Eventmodeling: Describing Business Rules with Given/When/Then

How to make business rules visual, understandable, and testable

October 2023 • 8 min read • Event Modeling

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In the last episode of the “Eventmodeling & Eventsourcing” Podcast, we mainly discussed “Given / When / Then” scenarios to describe business policies and touched on slice-based architectures, especially how they simplify concurrent development and version control.

Why Different Types of “Given / When / Then”?

We use business rules in three places:

1. When we change the system
2. When we query the system
3. When we perform automated background tasks

1. Rules to Change the System

To change the system, we use state change slices – typically consisting of:

• a screen
• a command (blue)
• and resulting event(s) (orange)

📌 Example: A customer clicks Submit Order on the orders page → this stores an event in the system.

These rules are described using Given / When / Then:

• Given – sets system into a defined state
• When – describes the action
• Then – describes the expected outcome

This approach makes rules:

• Visual
• Understandable
• Testable (e.g., Gherkin syntax)

Example

When the Submit Order command is executed → Expect Order Submitted event stored with same ID

Preventing duplicates:

Given: Order Submitted already exists When: trying to submit again → Then: error state is expected

2. Rules for Read Models and Automations

In the podcast, Adam and I explained our different takes:

My approach:

• Provide Given / Then for automations
• Optional When for context
• Emphasis: describe state & expected outcome

Given: Order Submitted Then: Customer Notified is triggered automatically

Adam’s approach:

• Treat automations like state change slices
• Use full Given / When / Then

e.g. Treat automated SNS notification like a human-triggered command

Why “Given / When / Then” Matters

They are:

• Often ignored at the start
• Crucial for explaining how your system works
• A natural way to bridge business and technical understanding

What Else You Should Know

• 🎓 Want hands-on learning? Join workshops on Event Modeling
• 📘 “Understanding Eventsourcing” provides deep insights into these concepts
• 🧩 Great workflow between Figma & Miro for Eventmodeling

🎧 Follow the Eventmodeling & Eventsourcing Podcast 👋 See you next week!