Windows Workflow Foundation Overview

Windows Workflow Foundation is the programming model, engine, and tools for quickly building workflow-enabled applications on Windows. It consists of a namespace, an in-process workflow engine, and designers for Visual Studio 2005. Windows Workflow Foundation is a framework which enables users to create system or human workflows in their applications written for Windows Vista, Windows XP, and the Windows Server 2003 family. Windows Workflow Foundation can be used to solve simple scenarios such as showing UI controls based on user input, or complex scenarios encountered by large enterprises, such as order processing and inventory control.
Scenarios that Windows Workflow Foundation addresses include:
• Enabling workflow within line-of-business applications
• User-interface page flows
• Document-centric workflow
• Human workflows
• Composite workflows for service-oriented applications
• Business rule-driven workflows
• Workflows for systems management
Windows Workflow Foundation provides a consistent and familiar development experience with other .NET Framework 3.0 technologies, such as Windows Communication Foundation and Windows Presentation Foundation. The Windows Workflow Foundation API provides full support for Visual Basic .NET and C#, a specialized workflow compiler, debugging within a workflow, a graphical workflow designer, and developing your workflow completely in code or in markup. Windows Workflow Foundation also provides an extensible model and designer to build custom activities that encapsulate workflow functionality for end users or for reuse across multiple projects.

The concepts in this section are inherent to Windows Workflow Foundation. They are briefly described here to provide a basic understanding. The Programming Guide section of this SDK provides implementation details and a deeper level of knowledge.
A workflow is a set of elemental units called activities that are stored as a model that describes a real-world process. Workflows provide a way of describing the order of execution and dependent relationships between pieces of short- or long-running work. This work passes through the model from start to finish, and activities might be executed by people or by system functions.
Workflow Runtime Engine
Every running workflow instance is created and maintained by an in-process runtime engine that is commonly referred to as the workflow runtime engine. There can be several workflow runtime engines within an application domain, and each instance of the runtime engine can support multiple workflow instances running concurrently.
When a workflow model is compiled, it can be executed inside any Windows process including console applications, forms-based applications, Windows Services, ASP.NET Web sites, and Web services. Because a workflow is hosted in process, a workflow can easily communicate with its host application.
The following illustration shows how workflows, activities, and the workflow runtime engine are all hosted in process with a host application.

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