How Do I Define a Capability?

Start with the responsibility the system exposes. A capability should read as a business or architectural responsibility, not as an endpoint, class, queue, or handler.

Model The Responsibility

Name the capability, then define the actor and input shape that express the intent. Add outcomes for every meaningful result path.

Add Semantics

Rules express constraints, effects express externally meaningful work, events express signals, and the when block explains why each outcome can happen.

language dcl 0.9

actor Customer is human

effect PersistRegistration is persistence
effect SendVerificationMessage is notification

shape RegistrationInput {
  email: Email required
  acceptedTerms: Boolean required
}

event VerificationMessageSent is {
  email: Email required
}

capability RegisterCustomer {
  intent RegistrationInput from Customer

  outcomes {
    RegistrationAccepted
    TermsRejected
    VerificationDeferred
  }

  rule TermsAccepted: input.acceptedTerms is true

  effects {
    PersistRegistration
    SendVerificationMessage after PersistRegistration
  }

  events {
    emits VerificationMessageSent
  }

  when {
    TermsAccepted violated then TermsRejected
    SendVerificationMessage unresolved then VerificationDeferred
    otherwise then RegistrationAccepted
  }
}

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