Transactional Delivery of Data

The previous section explored how the progress of a WF program instance, which unfolds episodically, is recorded using transactionsand, significantly, also how transaction-aware services utilized by activities can participate in these transactions. Essentially, what we learned boils down to a model for how WF program instances can send data to external systems in a transactional manner.

Now we can turn our attention to the inbound direction. How can data be handed from an external system to a WF program instance in a transactional manner? To answer this question, we return to the WorkflowInstance.EnqueueItem method that we introduced back in Chapter 3 for our ReadLine activity. The signature of this method is shown here:

 public void EnqueueItem(IComparable queueName,                         object item,                         IPendingWork service,                         object workItem); 

The queueName parameter is the name of a WF program instance queue where the WF program instance is ready to receive data. A WF program instance queue is nothing more than the data structure used by a bookmarka named location where the execution of a WF program instance (or, more specifically, an activity within a WF program instance) can be resumed.

The item parameter is the data that is being enqueued.

When we used the EnqueueItem method in the code snippets of earlier chapters, we only used the first two parameters and passed a value of null for the other two. Now, though, you can guess that it is precisely these latter two parameters that give us the ability to enqueue data in a transactional manner.

When an item is enqueued into a WF program instance queue, it is available for processing by the WF program instance. However, just as we discussed earlier, the WF program instancethough it can consume this datamay not reach its next persistence point before some failure condition occurs.

If we pass null as the value of the latter two parameters of EnqueueItem, and a failure occurs before the next WF program instance persistence point, the enqueued data is lost. Instead, if we provide a transaction-aware service and a work item when we enqueue the data, our service will get called when the next persistence point in the WF program instance is reached. A transaction will then be available so that our service can transactionally record the fact that the data was successfully handed to the WF program instance.