Use case diagram

The first question to ask is: "What will the software be used for?" ". The use case diagram. The use case diagram includes actors and use cases. It is important to provide with a usage diagram a detailed description if possible including a nominal scenario (typical sequences) and alternative sequences on particular cases.

Actor

The first are represented by figures. An actor is an entity external to the modeled system and which interacts directly with it. The actors are the users of the system: integrated software, external computer systems, an element external to the system with which it interacts.

Actors are therefore roles and not a physical person. A natural person can be represented by different actors. If several people play the same role vis-à-vis the system, they will be represented by a single actor.

Use case

A use case is a service rendered to an actor, it is a functionality by the latter. Actors requesting services from systems are called primary actors. When they are requested by the system they are called secondary actors.

Relationships

There are several types of relationship: between actors, between use cases or between actor and use case.

Relations between actors symbolize a generalization, and is represented by a solid line and an empty arrow. There is generalization between A and B if we can say "A is a kind of B". This would amount to inheritance in an object oriented language.

There are two possible relationships between use cases (we have already seen the generalization). The inclusion between A and B means that B is a mandatory part of A. In the other direction we speak of extension, B is an optional part of A. It is represented by a dotted arrow from A to B. We can read A includes B or B extends A.

the inclusion and extension relationships make it possible to isolate a reusable service as part of several other use cases, this is called reuse.

It may happen that a use case covers a set of exchanges and processes. In this case, the inclusion and extension relations make it easier to write the case.