Can one Activity Diagram Depict only One process

Question

I am Working on a project for Orders data visualization Dashboard.

This is the use case Diagram:

Currently I'm working on the activity diagram and my question is: Can the Manager and the Shipper login into the system and initiate their own activities in the same diagram?

Answer 1

An activity diagram "specifies behavior by sequencing subordinate units". It can perfectly have several initial nodes, e.g. one for Admin and one for Manager or Shipper. And when the activity is invoked, all those initial nodes would be activated at the same time, starting each a concurrent flow, each being performed at its own pace.

But this makes only sense if the Manager's actions and the Shipper's actions are really related, concurrent and somehow synchronized. E.g. every time an Admin login is performed, a Manager login would be expected.

If the Manager flow as well as the Shipper flow are both independent and in reality unsequenced, you should use separate activity diagrams. In this case, trying to squeeze them into a single diagram could even be misleading.

Additional remarks, unrelated to your question:

• Typically you'd have a separate activity diagram for each use case. It's not an obligation, but it's a common practice to describe what happens for a specific use-case. In your case, it would mean that only the Ship Order and the Update product stock would be in a situation where actions could be interdependent.
• When activity diagrams are used to design systems, it shows in principle what happens inside the system. The actors are outside the system. So the partitions (columns) would in principle not show what an actor does, but what the system does in relation with an actor. Of course, if you use activity diagrams for process modeling, it would be a different story.

Answer 2

Yes, they can. With each InitialNode a flow begins and the tokens run down the actions independently. Probably at a certain time you will have some synchronization between both (this is not mandatory but otherwise it would probably rather pointless to have two independent flows in the same diagram). In that case you either have a MergeNode (bar) to wait for both tokens to arrive or an action has two incoming edges waiting for both tokens in order to commence.