Event-Driven APIs with AsyncAPI — Complete Guide

DodaTech Updated 2026-06-28 4 min read

In this tutorial, you will learn about Event. We cover key concepts, practical examples, and best practices to help you master this topic.

Event-driven architectures use events to communicate between services. AsyncAPI provides the documentation standard for these systems. This lesson covers common event-driven patterns and how to represent them in AsyncAPI.

What You'll Learn

Event-driven architectural patterns
Event sourcing with AsyncAPI
CQRS pattern documentation
Saga pattern for distributed transactions
Pub-sub and message routing

Why It Matters

Event-driven patterns solve real problems in Distributed Systems. Documenting these patterns with AsyncAPI ensures consistency and enables automated tooling for complex architectural styles.

Real-World Use

An e-commerce platform uses the saga pattern for order processing. AsyncAPI documents each step of the saga: order created, payment authorized, inventory reserved, shipment scheduled. If any step fails, compensating events roll back the Transaction.

Flow Chart

flowchart LR
    A[Event-Driven Patterns] --> B[Event Sourcing]
    A --> C[CQRS]
    A --> D[Saga Pattern]
    A --> E[Pub-Sub]
    B --> F[Event Store]
    C --> G[Read Model]
    C --> H[Write Model]
    D --> I[Compensating Events]
    E --> J[Broker]

Code Examples

Example 1: Saga Pattern with AsyncAPI

asyncapi: '2.6.0'
info:
  title: Order Saga
  version: '1.0.0'

channels:
  saga/order/started:
    publish:
      message:
        payload:
          type: object
          properties:
            sagaId:
              type: string
            orderId:
              type: string
            steps:
              type: array
              items:
                type: string
                enum:
                  - authorize-payment
                  - reserve-inventory
                  - schedule-shipment

  saga/order/{step}/completed:
    parameters:
      step:
        schema:
          type: string
    publish:
      message:
        payload:
          type: object
          properties:
            sagaId:
              type: string
            orderId:
              type: string
            step:
              type: string
            result:
              type: object

  saga/order/{step}/failed:
    parameters:
      step:
        schema:
          type: string
    publish:
      message:
        payload:
          type: object
          properties:
            sagaId:
              type: string
            orderId:
              type: string
            step:
              type: string
            error:
              type: string
            compensationEvent:
              type: string

Expected output: AsyncAPI channels modeling a saga pattern with parameterized step channels for completed and failed events.

Example 2: Event Sourcing with AsyncAPI

asyncapi: '2.6.0'
info:
  title: Account Event Store
  version: '1.0.0'

channels:
  events/account/{accountId}:
    parameters:
      accountId:
        schema:
          type: string
    publish:
      operationId: appendAccountEvent
      message:
        payload:
          type: object
          required:
            - eventId
            - eventType
            - timestamp
            - data
          properties:
            eventId:
              type: string
              format: uuid
            eventType:
              type: string
              enum:
                - ACCOUNT_CREATED
                - ACCOUNT_CREDITED
                - ACCOUNT_DEBITED
                - ACCOUNT_FROZEN
                - ACCOUNT_CLOSED
            timestamp:
              type: string
              format: date-time
            data:
              type: object
            version:
              type: integer
              description: Aggregate version number
    subscribe:
      operationId: replayAccountEvents
      message:
        $ref: '#/components/messages/AccountEvent'

Expected output: An event sourcing channel that stores all account state changes as an append-only event stream.

Example 3: CQRS Pattern Documentation

asyncapi: '2.6.0'
info:
  title: CQRS Order System
  version: '1.0.0'

channels:
  commands/order:
    subscribe:
      operationId: handleOrderCommand
      summary: Command channel for order mutations
      message:
        oneOf:
          - $ref: '#/components/messages/CreateOrderCommand'
          - $ref: '#/components/messages/UpdateOrderCommand'
          - $ref: '#/components/messages/CancelOrderCommand'

  events/order:
    publish:
      operationId: emitOrderEvent
      summary: Event channel for order state changes
      message:
        oneOf:
          - $ref: '#/components/messages/OrderCreated'
          - $ref: '#/components/messages/OrderUpdated'
          - $ref: '#/components/messages/OrderCancelled'

  queries/order:
    subscribe:
      operationId: queryOrder
      summary: Query channel for read operations
      message:
        payload:
          type: object
          properties:
            queryType:
              type: string
            filters:
              type: object

Expected output: CQRS pattern with separate channels for commands, events, and queries.

Common Mistakes

Mistake	Explanation
Mixing commands and events	Commands are requests (imperative), events are facts (past tense). Keep them in separate channels
Ignoring compensating events	Sagas need documented compensation paths for every step that can fail
Overcomplicating event schemas	Event payloads should contain the minimum data needed, not entire aggregate snapshots
Not documenting idempotency	Event consumers need to know how to handle duplicate events safely
Forgetting schema evolution	Events live forever; plan for backward compatibility from the start

Practice Questions

How does AsyncAPI support the saga pattern?
What is the difference between a command channel and an event channel?
How do you document event sourcing with AsyncAPI?
What is the CQRS pattern and how do you represent it in AsyncAPI?
How do you handle compensating transactions in event-driven systems?

Challenge

Design a complete Event-Driven Architecture for a banking system using AsyncAPI. Include event sourcing for account transactions, CQRS for read/write separation, and a saga pattern for money transfers between accounts.

FAQ

Can AsyncAPI document CQRS systems?

Yes, use separate channels for commands, events, and queries to represent each CQRS path clearly.

How do I document event versioning?

Use message traits for version headers, or define different messages for different event schema versions.

What is the difference between pub-sub and event streaming?

Pub-sub is one-to-many broadcast. Event streaming is ordered, replayable, and typically uses an append-only log.

How do idempotency keys work in event-driven systems?

Include an idempotency key in event headers. Consumers check for duplicate keys before processing.

Can AsyncAPI handle dead letter queues?

Yes, document dead letter channels in your spec to show how failed messages are handled.

What is the best way to document compensating events?

Use correlation IDs to link compensating events to original events, and document the compensation flow in the channel description.

Mini Project

Design the complete event-driven architecture for a food delivery platform using AsyncAPI. Include event sourcing for order state, CQRS for menu queries vs order mutations, saga pattern for order fulfillment (prepare food, assign driver, deliver), and dead letter handling.

What's Next

Learn how to use AsyncAPI with Kafka

← Previous AsyncAPI Tools and Ecosystem — Complete Guide Next → AsyncAPI with Kafka — Complete Guide

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