activity diagrams

[Kalpesh Soni]

In SDLC
Why/When do we draw activity diagrams ?
(My guess- even before usecase or class diagrams
as they give you an idea of flow)
And why/when do we draw sequence diagrams ?
( , I think this is too fine grained,
unless my customer wants to understand my code,
I cannot afford to draw these !)

[Idly Vada]

In SDLC
Why/When do we draw activity diagrams ?

In systems modeling, activity diagrams can be used to document the
flow of logic through a use case.
Usually, activity diagrams are drawn before usecase and class diagrams. As a common practice activity diagrams are drawn for complex processes only.

And why/when do we draw sequence diagrams ?

Sequence diagrams are usually created to show the flow of functionality and control throughout the objects in the system.They are used to depict object interaction in the system.
I think two-pass approach to sequence diagrams should be followed. First pass focusses on the higher level info the customers are concerned with.While second pass diagrams are detail oriented.

[Stan James]

Well, the agile answer is when there is something you want to communicate or explore, use the diagram (and the medium) that does it best. That doesn't help much does it? But if you have an SDLC that says "You must produce this diagram for every use case, then this diagram for every public method, then ..." you are probably in trouble, focusing on diagrams instead of software.
I had to play with "communicate or explore" up there. There are plenty of reasons to make a diagram. Communicating is getting your idea across to somebody else. Does it have to be beautiful slideware, permanent CASE tool artifacts, or will a napkin or whiteboard do? Explore is developing a new idea, alone or in a group. Try a design, see if it works, try another one. I might go through a whole pad of paper before I understand the problem.
See Scott Ambler's Agile Modeling site for more thinking in this direction.
All that and I forgot to answer your question ... for myself, I use activity diagrams, um, never. But they might be good exploring the business processes with the customer. I like object interaction diagrams as a starting point to figure out what objects I have and what they do. I like sequence diagrams for geek-to-geek communications about how a series of steps works. We zoom out to service level communication between completely separate systems and zoom in to method level communication between classes.
Here's another Scott Ambler page about many artifacts, when you might use them, and interestingly how they are commonly misused.
http://www.agilemodeling.com/essays/modelingTechniques.htm
[ September 15, 2003: Message edited by: Stan James ]

[Jayadev Pulaparty]

I think sequence diagrams are drawn to figure out the responsibilities of a particular module (class, package, etc.). They are somewhat complementary to CRC diagrams.