The Use Case Model

The use case model is, essentially, a visual representation of all the people and computers that will interact with a system and all the tasks that the people and computers can do with the system. In other words, it's a high level view of what a system will do.

This blog describes how to draw a use case model (it's not hard) and, much more importantly, how to reason about a system using a use case model. Because use case models can be drawn very quickly at the beginning of a project, any reasoning we can do with a model can have huge pay-off down the line.

What is a Use Case Model?
The thing I like most about use case models is that they're damn easy to understand. Take my most often used example: a pizza company:

This model tells us that there are two key people using the system - someone who takes orders and someone who makes them. The person who takes the orders can do three things - order a pizza, cancel an order, and deliver a pizza. The person who makes the order can only do one thing - make a pizza.


For a bit extra reasoning power, I call the "people" in the diagram "user roles". This is because, in reality, one person might play both roles. Or there might be one role played by multiple people. To get technical, a "user role" is a role that a person takes on when using the system.


Are there any guidelines for drawing one?
In fact, yes. The use case model above is pretty damn awful. Here are some guidelines and how they're broken above:

• A user role describes a role that a particular user takes on when interacting with the system. In the diagram above, the "order taker" role can perform activities relating to manipulating orders in the system and activities related to delivering pizza. This is bad because when we come to do solution design, we want to be able to support the tasks for a user role on as few screens as possible - and having order delivery support on the same screen as entering new orders would most probably be dumb.
  
• The task names like "make pizza" relate to the overarching goals of the user as opposed to the goals from a particular interaction with the system. Much more sensible names would be things like "get next order to make" and "mark order as complete". In other words, the tasks performed at the system boundary are not well defined.
  
• The complete set of tasks described is insufficient to achieve the key business scenario - get the right food to the right customers within 30 minutes. To support that scenario we need tasks like "view list of waiting orders".
  

Here is an updated use case model with the first two points applied:

This is a much more precise model. We're now clearly defining the key functional requirements of the system in a much less ambiguous way. We've also identified three types of user roles that require different types of solution support. For example, the requirements of the order taker, a person who has to deal with a customer who will change their mind are quite different to the requirements of the order maker - a person who doesn't need a high degree of interactivity in any solution. We can reasonably expect that the order taker will become an expert user of the system as their goal is to get the orders into the system whereas, for the order maker, using the system is secondary to their main goal of making the orders!



What kinds of reasoning can we do with them?
There are several kinda of reasoning that we can now do. 

The first is identifying missing functionality. Given that our key business scenario is "get the right food to the right customers within 30 minutes", we might want to add in a "order delay manager" user role and use cases that let them examine the order queue and assign staff to particular parts of the shop. We might want to add in basic cash management and accounting functions - or even inventory control functions. 

The second is around scope and prioritization. With a simple quick diagram we can have a conversation with our customers about how they view the system being used. We can then talk about which functions are the most important - and if we're doing things in an agile way, we can start building those key functions straight away.


Finally, we can use the diagram as a jumping point for analysis. There will be business rules applying to all functions. For example,  there might be a cost to the customer of canceling an order. However, to read about how to best document these rules - and the process required for describing the requirements for a function - you'll have to wait for the "use case" blog post I've got lined up!