Saturday, July 31, 2010

Business Process Modeling (BPM) with .NET Workflow Foundation (WF) 4.0

I'm currently reading the book Pro WF: Windows Workflow in .NET 4. As a short intermediate review: It's a great resource! I'll write a complete review when I'm done with the rest of the book. I'll also come up with a post that describes a complete overview of Workflow Foundation (WF) 4.0.

This post provides a simple example how WF can be used by our clients to apply real Business Process Modeling (BPM).

Guess we are developing a sales order system. Depending on an orders net-amount the order gets a discount. The calculation of this discount is usually best done within source code (at least in my opinion). So here is a class that represents our default implementation.
using System.Activities;

namespace WfActivityLibrary1 {
   public sealed class DefaultDiscount : CodeActivity {
      public InArgument<Order> Order { get; set; }

      protected override void Execute(CodeActivityContext context) {
         Order order = Order.Get(context);

         if (order.NetAmount >= 1000)
            order.Discount = order.NetAmount * 0.2m;
         else if (order.NetAmount >= 500)
            order.Discount = order.NetAmount * 0.1m;
         else if (order.NetAmount >= 100)
            order.Discount = order.NetAmount * 0.05m;
            order.Discount = 0m;
The used base class CodeActivity represents one of the Workflow Foundation base classes that can be used to implement custom workflow activities. Usually I'd move the business logic into a class within the Business Logic Layer (BLL) and consume that class within the WF activity. But this would increase the complexity of this sample with additional layers and without any real worth about working with WF, what's the topic of this post.

The problem with discounting is, operational business is very dynamic and the client usually needs to be able to provide selling campaigns to her/his customer. A campaign can be "20% for everything" or "50% of for premium customers".

Until now, this was the point where we started to create dozens of mapping tables and much more classes, containing hundreds of if-statements, which covered the different types of clients discounts. The client got some new masks within the application to manage all those new master data or (worse) a way to import some outside maintained Excel sheets.

However, what if the client wants to provide a new discount related on information that are not yet covered by our "discount calculation"-subsystem within our software? We started to add more master data tables, more classes, more if-statements and more master data masks within the application.
Humans are creative. Especially sales people, when they try to find a new ways to sales their products. (What's a good thing!) So requirements like calculation of discounts for selling-out campaigns are often spirals that never end.

Business Process Modeling

Often clients do not for complicated master data table relations or huge complexity within their system (which is often expensive to realize). They don't understand why it is so complicated to combine discount rates dynamically in different ways. And from a workflow based point, they are right though.

Instead of all those master data tables, and mask, the client often would prefer some way to really dynamically define their discounts with some simple components. That's the point where an application, that supports those features, enters the space of Business Process Modeling (BPM). BPM enabled software represents a huge benefit to the client, since (s)he does not longer need to specify a new change request for the software vendor whenever business changes. Instead of immutable behavior, the software is able to be adopted into the changing business processes.

BPM with WF

Based on our initial sample, let's take our already existing "Default Discount" workflow activity and one additional activity called "Fix Rate Discount". The "Fix Rate Discount" gets two arguments:
  • Order: The order to be discounted
  • Percentage: The percentage rate to apply to the order
I kept the source code of the "Fix Rate Discount" activity away, since it is really simple and does not relate to business modeling at all.
Armed with those two (notice, only two) workflow activities it's time to review our two selling-out campaigns.
  • 20% for everything
  • 50% of for premium customers
WF provides the possibility to define new workflows not only in Visual Studio but in any kind of .NET client application, simply by hosting a special WF designer control. So we can include this control into our application (or an external BPM module) and give the client the possibility to model their business by utilizing any defined activities. Even more, each new workflow represents an activity too and can be reused within any other workflow.

WF provides two different kinds of diagrams. Sequence diagrams represent a sequence of activities, executed top down. Flowchart diagrams are like any kind of well known flow chart out of tools like common BPM tools, Microsoft Visio, and most other modeling tools. Both diagram types can be easily combined with each other. Both diagram types support activities like "if"-decisions or several different loops.

I've chosen a sequence diagram for our "20% for everything" selling-off.

Here you see an excerpt of the WF workflow designer and related toolboxes. Certainly the main component is the diagram designer. Activities can be dragged into the designer and configured. The left toolbox (in my configuration) shows all workflow activities that are currently available. The properties window on the right side can be used to configure distinct activities within a diagram and exchange data. The tool-window at the bottom is used to define input and output arguments of the current workflow. For instance, the current workflow expects an input argument of type "Order" called "OrderToDiscount". Those arguments are provided at runtime when the system calls the workflow. Within the diagram the "DefaultDiscount" activity represents the systems implemented default discount calculation. The "Add 20% Discount" activity represents a "FixRateDiscount" activity where I changed the name to be more meaningful within the diagram.
For the "50% of for premium customers" workflow, I've chosen a Flowchart diagram. Not because it wouldn't be possible to do this with a Sequence diagram, but to show the other diagram type.

The "Decision" activity represents an "if"-decision that determines if the customers category is "Premium". For sure, in a real system this information evaluation should be encapsulated in a custom activity. The "DefaultDiscount" represents our system default discount calculation. The "50% Discount" activity is another instance of our "FixRateDiscount" activity.

Instrumentation Instead of Development

Nevertheless, be careful with too many logic within those workflows. Due to some very powerful activities like "Assign" to assign any value to any variable or property of an object or "Invoke" to invoke any method on an object instance, you can use WF to develop almost everything. In my opinion, this is the wrong way to use workflow engines. Those tasks are usually better placed in source code.

I see a huge strength in workflows to enable clients to change the processing behavior of a system. I don't see a worth in letting the client develop her/his system. Usually they neither do have the skills to do this, nor they are interested in tiny low level tasks within their models.

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