In contrast to coupling, cohesion is the "act or state of sticking together" or "the logical agreement." Cohesively integrated source and target systems are independent from one another. Changes to any source or target system should not affect the others directly. In this scenario, information can be shared between systems without worrying about changes to the applications or databases, leveraging some type of loosely coupled middleware layer to move information between applications and make adjustments for differences in application semantics. Most data integration product leverage the notion of cohesion.

The advantages of using cohesion included:

The ability to avoid changing source and target systems just to facilitate integration. You no longer have to make changes to the systems because the points of integration are less invasive.

The fact that a single system failure won’t bring down all connected systems. Since the systems are not dependent, a failure typically won’t affect the integrated systems (at least immediately).

The disadvantages include:

The inability to provide visibility into the services layer, and thus gain value from encapsulated business services and tactical functions. This is simple information movement; there is usually no notion of services access, thus remote applications can only see information not behavior, thus no reuse of services.

You need to consider the tradeoffs. Coupling, or service-oriented integration, provides the greatest flexibility as the application integration solution moves into the future. The notion of leveraging services makes the application integration solution much more valuable than simple information movement.

However, cohesion has its advantages. Systems can be added to, changed, or removed from a cohesive application integration solution without typically requiring changes to any of the other systems in the problem domain.

Posted by Dave Linthicum on July 19, 2006 05:33 AM