C# 4 and .NET 4

Workflows ❘ 1327
persistence provider to load up an existing instance from the database. This instance is then resumed by
calling the ResumeBookmark function.
If you run the example, you’ll see a prompt onscreen. While that prompt is there the workflow is persisted
and unloaded, and you can check this by running SQL Server Management Studio and executing the
command shown in Figure 44-13.
figure 44-13
Workflow instances are stored within the InstancesTable of the System.Activities.DurableInstancing
schema. The entry shown in Figure 44-13 is the persisted instance of the workflow running on my machine.
When we continue the workflow it will eventually complete, and at that point the workflow will
be deleted from the instances table, as there’s an option on the instance store exposed as the
InstanceCompletionAction, which by default is set to be DeleteAll. This ensures that any data stored
in the database for a given workflow instance will be deleted once that workflow completes. This is a
sensible default, as once a workflow completes you would normally not expect any data to hang around.
You can change this option when you define the instance store by setting the instance completion action to
DeleteNothing.
If you now continue running the test application and then retry the SQL command from Figure 44-13, you
should notice that the workflow instance has been deleted.
Workflowservicehost
A long way back in this chapter we mentioned that there were three ways to host workflows — the last
is by using the WorkflowServiceHost class which exposes a workflow through WCF. One of the major
areas that Workflow is destined to be used for is as the back-end to WCF services. If you think about what
a typical WCF service does, it’s usually a bunch of related methods that are typically called in some sort of
order. The main problem here is that you could call these methods in any order, and usually you need to
define the ordering so that, for example, the order details are not uploaded before the order itself.
With Workflow you can easily expose WCF services that also have a notion of method ordering. The
main classes you’ll use here are the Receive and the Send activities. In the code for this example (which is
available in the 07_WorkflowsAsServices solution), the scenario used is an estate agent (a Realtor if you’re
in the United States) who wishes to upload information about a property to a web site.
The first call to the service will construct a new workflow instance, and this call will return a unique ID
that is used in subsequent calls to the service. We’ll then use this unique ID in subsequent calls, as this will
identify which workflow instance on the server we wish to communicate with. Workflow uses a facility
called “correlation” to map an incoming request to a workflow instance. Here, we’re using message-based
correlation to link a property of the incoming message with an existing workflow instance. The workflow
we’ll use for this example is outlined in Figure 44-14.
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