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Workflow Extension

One of the core features of WF, since WF3, has been that it is lightweight enough to be hosted in any .NET application domain.  Because the runtime can execute in different domains and may need customized execution semantics, various aspects of the runtime behaviors need to be externalized from the runtime.  That is where workflow extensions come into play.  Workflow extensions enable you as the developer writing the host code, if you so choose, to add behavior to the runtime by extending it with custom code. 

Two extension types that the WF runtime is aware of are the persistence and tracking extensions. The persistence extension provides the core functionality for saving the state of a workflow to a durable store and retrieving that state when it is needed.  The persistence extension shipping with the framework includes support for Microsoft SQL Server, but extensions can be written to support other database systems or durable stores. 

Persistence

In order to use the persistence extension, you must first setup a database to hold the state, which can be done using the SQL scripts found in %windir%\Microsoft.NET\Framework\v4.0.21006\sql\<language> (e.g. c:\windows\microsoft.net\framework\v4.0.20506\sql\en\). After creating a database, you execute the two scripts to create both the structure (SqlWorkflowInstanceStoreSchema.sql) and the stored procedures (SqlWorkflowInstanceStoreLogic.sql) within the database. 

Tracking

Persistence enables the host to support long running processes, load balance instances across hosts and other fault tolerant behaviors.  However, once the workflow instance has completed, the state in the database is often deleted as it is no longer useful.  In a production environment, having information about what a workflow is currently doing and what it has done is critical to both managing workflows and gaining insight into the business process.  Being able to track what is happening in your application is one of the compelling features of using the WF runtime.  

Tracking consists of two primary components: tracking participants and tracking profiles.  A tracking profile defines what events and data you want the runtime to track.  Profiles can include three primary types of queries:

- ActivityStateQuery – used to identify activity states (e.g. closed) and variables or arguments to extract data
- WorkflowInstanceQuery – used to identify workflow events
- CustomTrackingQuery – used to identify explicit calls to track data, usually within custom activities

Creating Custom Activities

While the base activity library includes a rich palette of activities for interacting with services, objects, and collections, it does not provide activities for interacting with subsystems such as databases, email servers, or your custom domain objects and systems.  Part of getting started with WF4 will be to figure out what core activities you might need or want when building workflows.  The great thing is that activities are composable so as you build core units of work, you can compose them into more coarse grained activities that developers can use in their workflows. 

Activity class hierarcy

For most custom activities, you will either derive from AsyncCodeActivity, CodeActivity, or NativeActivity (or one of the generic variants), or model your activity declaratively.  At a high level, the four base classes can be described as follows: 

- Activity – used to model activities by composing other activities, usually defined using XAML.
- CodeActivity – a simplified base class when you need to write some code to get work done.
- AsyncCodeActivity – used when your activity perform some work asynchronously.
- NativeActivity – when your activity needs access to the runtime internals, for example to schedule other activities or create bookmarks.

Composing activities using Activity designer

When you create a new activity library project, or when you add a new item to a WF project and select the Activity template, what you get is a XAML file with an empty Activity element in it.  In the designer, this presents itself as a design surface where you can create the body of the activity.  To get started with an activity that will have more than one step, it usually helps to drag a Sequence activity in as the Body and then populate that with your actual activity logic as the body represents a single child activity. 

You can think of the activity much like you would a method on a component with arguments.  On the activity itself, you can define arguments, along with their directionality, to define the interface of the activity.  Variables that you want to use within the activity will need to be defined in the activities that comprise the body, such as the root sequence we mentioned previously.  As an example, you can build a NotifyManager activity that composes two simpler activities: GetManager and SendMail. 

Writing custom activity class

If you determine that your activity logic cannot be accomplished by composing other activities, then you can write a code based activity.  When writing activities in code you derive from the appropriate class, define arguments, and then override the Execute method.  The Execute method gets called by the runtime when it is time for the activity to do its work.

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