Component diagram before class diagram?

Question

I would like to know which diagram from those two should be created first when designing large IT system. In every article I read about UML, the component diagram is mentioned as almost last. I think that, when designing a larger system is better to divide it to smaller parts/modules on the component diagram and then for every component create a detailed class diagram. Is that correct?

How it look in practice from your experiences? Do you know of any article/tutorial that shows from beginning to end, the design of system using UML?

Answer 1

There is no mandatory order for UML diagram design - in fact, you will often find yourself creating several simple diagrams of different types, and refining them later. The process is generally iterative but doesn't have a follow a particular order.

That being said, some UML diagrams provide a more abstract view than others, and it may sometimes make sense to start with the most abstract views and work your way down to the most specialised ones. Use case diagrams typically come early, while object diagrams or state diagrams often come later in the design process, once the original, abstract solution has been refined.

You will also want to take a look at consistency between your diagrams: indeed, with several views you will introduce redundant elements, and it is important to make sure that they do not contradict each other. For example, if you have a sequence diagram and a class diagram, any call message in the former should correspond to an operation in the latter. See Alexander Egyed et al's work for automated consistency management and correction in UML models.