What is a component?

From use cases to sequence diagrams and now component diagrams, no doubt you have already gathered that we have implementation on our mind. There is a fundamental difference between component diagrams and the other two types though. That is to say that use case and sequence diagrams are referred to as behavioural diagrams in UML and in the same family of diagrams as activity or collaboration diagrams. In contrast component diagrams are termed structural in the same category as class and object diagrams.

This is the kind of diagram that models the actual software components that are to enable use cases. By software component we mean an executable file or a source file. A file containing code that can be saved to memory and ultimately placed on the client, or on the Web server hardware, or even on an application server hardware is hereby referred to as a component. Other objects like resource files and documents are also referred to as components but we are going to concentrate on software code.

Strictly speaking we are talking about one set of components for one use case realisation. Diagram depicts the relationship between the software components. It should also demonstrate the location of the components, communication between the components, and their dependencies.

Examples of components could be HTML documents, JSP and ASP scripts, database, etc. So based on the choice of technology, software architect models solution components as Enterprise JavaBeans (EJBs), Component Object Model/Distributed Component Object Model (COM/DCOM) components. These are industry standard component models that solution could be based on. Reusability is the major driving factor for implementing components based on these standards use to their extensive use.