What's In a Workflow Queue Name?
There is a queuing infrastructure in Windows Workflow that facilitates
communications between workflows and the outside world. Event related
activities, like the HandleExternalEvent and Delay activities, provide
a formalized layer of abstraction over this queuing infrastructure. You
might never need to know that a queuing service exists if you can get
100% of the job done with components from the base activity library.
If you need more flexible messaging, or like Harry, want to work with
a low level API, you can use the queuing service to create queues and
en-queue messages for your own purposes. Before jumping in, it's
worthwhile to study how the built-in activities make use of workflow
queues, particularly workflow queue names.
If we look at the correlated local service example in the Windows SDK,
we'll find a Parallel activity with two sequences inside. Both
sequences call a CreateTask method and expect a TaskCompleted event to
arrive. A local service we've implemented has to raise this
TaskCompleted event, but the event isn't delivered directly to the
running workflow. Instead, the workflow runtime catches the event. The
workflow instance might be unloaded from memory and living as a
serialized blob in a database table. The runtime will reload the proper
workflow and deliver the event.
If there are multiple workflow instances active, it's easy for the
runtime to find the right instance because of the InstanceId property
on all incoming event arguments (local service communications might
define event arguments that derive from the ExternalDataEventArgs
class).
But how does the workflow know what activity is waiting for the event?
This isn't hard if there is only one activity waiting for an event, but
in this sample we have two activities both waiting for the same event
from the same service.The secret is in the workflow queue names the individual activities
create to wait for the events. If we use GetWorkflowQueueData on the
workflow instance, we can inspect the queue names. A queue name in the
this SDK sample will look like the following:
Message Properties
Interface Type:ITaskService
Method Name:TaskCompleted
CorrelationValues:
001
What we see is that the workflow queue "name" actually contains all
of the information the runtime needs to deliver events to their proper
destination. The name includes the interface and event name, as well as
correlation values when the associated activities have correlation
tokens.
Queue names in windows Workflow are more than just friendly
identifiers. As Harry pointed out, they implement IComparable and are
actually a key piece to how Windows Workflow works with data exchange
services.