Emlang

Event Modeling in YAML

Get Started Try it live

What is Emlang?

Emlang is a text-based DSL for Event Modeling that lets you write event models in YAML. It's also a complete toolchain that validates syntax and generates HTML diagrams.

Event Modeling traditionally uses walls and visual tools for collaborative workshops. Emlang complements these practices by letting you maintain models as code—useful for documenting existing systems, writing normalized specs, or when text-based workflows fit better. See the language specification for the full syntax.

Examples

User Registration with Test Try it live
slices:
  User registration:
    steps:
      - t: Customer / Registration form     # trigger on the Customer swimlane
      - c: Register user                    # command
        props:                              # free-form properties
          email: string
          password: string
      - e: User / User registered           # event on the User swimlane
        props:
          userId: uuid
          email: string
          registeredAt: datetime
      - v: User profile                     # view (read model)
    tests:
      Email must be unique:                 # Given-When-Then test
        given:
          - e: User / User registered
            props:
              email: joe@example.com
        when:
          - c: Register user
            props:
              email: joe@example.com
        then:
          - x: Email already in use         # exception
Invoice Generation Try it live
slices:
  Invoice generation:          # processor pattern (direct form, no tests)
    - e: Shipping / Order shipped
    - v: Pending invoices
    - t: Invoice generation process
    - c: Generate invoice
      props:
        orderId: uuid
        customerId: uuid
    - e: Billing / Invoice generated
      props:
        invoiceId: uuid
        amount: decimal
        generatedAt: datetime
    - v: Pending invoices
Exploratory Brainstorming - Order Flow Try it live
slices:
  Order flow:              # just events, no commands yet
    - e: Order placed
    - e: Payment received
    - e: Items reserved
    - e: Shipment scheduled
    - e: Order shipped
    - e: Order delivered
    - e: Feedback requested
    - e: Order completed

Getting Started

Install with Go:

go install github.com/emlang-project/emlang@latest

Or download a precompiled binary from the releases page.

Generate your first diagram:

emlang diagram model.yaml -o diagram.html

See the CLI Reference for the full list of commands and options.

Use Cases