A use case is a statement about how an entity uses a system. Entity here refers to a type of user or a component interacting with the system. In use case definitions, entities can also be referred to as actors.

We start off by identifying the actors in the messenger system. A glaringly obvious actor is the user of the application (the person that uses the application to satisfy their messaging needs). Another actor that ideally should be considered is the admin. For the purpose of this simple messenger application though, we will cater for the single user actor. The use cases of the user are as follows:

• The user uses the messenger platform to send and receive messages
• The user uses the messenger platform to view other users on the messenger app
• The user uses the messenger platform to set and update his status
• The user can sign up to the messenger platform
• The user can log in to the messenger platform

The preceding use cases are enough to get us going. If, at any point during the course of system development, we come across a new use case, we can easily add that to the system. Now that we have identified the use cases of the system, we must properly describe the behavior of the system in catering for these use cases.