CODEWITHSUNDEEP
umlactivity-diagram
Robertcode
Robertcode

Reputation: 1001

Are return arrows allowed in a UML Activity Diagram

I would like to represent the following in a UML Activity Diagram

1) User requests a product list from Application

2) Application requests product list from Database

3) Database returns product list to Application

4) Application displays list to User

Originally I thought this would be a simple case of arrows travelling from left to right connecting to each activity boxes then drawing arrows back all the way to the where the request started as follows:

User-->Application-->Database

User<--Application<--Database

But then I noticed I see no examples of UML Activity Diagrams with arrows showing the return of anything back to a requester. They all show a one way path to an end point. Is a return always implied if it started with a request to get something? Are return arrows incorrect to use?

Thanks in advance.

Update --- After some looking around it appears paths are one way. When you reach the end point that is to provide something like a list instead of drawing arrows all the way back to the requester it looks to me like you just add a phrase to the end node saying something like the following: "Return list".

Upvotes: 2

Views: 2267

Answers (2)

Jim L.
Jim L.

Reputation: 6529

It seems you may not understand the token semantics in activities. An activity diagram does not represent a call stack where one action returns to the previous action. However, one action calling another Call Behavior Action does work like a call stack.

The way an activity diagram works is that each action is a step. A completed action offers tokens to outgoing edges. As soon as any action is offered all the tokens it needs to start, it actually starts. Data and control both work this way.

You can think of each action like a function call. When all tokens are ready on all incoming edges, the function is invoked. When a function finishes, all the outgoing tokens are offered to all enabled outgoing edges. (Outgoing edges are enabled when an optional guard is not false.)

I recommend you read the excellent series by Conrad Bock to be sure you understand how activities work. Otherwise, you are going to frustrate yourself and others trying to make it fit your preconceptions.

Upvotes: 2

Uffe
Uffe

Reputation: 10504

There's nothing stopping you from introducing loops in an activity diagram, although for clarity, whenever you've got two or more possible exits from one activity, you should use guards on your connectors to specify under which condition that connector may be followed (eg [x < 10] on one and [x >= 10] on the other).

You can also use decisions (rhomboid) if you want to make the branching condition explicit (eg a decision named "x < 10 ?"); you'd then draw a single connector from the preceding activity to the decision, and the two guarded connectors from the decision.

But from your example it seems you're not drawing a proper activity diagram at all. In an activity diagram, the nodes must be activities -- not actors. There should always be a verb in the name of every activity. So "User requests a product list from Application" is a good activity, but if you've got an activity named "User" you're doing it wrong.

Upvotes: 0

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