Tuesday, February 10, 2009

SOA: Business Event Message Models

InfoQ has published an article about SOA Message Type Architecture by Jean-Jacques Dubray. The article shows how to model 'message type' artifacts based on an enterprise common data model using a DSL, and also outlines how the modeled artifacts can be used to generate XML schemas for use in your service contracts. The message type DSL is not for modeling messages, it is just for modeling the types used as message payloads. In WCF terms, a message type is a [DataContract].

Note that even if the message type model contains a set of standardized verbs, the model does not cover business process aspects such as flow and actions. You will still need to analyze the business processes that pass those message types around to model the action, query and notification events that drive the processes. I've written several times about creating such a model that comprise both the business events and the message types, a business process information model (BPIM). I like JJ's approach; I just think that we need to model also the business capabilities and interactions that utilize the message types to get a complete set of artifacts for service contracts.

One difference is that I prefer using a common information model (CIM) as the basis for modeling the message types, rather than an enterprise data model (EDM). It is a lot of effort to create an EDM that covers all information in all systems-of-record in a company; and the moment you have completed the all-encompassing model, your CxO will inform you that parts of the business have been outsourced or that a new business will have to be incorporated, or even just that the CRM system is to be replaced. Change is the only constant. Thus I prefer starting small by creating a CIM that covers only the business entities comprised by the business processes that are about to be service-oriented. As there will be multiple resource domains in your architecture, there will be multiple CIM models as your SOA grows. Federate these domain models by creating context maps for cross-domain capabilities and logic only.

Create a CIM according to the Domain-Driven Design (DDD) principles and use the domain entity objects when implementing the business logic underlying your services. Design the BPIM based on the CIM, ensuring that the model is canonical for each process domain. As the BPIM is a projection of the CIM, the service interface messages will have an unambiguous mapping to/from the underlying domain resources (entities and aggregates). Mapping data between the message types and objects will be required in the service implementation (provider container).

The 'message type' artifacts should also be partitioned into bounded contexts according to business area as in Domain-Driven Design; where each service domain comprise a set of cohesive services based on the same underlying CIM, delivering an BPIM for the specific business domain. Having clear domain boundaries make it easier to analyze, model and design, and version the artifacts - it also aids the discoverability by the consumers by providing a clear business context for the provided services.

Your SOA solution will, as it grows, encompass multiple service domains each with its own BPIM. Composite services that compose business capabilities across two or more domains will require mediation between the message models. Even if "transformation avoidance" is considered a SOA best practice, it is unrealistic that you will be able to avoid mediation completely in your service bus (composition container). In addition, you cannot expect to enforce your model upon the extended enterprise, think of e.g. third-party and outsourced capabilities.

The message types exposed by a service domain implicitly become the "canonical schema". They enable better service discoverability, reuse and composition as they all share the same underlying data model - which also ensures that the message payloads have common semantics within the service domain. In combination with business functions, the schemas provide a complete standardized service contract.

As you have seen, the BPIM and CIM approach fits well with DDD. It also fits well with the middle-out SOA approach - including the recommendation to start small and think big picture, not big bang.

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