Question

there would be users with different privilages and as a consequence there would be a lot of conditions, so how can i model the conditions in the use case diagrams? for example, the forum manager may create a post or update a post or even delete a post, create a threads or update avialable threads, delete threads, etc.

1- should i have different “use case” elements for each task? i mean add an oval/circle element for each task?? like creating an oval for “create a post”, an oval for “updating the post”,…..

2- is it correct to have one actor for each privilage?? like one for anonymuse user, one for logged in user, …..

thanks.

Answer 1

You shouldn’t get into conditions and ifs and buts in use cases diagrams. A set of use cases is intended to provide an overview of the system’s functionality, and each use case describes an interaction between the system and one or more actors. You want to keep that description simple and succinct.

Each use case should in some way make sense to the actor. To a person using a forum, it does make sense that creating a post is a separate activity from updating one (or responding to one), so that seems like a sensible start to me. You should not be overly concerned with the number of use cases at this stage. The number of use cases does not translate directly into system complexity, and a large number of clearly defined use cases is better than a small number of large, ambiguous ones.

The next step is to elaborate your use cases, and that’s where you can start talking about conditions. Elaboration is typically done using an activity diagram which describes how the interaction between the actor and the system proceeds, eg Poster initiates post; System checks poster’s privileges; System rejects post if privileges are insufficient; etc.

There is of course no right or wrong, but generally speaking it’s a bad idea to use actors such as “logged in user” etc, and in fact you should avoid employing a “user” actor at all. Why? Because the interaction is actually between the system and a person, whereas a user (account) is an in-system representation of a person’s privileges.

In other words, if you find yourself using actors which are in fact concepts from within the system, you’ve taken a wrong turn somewhere. Every use case must involve a system-external actor, otherwise you’re not describing the system from the outside.

A better set of actors for a forum system would probably be Poster, Reader and Manager (and possibly a System Administrator as well).