Interaction

An interaction defines a single step within a Use Case, describing how one participant communicates with or acts upon another. Interactions are the building blocks of use cases, capturing the flow of actions between users, entities, and other system components.

Purpose

Interactions serve to:

• Document step-by-step flows: Show exactly what happens at each point
• Enable sequence diagram generation: Provide data for visual documentation
• Define acceptance criteria: Each interaction can be verified in testing
• Clarify responsibilities: Make explicit which component handles each action

Syntax

Interactions use a natural language-like syntax within use cases:

case PlaceOrder is {
  user Customer "selects items and clicks checkout"
  then entity ShoppingCart "validates item availability"
  then entity PaymentService "processes payment"
  then entity OrderService "creates order record"
  then user Customer "receives confirmation"
}

Interaction Keywords

Keyword	Description	Example
user ... then	User initiates an action	user Customer "clicks submit"
then	Continues the flow to next step	then entity Service "processes request"
tell	Send a command to an entity	tell entity Order to CreateOrder
publish	Send a message to a pipe	publish event OrderCreated to OrderEvents
saga	Initiate a saga process	saga PaymentSaga "processes distributed transaction"

Participants

Interactions can involve:

• Users: Human or system actors defined at the domain level
• Entities: Stateful business objects that process commands
• Applications: UI components that present information
• Sagas: Multi-step processes that coordinate across components

Example: Complete Flow

epic UserRegistration is {
  user NewUser wants to "create an account"
  so that "they can access the platform"

  case SuccessfulRegistration is {
    user NewUser "enters email and password on signup form"
    then application SignupForm "validates input format"
    then entity UserService "checks email availability"
    then entity UserService "creates new user record"
    then entity EmailService "sends verification email"
    then user NewUser "sees success message"
  }

  case EmailAlreadyExists is {
    user NewUser "enters existing email"
    then application SignupForm "validates input format"
    then entity UserService "finds email already registered"
    then user NewUser "sees error suggesting login instead"
  }
}

Occurs In

• Use Case

Contains

Nothing (leaf element within use cases)