Usecase Definitions in AaC

One of AaC’s goals is to allow for a system to be modeled in a way that various disciplined users are able to understand and implement the model for their area of responsibility in the overall system. One of the system model aspects captured within AaC is the behavior of the system, which has a root key of usecase. The usecase describes a system behavior, who is involved with the system behavior, and outlines the steps for achieving the described system behavior.

Utilizing Usecase Definitions

There is an example usecase within our alarm clock model example for setting the alarm time.

  description: The user sets the time for the alarm clock
  participants:
    - user
    - alarm_clock
    - timer
  steps:
    - The user sets the time on system
    - The alarm clock stores the time in the timer
---
usecase_step:

By incorporating the usecases for various system behaviors, the alarm clock is able to be evaluated for functional completeness that cannot be seen in the alarm clock model alone.

Some questions that come up when evaluating the system for functional completeness are outlined below. They can be addressed with the incorporation of a usecase definition for the system.

  1. Does the system define the necessary participants?

  2. Are all necessary functionalities feasible with the current system components?

  3. Can the system be understood enough to be tested?

Crafting Usecase Participants

The participants of the usecase are included as a portion of the usecase definition, highlighted below.

  description: The user sets the time for the alarm clock
  participants:
    - user
    - alarm_clock
    - timer
  steps:
    - The user sets the time on system
    - The alarm clock stores the time in the timer
---
usecase_step:

participants are actors (who) can perform the action on the system and what part(s) of the system are necessary for the action to be accomplished. Therefore we see three participants in the above example: user, alarm, and timer.

The user is the actor that wants to make a change in the alarm clock system.

The alarm is what the user wants to change within the alarm clock system.

The timer is for the alarm clock system to know when to trigger the alarm.

  name: user
---
actor:
  name: alarm_clock
  model: AlarmClock
---
actor:
  name: timer
  model: ClockTimer
---
actor:

Crafting Usecase Steps

The steps of the usecase are included as a portion of the usecase definition, highlighted below.

  description: The user sets the time for the alarm clock
  participants:
    - user
    - alarm_clock
    - timer
  steps:
    - The user sets the time on system
    - The alarm clock stores the time in the timer
---
usecase_step:

steps are actions that the system follows to accomplish the desired described behavior of the usecase. The number of steps varies based on how many interactions between the participants need to happen for the behavior to be accomplished. Each of the steps should involve only two participants. One participant (source) performing an action on another participant (target). Each action must be defined as a behavior in the target. For example, the model AlarmClock must have a behavior named setAlarm that can be invoked by the source user.

  source: user
  target: alarm_clock
  action: setAlarm
---
usecase_step:
  name: The alarm clock stores the time in the timer
  source: alarm
  target: timer
  action: setTime
---
usecase:

  behavior:
    - name: setAlarm
      description: Set the alarm timer
      input:
        - name: targetTime
          type: Timestamp