This week I have found two commentators redefining loose coupling. The first is described in the discussion about virtual endpoints that was the subject of my previous posting and the second was this Zapthink article. I think it is important we clarify our definitions around SOA to avoid the concept being hijacked by commercial interests. So in this posting and the last I have attempted to sort out whether the discussion is adding to our understanding or muddying the waters.
In a previous posting I have identified 18 different types of coupling and I am still finding more. The first thing to realise about the seven levels of coupling is they are looking at a different attribute of coupling than what I refer to as 'types of coupling'. The levels refer to the entities that are being coupled. My discussion on coupling types looks at the coupling of two modules of program code. This code could be in calling programs, objects, subroutines or services.
The following table lists the seven levels. This table makes the distinction between 'Levels of Coupling' and 'Types of Coupling' clear. The core tenet that Zapthink is discussing is the "loose coupling of Service consumers and providers". This has a different meaning than loose coupling between program code in different services. The main difference is that aspects of the service provider, with the exception of the implementation are not program code at all. Another difference is that a service consumer may not be a service either and it also might not contain program code.
| Aspects of the Service Provider | Possible Types of Coupling | |
| Loose Coupling the Implementation | The service implementation | 18 types and still counting |
| Loose Coupling the Service Contract | Service contract | Interface Coupling, Bind-Time Coupling, Service Intermediary Coupling, Infrastructure configuration Coupling |
| Loose Coupling the Service Policy | Service policy | Bind-Time Coupling, Service Intermediary Coupling, Infrastructure configuration Coupling |
| Loose Coupling the Process | Service-oriented process (Trivial if this is a service) | 18 types and still counting |
| Loose Coupling the Data Schema | Data schema of provider dataset and data schema of interface. | Data Services Layer Coupling, Data Transformation Service Coupling |
| Loose Coupling the Infrastructure | Infrastructure of provider | Single-vendor SOA platform Coupling |
| Loose Coupling at the Semantic Layer | Meaning of message attributes | Dynamic service definition Coupling |
The seven layers are not as clearly layered as the phrase 'Levels' might suggest. Certainly policies could be considered a higher level than contracts and processes considered at higher level to implementation. The other levels could be considered to sit atop the infrastructure level and under the semantic level but normally data and process would sit beside each other with neither the master. I also see contracts sitting beside data and process as equal partners.
Coupling takes account of the dependency between two entities and the clarity of this dependency. Thus two services exchanging a message are loosely coupled if every dependency is clear from the format of the message. Services have different reasons for being dependent. These reasons are what we mean by 'Types of coupling'.
The types of coupling for the seven levels are described in the third column of table above. The first level describes service to service coupling which I have described earlier. The fourth level is interesting because the solution is almost trivial. The business process should look like a service and therefore all the coupling types for coupling between services also apply for coupling between a consumer service and provider service that is a composition of services used to implement a process. The sort of things that might cause dependencies with contacts, polices, infrastructure, data schemas and semantics are different to the program code dependencies.
Loose coupling of data schema is partly addressed by another Zapthink article which advocates the building of data transformation services rather than incorporating transformation logic inside services used to contain business logic.
Tight coupling between the consumer service and the infrastructure of the provider could be based around a number of proprietary technologies. The ones highlighted by Zapthink are the non-standard ESB and the single vendor SOA platform.
The types listed in the third column of the above table are those implied in the Zapthink article. I expect that a thorough investigation would uncover other types of coupling as well. The point of listing them in the table is to suggest that each coupling level will have its own types and each level (with the possible exception of the process level) needs to be investigated separately.
We can find coupling examples everywhere. For example Carlos Perez and David Walend discuss whether a DBMS and its application are loosely coupled if the application is accessing the DBMS using SQL. This is interesting but not relevant to SOA. For the point of view of defining SOA, I do not believe the seven levels of loose coupling matter. You can have an SOA as long as the services are loosely coupled. However as a model to assist good practice, I think the seven levels will provide value even if they do overload the 'loose coupling' term.
The concepts discussed in relation to levels of coupling are certainly valid. We do not want service consumers dying or behaving incorrectly due to changes with service provider's infrastructure, policies or any of the other aspects described in the seven levels. Zapthink continue to guide us on the path to software heaven, however in the process of introducing 'levels of coupling' Zapthink commit the sin of causing confusion around the important 'loose coupling' term. I recommend the "Seven Levels" article to SOA developers and I suggest developers need to be clear on what is being coupled whenever they use or encounter the term 'loose coupling'.
End Note:
One way to distinguish other levels of coupling from service to service implementation coupling would be to call the different levels something different other than 'loose coupling'. The names that occurred to me were 'loose buckling' or even 'loose zipping' which would be reminiscent of the zapthink integration zipper. Unfortunately both these terms have implications of embarrassing failure if the buckling or zipping is too loose. We do not want our metaphorical trousers around our ankles.
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