Coupling with Contracts

I listened to a podcast by Thomas Erl yesterday which gave yet another slant to the subject of coupling in SOA. My previous posting about coupling was inspired by a Zapthink arcticle on "The Seven Levels of Loose Coupling" seven levels of coupling. The Thomas Erl podcast provides more information on at least three of the seven levels described by Ronald Schmelzer in the Zapthink article. These are

• the coupling between the service consumer and the services implementation,
• the coupling between the service consumer and the service contract, and
• the coupling between the service consumer and the business process

In his podcast Erl asks the listener to refer to a diagram on his "SOA Principles" web-site. The types of coupling he focuses on in the podcast and the diagram are:

Service consumers are coupled to the service contract

• The service contract can be coupled to a parent business process
• The service contract can be coupled to the service logic
• The service logic can be coupled to the service contract
• The service logic may be coupled to proprietary vendor technology
• The service logic may be coupled to multiple services it may need to compose
• The service logic may be coupled to various resources that are part of the overall implementation environment

These coupling relationships broaden the possibilities explained in the Zapthink article. Firstly all the coupling relations are not based around the service consumer and secondly the coupling is not a symmetric relationship. Erl explains that it is good to couple the logic to the contract as this is contract-driven development but poor practice to have the contract tied to the logic of the service.

The multitude of coupling relationships was referred to in my earlier posting. When discussing coupling, it is important to be clear on what is being coupled when explaining service coupling. Zapthink discusses implementation, contract, policy, business process, data schema, infrastructure and semantic layer. What Erl describes is the service logic is part of what Zapthink describes as the implementation. Erl does not describe policy coupling. The data schema is part of the implementation environment and the infrastructure could be vendor technology.

I would prefer to keep coupling as a symmetric relationship. In other words if logic is coupled to contract then contract is coupled to logic. Erl is trying to cover methodological issues when he talks about contract-driven development. This is an important 'pattern' but it is not about coupling. It is about what a developer does first. The fact is that the contract and the logic are coupled and if this coupling can be made loose then that will provided more flexibility.

The focus of the Zapthink article is on the coupling with the service consumer. Erl has a twin focus on the contract and the service logic. My previous discussions on coupling tended to focus on coupling between services (or program modules). I tend to think that most important coupling concern in SOA is between the consumer and the service implementation in SOA. The consumer may be a user interface or another service. Contract coupling is important but it is more of a vehicle for loosening the coupling between consumer and provider.

One strength of the Zapthink article is that it has enabled me to analyse the Erl model. As is usual for Thomas Erl, his illustrations provide some clarity to a complex interaction of concepts. In this case his diagram should be taken as a sample of the coupling issues rather than a representation of all of the coupling issues relevant to SOA. Also as discussed above I suggest we stick with a symmetric notion of coupling to keep the world of SOA a simpler place.