What do notifications look like?

[David Hull <dmh@tibco.com>]

(This is a shorter and more concrete intro to the general WSDL issue from my previous message).

If I produce notifications, how do I tell potential consumers what they look like?

One approach (taken from the WSRF interop scenario document :-) is to say something like "

[...] the notification message generated will contain:

   <widget:TestNotification/>"

in a text document.

This seems somewhat informal.  The obvious machine-readable format is WSDL.  If I produce, say, stock quote notifications, I should advertise a Notification operation with an appropriate message type.  Note that this WSDL is associated with the producer and not with the consumer.

Also note that the WSDL need not be associated with a service that handles Subscribe operations.  There are two reasons for this

1. There is no reason to require that WSN be the only way to subscribe to notifications advertised in a WSDL -- the binding might just as well give information for some native MOM.
2. Even if WSN is the mechanism, the WSN "NotificationProducer" might not be the same service as the, erm, producer of notifications.

This seems like further evidence that "NotificationProducer" is actually more like "SubscriptionFactory," and it even gives a place to put the GetCurrentMessage operation: A (proposed) NotificationProducer advertises a Notification operation with a given message type, and may also advertise a GetCurrentMessage Request/Response operation with the same message type as its response.  The binding information for the Notification operation might include and endpoint for a (proposed) SubscriptionFactory implementing WSN, or it might specify some other mechanism.