Context Diagram | UML 2 For Dummies

The first diagram that you need on any project is a context diagram. UML does not have a context diagram per se. We use the use-case diagram of UML to show the context of the system or software that we are developing. So, we give this special use-case diagram a solution-oriented name: context diagram. If you already have one, that’s great. But in our experience, most software- or systems-development projects start out without a context diagram, blissfully unaware that they need one.

For your system or software development to be successful, you have to know the answers to the following questions:

Who uses your system?
What data must go into your system?
What information and objects must your system produce as output to users and other systems?

A context diagram answers these questions because it shows your system in a setting (context) defined by its interactions. The diagram helps define the boundaries of your system or software application by showing all the users and systems that your application must interact with.

Tip A context diagram provides a good starting point for your work on use cases. In fact, you build a context diagram based on the use-case diagram provided by UML. Use the following steps to construct such a diagram:

Place a large rectangle in the center of the diagram.

This represents your system or software application.
Place the name of the system at top-center, just inside the rectangle.
Identify and name each of the actors that you expect to interact with your system.

Remember Actors can be human users, other systems, hardware, or the clock.
Place the actors around the outside of the rectangle representing your system.

Use “stick figures” (including the name of each actor) for human users. If the actor is not a human, use class notation that uses the name of the actor as the class name and give the class the «actor» stereotype.
Draw a line between the actor and the system rectangle.

This shows that the actor interacts with your system.
Show the information, data, and/or objects that flow into your system from an actor above the line connecting that actor to the system.

You show this as text with a small arrow pointing from the text toward the system rectangle.
Show the information, data, and/or objects that flow out of your system to an actor below the line connecting that actor to the system.

Again, you show this as text with a small arrow pointing from the text toward the actor receiving the system output.
Repeat Steps 6 and 7 for each actor that sends data into the system or receives data from the system.

Remember The context diagram helps you set boundaries for the scope of your project. You know from the context whom your system must satisfy, what data your system must accept, and what data your system must generate.