Tight coupling versus loose coupling

Most large, complex systems are built as small collections of large subsystems instead of as large collections of small, independent subsystems. This is because of the potential for increased performance, security, economy, or some other key property that you can't get by decoupling the system into relatively independent, small elements. The tight coupling characteristics of large-scale systems generally result from optimizing the overall design and from minimizing redundancies and inefficiencies among the system's components. This results in closer coupling among the system's components and large numbers of critical interdependencies.

One disadvantage of tight coupling among a system's components is that failures within the individual components tend to disable the entire system. Loosely coupled Web services are seen as a better alternative; a Web service failure doesn't disable the entire system, provided a failover Web service server is in place.

You can change details in loosely coupled Web services as long as those changes don't affect the functionality of the called Web services. The tight-coupled systems can be difficult to maintain, because changes in one system subcomponent usually require the other subcomponent to adapt immediately.

Loosely coupled Web services require substantial redundancies unlike tight coupling between clients and service, which minimizes redundancies. The listening Web service and the requesting Web service might not trust each other. This means security and trust standards must be added to get both the listener and requester to trust each other. On the other hand, tightly calling and called coupled systems assume that both have the knowledge of what each requires to trust one another.