Coupling refers to the interrelatedness and interdependencies between two or more components or web services. One simple way to think about coupling is this: how easy is it to change something about web service A without having to change web service B. If many changes in A require something in B to be changed, they are tightly coupled. If A can be changed easily without B being touched, then they are loosely coupled.

Tightly coupled architectures are much harder to maintain and reuse, so loose coupling is a major goal in a service oriented architecture. Coupling can occur in many ways.

The most obvious form of coupling is a dependency on a specific commonly understood interface between the requestor and service provider. This can be avoided by using WSDL or other interface definition language to describe the interface.

Another common form of coupling is created when the requestor expects the service provider to be at a specific location. UDDI and other discovery vehicles services are meant to avoid this form of coupling.

Two or more components can be coupled by requiring a common language or common platform. To prevent this form of coupling, web services use SOAP or some other form of platform independent remote protocol.

Components can also be coupled by putting responsiveness constraints on the produced. The request response invocation style, for instance, requires that the service provider respond to a request quickly. The one way style of method invocation generally creates a looser form of coupling since it allows the service provider to respond “eventually.”

Finally, and most subtlety, components can be semantically coupled. Web services has no magic answer to prevent this – that’s why they pay software designers the big bucks. Semantic coupling occurs when two web services are so dependent on each other that it’s virtually impossible to use web service A without using web service B (or a nearly identical implementation of B). Semantic coupling is usually caused by web service A being too dependent on aspects that really should be considered implementation details of B.