The Windows Workflow Foundation (WF) ships with a Policy Activity that allows you to execute a set of rules against your workflow. This activity contains a design time rules editor that allows you to create a set of rules. At run time, the Policy Activity runs these rules using the WF Rules engine.

Among other features, the rules engine allows you to prioritize rules and to set a chaining policy to govern rules evaluation.  The rules engine uses a set of Code DOM expressions to represent the rules. These rules can be run against any managed object, not just a workflow. Hence, the mechanisms of the rules engine have nothing to do with workflow. You can actually instantiate and use this rules engine without having to embed it inside of a workflow. You can use this rules engine to build rules-driven .NET applications.

I gave a talk at the last Las Vegas VSLive! that demonstrates how to do this. The first sample in the talk uses a workflow to demonstrate the power of the rules engine. The second and third samples use a very simple example to demonstrate how to use the engine outside of a workflow.

Two problems have to be solved.  You have to create a set of Code DOM expressions for the rules. You have to host the engine and supply it the rules and the object to run the rules against.

While the details are in the slides and the examples, here is the gist of the solution.

To use the rules engine at runtime, you pull the workflow rules out of some storage mechanism. The first sample uses a file. A WorkflowMarkupSerializer instance deserializes the stored rules to an instance of the RuleSet class.  A RuleValidation instance validates the rules against the type of the business object against which you will run the rules against. The Execute method on the RuleExecution class is used to invoke the rules engine and run the rules.

How do you create the rules? Ideally you would use some domain language, or domain based application, that would generate the rules as Code DOM expressions. If you were masochistic enough, you could create those expressions by hand.

As an alternative, the second sample hosts the Workflow rules editor dialog (RuleSetDialog class) to let you create the rules. Unfortunately, like the workflow designer, this is a programmer's tool, not a business analyst's tool. A WorkflowMarkupSerializer instance is used to serialize the rules to the appropriate storage.

I would be interested in hearing about how people use this engine to build rules driven applications.

David Chappell (http://www.davidchappell.com/HTML_email/Opinari_No16_8_06.html) argues that SOA may not foster the service reuse that everyone has been hoping for. I think his analysis is correct, but I think with business services we at least have a reasonable hope of achieving reuse. Here we are least dealing with the way things actually happen in the world as opposed to programmer abstractions such as objects or components. That combined with the looser coupling of services gives me some hope.

The reason why frameworks like .NET are successful is they reflect years and years of experience with programming problems. Many examples of reuse (such as file systems and compilers) are so embedded in our experience that we no longer see them for what they are.

Reuse may fail here as well for all the reasons mentioned in David Chappell's analysis. At least now I feel we are on the right track.

The Reference Model for Service Oriented Architecture defines a vocabulary for building service-oriented systems. Put together by a technical committee operating under the auspices of the OASIS standards organization, it is the result of individuals and organizations representing vendors, users, governments, consulting organizations, and academic institutions.

The Reference Model (RM) sees SOA as a means for organizing and using distributed capabilities that may be under the control of different ownership domains. The RM is not an architecture. It does not attempt to make any architecture normative. It does not try to make any standard or set of standards normative.

It does provide a common set of semantics that can be used across different implementations. This does sound rather fancy. Nonetheless, just like Moliere's bourgeois gentlemen that found out he was speaking prose all his life, many industries have been using reference models all along. They just never had to define them explicitly.

An architect for a residential dwelling knows that if they use the term door or window, the builder will understand what is meant. There are widely varied implementations of doors and windows depending, for example, if you are building a space station or an igloo. Nonetheless, everyone knows what the terms mean. Many of these terms are codified in building codes, and by standards bodies, and have evolved over the years. The software architecture community moves too quickly for such evolution; this is where standards organizations can help.

Software architectures, for sure, can have views and viewpoints, but the terms in which they are discussed have to be understood.

The core concepts that the RM discusses are service, visibility, execution context,  service description, real world effect,  interaction, and contract and policy.

I will discuss these core concepts over the next few posts.

None of this work is going on in isolation, or is it intended to denigrate other work such as the WS* specifications, or organizations such as the ISO, IEEE, IETF, the Ontolog Forum or other groups. The reference model just supplies standard definitions so that it becomes easier for each group to communicate with the others.