Designing Event-Driven Sagas
Design the flow of a choreography saga using events to trigger actions across different microservices.
Designing Event-Driven Sagas is a free Microservices Communication Patterns (Saga, Circuit Breaker) lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Microservices Communication Patterns (Saga, Circuit Breaker) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Event-Driven Saga Design
Welcome! In this lesson, we'll learn how to design the flow of a Choreography Saga. This pattern helps manage complex business transactions that span multiple services.
We'll focus on using events as the primary way services communicate and trigger each other's actions.
Choreography Refresher
Remember Choreography Sagas? In this style, services communicate directly by publishing and subscribing to events.
- No central orchestrator: Services react to events from others.
- Decentralized decisions: Each service decides its next step based on the event it receives.
Our Business Scenario
Let's design a saga for an "Online Order Placement" system. This common scenario involves several steps across different microservices:
- Customer places order.
- Inventory is checked and reserved.
- Payment is processed.
- Order is shipped.
Identifying Core Saga Steps
For our "Order Placement" saga, we can break it down into these main steps, each potentially handled by a different service:
- Order Service: Creates the order.
- Inventory Service: Reserves items.
- Payment Service: Processes payment.
- Shipping Service: Schedules shipment.
Events Drive the Flow
In a choreography saga, each step is initiated by an event. When one service completes its task, it publishes an event.
Other services, interested in that event, subscribe to it and react accordingly. This creates a chain reaction.
Kicking Off the Saga
Every saga needs a starting point. For our "Order Placement," the Order Service initiates the process.
When a customer places an order, the Order Service creates it and publishes an OrderCreatedEvent.
Inventory Reacts to Order
The Inventory Service subscribes to OrderCreatedEvent. Upon receiving it, it attempts to reserve the items for the order.
After processing, it publishes either an InventoryReservedEvent or an InventoryFailedEvent.
class OrderCreatedEvent {
String orderId;
String customerId;
// ... item details
}
class InventoryService {
public void handleOrderCreated(OrderCreatedEvent event) {
System.out.println("Inventory Service: Received OrderCreatedEvent for " + event.orderId);
// Logic to reserve items...
boolean success = true; // Assume success for now
if (success) {
System.out.println("Inventory Service: Items reserved. Publishing InventoryReservedEvent.");
// In a real system, publish InventoryReservedEvent
} else {
System.out.println("Inventory Service: Inventory failed. Publishing InventoryFailedEvent.");
// In a real system, publish InventoryFailedEvent
}
}
}
public class Main {
public static void main(String[] args) {
InventoryService inventory = new InventoryService();
OrderCreatedEvent newOrder = new OrderCreatedEvent();
newOrder.orderId = "ORD123";
newOrder.customerId = "CUST456";
inventory.handleOrderCreated(newOrder);
}
}Payment Reacts to Inventory
The Payment Service subscribes to InventoryReservedEvent. This means payment only proceeds if inventory is successfully reserved.
It processes the payment and then publishes either a PaymentProcessedEvent or a PaymentFailedEvent.
Shipping Reacts to Payment
Finally, the Shipping Service subscribes to PaymentProcessedEvent. It schedules the shipment only after payment is confirmed.
It then publishes an OrderShippedEvent to complete the main saga flow.
Planning for Rollbacks
What if a step fails? For example, if payment fails after inventory is reserved? In a choreography saga, we use compensation events.
The Payment Service would publish a PaymentFailedEvent. The Inventory Service would subscribe to this event and publish an InventoryReleasedEvent to undo its previous action.
Saga Flow Check
Consider our Order Placement saga. What is the correct sequence of events if the entire process is successful?
Recap: Designing Event Flows
We've learned how to design a choreography saga by breaking down a business transaction into steps.
- Each step publishes an event to trigger the next.
- We identified the initial event and subsequent events for a successful flow.
- We also briefly considered how compensation events are used to handle failures.
Next, we'll explore how message brokers facilitate this communication!
Frequently asked questions
Is the “Designing Event-Driven Sagas” lesson free?
Yes — the full text of “Designing Event-Driven Sagas” is free to read here on the web, and the Microservices Communication Patterns (Saga, Circuit Breaker) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Microservices Communication Patterns (Saga, Circuit Breaker) course, upgrade to CoddyKit PRO.
What will I learn in “Designing Event-Driven Sagas”?
Design the flow of a choreography saga using events to trigger actions across different microservices. You practise Microservices Communication Patterns (Saga, Circuit Breaker) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Microservices Communication Patterns (Saga, Circuit Breaker)?
No prior experience is required. Microservices Communication Patterns (Saga, Circuit Breaker) on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Designing Event-Driven Sagas” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Microservices Communication Patterns (Saga, Circuit Breaker) lesson?
Yes. Every Microservices Communication Patterns (Saga, Circuit Breaker) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Designing Event-Driven Sagas
- Event Bus and Message Brokers
- Handling Compensation with Events
- Building Idempotent Event Consumers