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Event Storming is a fun, collaborative workshop technique to understand complex business domains. It helps teams visualize how things happen in a system.

Imagine a room full of sticky notes! Teams map out a business process by focusing on the "events" that occur, making it easier to see the big picture.

For SaaS, especially with microservices, understanding domain boundaries is key. Event Storming helps:

• Break down monoliths: Clearly identifies areas to split.
• Define service boundaries: Groups related events and actions into potential services.
• Improve communication: Gets everyone on the same page, from business to tech.

Event Storming uses different colored sticky notes to represent various elements of your domain:

• Orange: Domain Events (what happened?)
• Blue: Commands (what triggers an event?)
• Yellow: Aggregates (who performs a command?)
• Green: Read Models (how users view data?)
• Purple: Policies (rules reacting to events)

This is the starting point! Think about "what happened?" in your business process. Events are facts from the past, always written in past tense.

Examples:

• OrderPlaced
• PaymentReceived
• UserRegistered
• SubscriptionCancelled

These are the core truths of your system.

While Event Storming is visual, the events we discover often translate into code. Here's how a simple OrderPlaced event might look in a Java application:

Try running this example to see how an event could be represented and "published" (here, just printed).

After events, we look for commands. A command is an explicit request to do something, which usually leads to an event (or multiple events).

Ask: "What action caused this event to happen?" Commands are usually imperative verbs.

• PlaceOrder (leads to OrderPlaced)
• ProcessPayment (leads to PaymentReceived)
• RegisterUser (leads to UserRegistered)

Aggregates are the "who" or "what" that receives a command and produces an event. They are transactional consistency boundaries – meaning everything within an aggregate changes together.

Think of them as the "model" or "entity" in your code that holds state and enforces business rules.

• Order aggregate handles PlaceOrder.
• PaymentProcessor handles ProcessPayment.

Read Models (Green): These are how users or other systems query information. They are optimized for reading, often denormalized views built from events.

Policies (Purple): These are business rules that react to events. For example, "when an OrderPlaced event occurs, then send a confirmation email."

As you map out your domain, you'll see natural clusters of events, commands, and aggregates. These clusters often define your Bounded Contexts.

A Bounded Context is a logical boundary within which a specific domain model is defined and applicable. This is crucial for designing independent microservices!

Event Storming doesn't just help with technical design; it fosters a shared understanding across the entire team.

• Shared Language: Everyone uses the same terms.
• Knowledge Transfer: New team members quickly grasp complex flows.
• Problem Identification: Uncovers hidden assumptions and issues early.

Event Storming uses different colored sticky notes to represent various domain elements. Match the sticky note color to its correct description.

You've learned that Event Storming is a powerful, visual technique to understand complex domains. By focusing on Domain Events, you can identify Commands, Aggregates, Read Models, and Policies.

This collaborative approach helps in defining clear Bounded Contexts, making it an invaluable tool for designing robust and scalable microservices in your SaaS architecture.