Difference between Sequence Diagram (SD) and a System Sequence Diagram (SSD)?

Question

I'm working on a project for a grad class and still having trouble wrapping my head around them.

What is the difference between a sequence diagram (SD) and a system sequence diagram (SSD)?

And in what order should they be developed when working on a systems development project?

Solution 1

A System sequence diagram visualizes a use case, while a sequence diagram visualizes a method of a class.

The elements participating (exchanging messages) in a system sequence diagram are Actors and Systems. The messages exchanged by these elements could be any type depending on the systems (from web service calls to data input from a human).

The elements participating in a sequence diagram are objects (instances of various classes). The messages exchanged by these elements are method invocations.

First you would analyze and document the use cases. Each use case describes a main business task required. The system use case diagram is part of this activity.

Later you come to design each subsystem, namely each element participating in the system use case diagram. You draw one or more class diagrams depicting the classes participating to the solution. Then you would draw a sequence diagram for the most important (or difficult to understand) methods.

EDIT: Some examples can be found in the agile modelling site.

Solution 2

Sequence Diagram: It shows interaction between two objects. System Sequence Diagram: It shows interaction between system & actors.

Solution 3

The sequence diagram is merely a more common term.

A sequence diagram is used primarily to show the interactions between objects that are represented as lifelines in a sequential order.

(from VP ULM help) So, BTW, you can see that it is NOT a visualization of one method as @nakosspy writes.

It can be done for any level: methods, services, components, system.

As for System sequence diagram, it is a sequence diagram made for the highest level. It could describe even a group of use cases, not only a single one, as Wiki says. But what is much more important, elements of System SD should be described in user terms, without IT slang. Because it belongs to the requirements part of the project documentation.

Comments

[Fuhrmanator]

I'd add to this excellent answer that since some actors (human ones) can't really send a message to a system, there's a presentation layer whose job is to recognize some user gesture and which translates it into a message sent to the system. Web forms use fields and buttons, Siri/Google/Alexa/Cortana uses voice recognition, and there's even now the Amazon Go store that uses cameras to recognize gestures and send messages. SSDs abstract that layer, but it kinda has to be there on the Actor side.