Build an Event-Sourced Order Service

Build an Event-Sourced Order Service

What You’ll Build: A complete event-sourced order management system where all state changes are captured as immutable events.

Overview

This tutorial teaches event sourcing by building an order management service. You’ll learn to model business logic as aggregates that emit events, project those events into read models, and leverage the power of event replay.

By the end, you’ll have a service that:

• Creates orders as aggregates
• Adds/removes items with business rule validation
• Places and cancels orders
• Projects events into queryable read models
• Supports complete audit trails
• Enables time-travel queries

Learning Objectives

By the end of this tutorial, you will be able to:

• Design aggregates that enforce business invariants
• Create domain events that capture state changes
• Build event handlers that update read models
• Use the event store for persistence
• Replay events to rebuild state
• Implement event-driven workflows

Prerequisites

Before starting this tutorial, you should:

• Complete Todo Service and User Management tutorials
• Understand domain-driven design basics
• Understand the CQRS pattern
• Have PostgreSQL running (for event store)

What We’re Building

An order management service with:

Aggregates:

• Order - Root aggregate managing order lifecycle

Commands:

• CreateOrder - Initialize new order
• AddItem - Add product to order
• RemoveItem - Remove product from order
• PlaceOrder - Submit order for processing
• CancelOrder - Cancel a pending order

Domain Events:

• OrderCreated
• ItemAdded
• ItemRemoved
• OrderPlaced
• OrderCancelled

Queries:

• GetOrder - Retrieve order details
• ListOrders - List orders for customer

Read Models:

• OrderReadModel - Projected from events for queries

For the complete tutorial with all implementation details, see the platform example at:

/docs-new/06-examples/event-sourced-service/README.md

This tutorial will guide you through:

1. Setting up event sourcing infrastructure
2. Creating domain events
3. Building the Order aggregate
4. Implementing command handlers with event store
5. Creating read model projections
6. Testing event sourcing flows
7. Using event replay and catchup

The key difference from CRUD services is that instead of directly modifying state, we:

1. Load aggregate from event history
2. Execute business logic (emits events)
3. Append events to event store
4. Events automatically update read models

Refer to the detailed event-sourced service example for complete code and explanations.

Additional Resources

• Event Sourcing Concepts
• Aggregates Guide
• Event Store
• Read Model Projections