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Your First Bounded Context

Planning Your Domain

Before writing .ddd code, identify:

  1. The bounded context — what area of the business are you modeling?
  2. The commands — what actions can users take?
  3. The events — what facts are recorded?
  4. The states — what lifecycle does the aggregate go through?
  5. The invariants — what rules must hold?

Example: User Registration

Step 1: Define the Context

context Registration {

Step 2: Declare Types

  type UserId = String
  type Email = String
  type DisplayName = String

Step 3: Define Commands

  command Register {
    userId: UserId
    email: Email
    name: DisplayName
  }

Step 4: Define Events

  event Registered {
    userId: UserId
    email: Email
    name: DisplayName
  }

Step 5: Build the Decider

  decider User {
    commands: Register
    events: Registered
    state: Unregistered | Active(email: Email, name: DisplayName)
    initial: Unregistered


    decide(Register, Unregistered) -> [Registered]


    evolve(Unregistered, Registered) -> Active {
      email: email
      name: name
    }
  }
}

What the Compiler Will Check (Phase 3)

  • Exhaustiveness: Register is handled for Unregistered. Since Active is not listed in terminal:, additional decide clauses would be needed if more commands existed.
  • Evolve totality: Registered is handled for state Unregistered.
  • Type safety: All fields reference declared types.

Next Steps

  • Add validation: validate email contains "@"
  • Add more commands: ChangeEmail, Deactivate
  • Add business rules: require guards in decide clauses