A single sequence diagram for the whole system?

Question

Sequence diagrams are used to model use case scenarios i.e., main success scenarios and alternate scenarios. That's what I understand from Craig Larman's book. I understand that it depends a lot on the practitioner. I mean a practitioner may decide not to model unimportant/obvious alternate scenarios etc.

Is there any accepted practice in the wild to draw a single sequence diagram for the whole system (having multiple use cases)? To me, it sounds a bit odd but I would like to explore if there, indeed, is something like this. It will be helpful if you could share references/links in support of this. I haven't found any yet!

Answer 1

One common misconceptions of people who learn (or get taught about) UML diagrams is that a single diagram is supposed to explain the whole system. With the exception of extremely simple systems, that's usually not the case. Any system with realistic complexity would end up with diagrams which are so complicated that they would cause more confusion than they would resolve.

Theoretically it would be possible to do that, but it would take a tremendous amount of effort for very questionable benefit.

In practice, UML diagrams only show a small part of the system, and quite often in a simplified manner.

Answer 2

In short

Complexity is mastered by breaking complex matters down into simpler ones. Based on this principe, an UML model of a complex system is made of many simpler diagrams. So keep each of it as simple as possible but not more.

Golden modeling rule

Independently of UML, there is a limit to graphical visualisation: any diagram with more than 6-7 elements will appear difficult to understand to the average reader. Keep your diagrams below this limit.

Urban legend? No! Experiments on short term memory by AI pioneer Herbert Simon proved in the 70s this limitation of the human brain. Modeling experts can easily read more complex diagrams, but only because their experience trained them to cluster items, and with this trick they can go up to 6-7 clusters.

Still not convinced?

A sequence diagram only show one specific scenario under the viewpoint of interactions (message exchanges) between objects/parts inside the system.

For alternative scenarios in the same diagram, combined fragments are needed. Your diagram will grow very big, have nested messages, and might quickly become unreadable. I let you extrapolate what might happen if you'd add all the scenarios of all the use-cases, with all the objects...

Last but not least, interactions are not always the best choice to explain what happens. Activity diagrams or state diagrams are very interesting alternatives in many situations.

Answer 3

Sequence diagrams can be used to model use case scenarios, but they can also be used at other levels of detail, for example to show an interaction scenario of multiple classes in a detailed design.

As the vertical axis of a sequence diagram depicts time, sequence diagrams are not suitable to show multiple, independent scenarios in a single diagram. And that becomes even stronger if the scenarios can be executed in parallel.

In practice, it will just be impossible to draw a single sequence diagram that accurately depicts the multiple use cases of an entire system, while still being readable and understandable. Even with a single use case that has multiple alternate and/or error scenarios it can be a real challenge to capture them all in a single readable sequence diagram.

Answer 4

I cannot imagine how a single sequence diagram could work. It would be most probably less readable than the code itself, though maybe (just maybe) it's doable. Yet, doesn't bring a value so no, don't expect to have such standard.

However, there is one diagram you may consider, assuming you want to stay on a high enough level - and that is the interaction overview diagram. It will work only for a system with relatively few function though (as a single diagram of course, if you split it into smaller chunks you may be successful even for large-scale systems).

The interaction overview diagram kind of combines activity diagram with a sequence diagram. You can think of it as a large activity diagram, where for some activities you use actual sequence diagrams to depict those.

I need to warn you though - even relatively simple system produces extensive interaction overview diagram. You really need to stay focused and draw a fine line how detailed the diagram should be to achieve expected results.

Yet, I attempted it once. I didn't complete it but it had at least a chance to succeed so you may want to give it a try as well.

In case you wanted more details, you may take a look here