Okay, so where do subscriptions come from?
So where do subscriptions come from or more importantly when do they appear. As we described in our earlier post, there are two types of subscriptions, activation subscriptions and instance (correlation) subscriptions. All activation subscriptions are created by admin tools like BizTalk Explorer or BizTalk Admin MMC. It does not make sense for the engine to create an activation subscription. (there is one exception to this and that is our caching service, but that is really not so important I just like like to through excessive amounts of detail at you). So these activation subscriptions are created when you do enlistment of your services (sendports and schedules). When you enlist a service, its activation subscription is put in the stopped state. This simply means that all messages routed via this subscription are sent immediately to the “suspendedQ”. Why is this state usefull? What happens if your backend database is down and so you can't send any messages to it. Technically, you could just let them keep failing and throwing errors and wasting your CPU cycles and filling your event log. Or you can stop your sendport and start it back up when the back-end system is backup. When you start a service, the subscription goes active. That means all messsages which are routed via this subscripton are sent to the “workQ” where they can be dequeued as soon as someone is available to process them. Also, when you start a service, we automatically resume any messages which were suspended because the service was previously stopped. Hence you can do what I described above. You could have a service running, the backend system goes down, you can stop the service, causing all messages to be routed to the suspended queue, and when the backend system is backup, you restart the service. Unenlisting a service causes the subscription to “go away”. In most cases, this means we delete it, but in all cases, this means that our no messages will be routed via this subscription. So when you are enlisting, stopping, starting, and unelisting your services (sendports and schedules) what you are really doing is playing around with the state of their activation subscriptions. It is really as simple as that.
Instance subscriptions are always generated by the runtime. Instance subscriptions are created by orchestrations which have a non-activation recieve on some correlated property. I can tell you how it works now in the runtime, but you should know that this is no guarantee as to how it will work in the future and really, it doesn't matter to you as long as it works. Instance subscriptions are created at the next persistence point after all correlation sets involved in the subscription have been initialized. If you have an activation receive which initializes correlation set C1, and a subsequent send (S1) which follows C1 followed by two more receives on C1, the two subscriptions would be created when we send the message out on S1 (assuming no internmediate persistence points like a random atomic scope). To go into more details on this, I really need to describe what a persistence point is (assuming you did not see my tech-ed talk which talked in detail about these). I guess I will have to save that for next time.
Note that convoys are as always a bit wierd and you should read the post on convoys to understand how their subscriptions are generated.
Have a good one.