ and name, so the names will
   really name classes of subscribers.

- A notifier could support ordered subscribers by allowing a sequence
   of subscriber classes to be specified. It will then call the
   subscribers in each class in the order the classes were configured.

   For example, the pluggable authentication service (PAS) might use
   this method to order its principal-creation subscribers.

- Alternatively, we could register an unnamed subscriber (for a
   subscriber type) that had a list of subscriber (class) named
   and called the subscribers for each class in order.

This feels like a conceptually straightforward model and is
sufficiently powerful, for example to handle PAS's subscription
requirements. [3]_

Thoughts?

Jim

.. [1] The implementation happens to implement a fixed order.  More
        specific subscribers are invoked before less-specific
        subscribers and subscribers of the same type are called in
        registration order.

.. [2] A subscriber type, which is also an adapter type, which is the
        combination of the required (adapted) specifications and the
        provided interface.

.. [3] It would also make it possible to use subscribers rather than
        utilities for credential extraction and challenge, which would,
        I think, be cleaner, as it would allow components to be
        registered by request type.

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