Overview
As software developers know, writing applications can be challenging, and we are constantly looking for tools and frameworks to simplify the process and help us focus on the business challenges we are trying to solve. We have moved from writing code in machine languages such as assembler to higher level languages like C# and Visual Basic that ease our development, remove lower level concerns such as memory management, and increase our productivity as developers. For Microsoft developers, the move to .NET allows the Common Language Runtime (CLR) to allocate memory, cleanup unneeded objects and handle low level constructs like pointers.
Much of the complexity of an application lives in the logic and processing that goes on behind the scenes. Issues such as asynchronous or parallel execution and generally coordinating the tasks to respond to user requests or service requests can quickly lead application developers back down into the low level coding of handles, callbacks, synchronization etc. As developers, we need the same power and flexibility of a declarative programming model for the internals of an application as we have for the user interface in Windows Presentation Foundation (WPF). Windows Workflow Foundation (WF) provides the declarative framework for building application and service logic and gives developers a higher level language for handling asynchronous, parallel tasks and other complex processing.
Having a runtime to manage memory and objects has freed us to focus more on the important business aspects of writing code. Likewise, having a runtime that can manage the complexities of coordinating asynchronous work provides a set of features that improves developer productivity. WF is a set of tools for declaring your workflow (your business logic), activities to help define the logic and control flow, and a runtime for executing the resulting application definition. In short, WF is about using a higher level language for writing applications, with the goal of making developers more productive, applications easier to manage, and change quicker to implement. The WF runtime not only executes your workflows for you, it also provides services and features important when writing application logic such persistence of state, bookmarking and resumption of business logic, all of which lead to thread and process agility enabling scale up and scale out of business processes.
What’s New in WF 4
In version 4 of the Microsoft® .NET Framework, Windows Workflow Foundation introduces a significant amount of change from the previous versions of the technology that shipped as part of .NET 3.0 and 3.5. In fact, the team revisited the core of the programming model, runtime and tooling and has re-architected each one to increase performance and productivity as well as to address the important feedback garnered from customer engagements using the previous versions. The significant changes made were necessary to provide the best experience for developers adopting WF and to enable WF to continue to be a strong foundational component that you can build on in your applications. I will introduce the high level changes here, and throughout the paper each topic will get more in depth treatment.
Before we continue, it is important to understand that backwards compatibility was also a key goal in this release. The new framework components are found primarily in the System.Activities.* assemblies while the backwards compatible framework components are found in the System.Workflow.* assemblies. The System.Workflow.* assemblies are part of the .NET Framework 4 and provide complete backward compatibility so you can migrate your application to .NET 4 with no changes to your workflow code. Throughout this paper we will use the name WF4 to refer to the new components found in the System.Activities.* assemblies and WF3 to refer to the components found in the System.Workflow.* assemblies.
Designers
One of the most visible areas of improvement is in the workflow designer. Usability and performance were key goals for the team for the VS 2010 release. The designer now supports the ability to work with much larger workflows without a degradation in performance and designers are all based on Windows Presentation Foundation (WPF), taking full advantage of the rich user experience one can build with the declarative UI framework. Activity developers will use XAML to define the way their activities look and interact with users in a visual design environment. In addition, rehosting the workflow designer in your own applications to enable non-developers to view and interact with your workflows is now much easier.
Data Flow
In WF3, the flow of data in a workflow was opaque. WF4 provides a clear, concise model for data flow and scoping in the use of arguments and variables. These concepts, familiar to all developers, simplify both the definition of data storage, as well as the flow of the data into and out of workflows and activities. The data flow model also makes more obvious the expected inputs and outputs of a given activity and improves performance of the runtime as data is more easily managed.
Flowchart
A new control flow activity called Flowchart has been added to make it possible for developers to use the Flowchart model to define a workflow. The Flowchart more closely resembles the concepts and thought processes that many analysts and developers go through when creating solutions or designing business processes. Therefore, it made sense to provide an activity to make it easy to model the conceptual thinking and planning that had already been done. The Flowchart enables concepts such as returning to previous steps and splitting logic based on a single condition, or a Switch / Case logic.
Programming Model
The WF programming model has been revamped to make it both simpler and more robust. Activity is the core base type in the programming model and represents both workflows and activities. In addition, you no longer need to create a WorkflowRuntime to invoke a workflow, you can simply create an instance and execute it, simplifying unit testing and application scenarios where you do not want to go through the trouble of setting up a specific environment. Finally, the workflow programming model becomes a fully declarative composition of activities, with no code-beside, simplifying workflow authoring.
Windows Communication Foundation (WCF) Integration
The benefits of WF most certainly apply to both creating services and consuming or coordinating service interactions. A great deal of effort went into enhancing the integration between WCF and WF. New messaging activities, message correlation, and improved hosting support, along with fully declarative service definition are the major areas of improvement.
Getting Started with Workflow
The best way to understand WF is to start using it and applying the concepts. We will cover several core concepts around the underpinnings of workflow, and then walk through creating a few simple workflows to illustrate how those concepts relate to each other.
Workflow Structure
Activities are the building blocks of WF and all activities ultimately derive from Activity. A terminology note - Activities are a unit of work in WF. Activities can be composed together into larger Activities. When an Activity is used as a top-level entry point, it is called a "Workflow", just like Main is simply another function that represents a top level entry point to CLR programs. This for example:
Sequence s = new Sequence
{
Activities = {
new WriteLine {Text = "Hello"},
new Sequence {
Activities =
{
new WriteLine {Text = "Workflow"},
new WriteLine {Text = "World"}
}
}
}
};
Data flow in workflows
The first thought most people have when they think about workflow is the business process or the flow of the application. However, just as critical as the flow is the data that drives the process and the information that is collected and stored during the execution of the workflow. In WF4, the storage and management of data has been a prime area of design consideration.
There are three main concepts to understand with regard to data: Variables, Arguments, and Expressions. The simple definitions for each are that variables are for storing data, arguments are for passing data, and expressions are for manipulating data.
Variables-storing data
Variables in workflows are very much like the variables you are used to in imperative languages: they describe a named location for data to be stored and they follow certain scoping rules. To create a variable, you first determine at what scope the variable needs to be available. Just as you might have variables in code that are available at the class or method level, your workflow variables can be defined at different scopes.
Arguments-passing data
Arguments are defined on activities and define the flow of data into and out of the activity. You can think of arguments to activities much like you use arguments to methods in imperative code. Arguments can be In, Out, or In/Out and have a name and type. When building a workflow, you can define arguments on the root activity which enables data to be passed into your workflow when it is invoked. You define arguments on the workflow much as you do variables using the argument window.
As you add activities to your workflow, you will need to configure the arguments for the activities, and this is primarily done by referencing in-scope variables or using expressions, which I will discuss next. In fact, the Argument base class contains an Expression property which is an Activity that returns a value of the argument type, so all of these options are related and rely on Activity.
Expression-acting on data
Expressions are activities you can use in your workflow to operate on data. Expressions can be used in places where you use Activity and are interested in a return value, which means you can use expressions in setting arguments or to define conditions on activities like the While or If activities. Remember that most things in WF4 derive from Activity and expressions are no different, they are a derivative of Activity<TResult> meaning they return a value of a specific type. In addition, WF4 includes several common expressions for referring to variables and arguments as well as Visual Basic expressions. Because of this layer of specialized expressions, the expressions you use to define arguments can include code, literal values, and variable references.
TO BE CONTINUED
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