What is AsyncAPI and how does it differ from OpenAPI?

What is AsyncAPI and how does it differ from OpenAPI? #

As event-driven architectures, message queues, and real-time communication patterns have become mainstream in modern software systems, the need for a standard way to describe asynchronous APIs has grown. AsyncAPI is the answer: an open specification for event-driven APIs that parallels what OpenAPI does for REST APIs. Understanding the relationship between the two standards — their similarities, differences, and the scenarios where each is appropriate — is essential for any team building or consuming modern APIs.

What is AsyncAPI? #

AsyncAPI is an open specification maintained by the AsyncAPI Initiative, a Linux Foundation project. Like OpenAPI, it uses YAML or JSON to describe API interfaces in a machine-readable format. But where OpenAPI describes synchronous request-response HTTP interactions, AsyncAPI describes asynchronous message-driven interactions — the events, messages, and channels through which systems communicate over protocols like:

AsyncAPI 2.x introduced the concept of channels (topics, queues, subjects) and operations (publish or subscribe) as the primary organizing concepts, replacing the path/method model of OpenAPI.

AsyncAPI 3.0, released in 2024, refined the model further — separating the channel structure from the application’s operations more cleanly and improving multi-protocol support.

The Core Difference: Synchronous vs. Asynchronous #

The fundamental distinction between OpenAPI and AsyncAPI is the communication model they describe:

Aspect	OpenAPI	AsyncAPI
Communication model	Synchronous request-response	Asynchronous message-driven
Initiator	Client always initiates	Either party can send
Protocol focus	HTTP/HTTPS	Any messaging protocol
Response timing	Immediate	Decoupled, eventual
Interaction unit	Operation (path + method)	Channel + message
Primary use case	REST APIs	Event streams, queues, pub/sub

In OpenAPI, a client sends a request and waits for the server’s response. In AsyncAPI, a producer sends a message to a channel, and one or more consumers receive it — potentially much later, independently, and without a direct connection to the producer.

AsyncAPI Document Structure #

An AsyncAPI 3.0 document shares structural similarities with OpenAPI but uses different top-level fields:

asyncapi: 3.0.0
info:
  title: Order Events API
  version: 1.0.0
  description: Events emitted when order status changes

servers:
  production:
    host: kafka.example.com:9092
    protocol: kafka

channels:
  orderPlaced:
    address: orders.placed
    messages:
      orderPlacedMessage:
        $ref: '#/components/messages/OrderPlaced'

  orderShipped:
    address: orders.shipped
    messages:
      orderShippedMessage:
        $ref: '#/components/messages/OrderShipped'

operations:
  receiveOrderPlaced:
    action: receive
    channel:
      $ref: '#/channels/orderPlaced'

  sendOrderShipped:
    action: send
    channel:
      $ref: '#/channels/orderShipped'

components:
  messages:
    OrderPlaced:
      payload:
        type: object
        required: [orderId, customerId, timestamp]
        properties:
          orderId:
            type: string
            format: uuid
          customerId:
            type: string
          timestamp:
            type: string
            format: date-time

Key structural elements:

channels — the communication channels (Kafka topics, AMQP queues, WebSocket streams).
operations — what the application does on those channels: send (produce) or receive (consume).
servers — the broker or server connection details, including the protocol.
components/messages — reusable message definitions, including payload schemas.

Shared Concepts: JSON Schema and Security #

Both AsyncAPI and OpenAPI use JSON Schema for defining payload schemas. This means teams with OpenAPI experience can immediately apply their schema knowledge to AsyncAPI documents. AsyncAPI 3.x aligns with JSON Schema Draft 2020-12, matching OpenAPI 3.1’s schema vocabulary.

Both specifications also support security scheme definitions, though the authentication mechanisms relevant to message brokers (SASL, mTLS, API keys) differ from those commonly used in HTTP APIs.

Tooling Ecosystem #

AsyncAPI has a growing ecosystem of tools:

AsyncAPI Studio — a web-based editor for AsyncAPI documents, similar to Swagger Editor for OpenAPI.
AsyncAPI Generator — generates documentation, server code, and client code from AsyncAPI documents. Supports templates for Node.js, Python, Java, and more.
Microcks — mocking and contract testing for both OpenAPI and AsyncAPI in a single platform.
Spectral — linting support for AsyncAPI documents alongside OpenAPI.
AsyncAPI Diff — detects breaking changes between AsyncAPI document versions.

The AsyncAPI tooling catalog lists the full ecosystem.

When to Use OpenAPI vs. AsyncAPI #

The choice between OpenAPI and AsyncAPI is determined by the communication pattern being described, not a preference:

Use OpenAPI when:

Describing REST APIs with HTTP request-response semantics.
Building CRUD APIs, query APIs, or RPC-over-HTTP patterns.
Generating HTTP client SDKs or server stubs.
Documenting webhooks (supported natively in OpenAPI 3.1).

Use AsyncAPI when:

Describing Kafka topics, RabbitMQ queues, or MQTT topics.
Documenting WebSocket streams or Server-Sent Event channels.
Building event-driven microservices that communicate through a message broker.
Describing the pub/sub interface of a service in an event-driven architecture.

Many real-world systems use both: a REST API described with OpenAPI for synchronous interactions, and event streams described with AsyncAPI for asynchronous notifications. Microcks and platforms like Apicurio support both specifications in the same governance and testing workflow.

The AsyncAPI and OpenAPI Collaboration #

The AsyncAPI and OpenAPI initiatives collaborate actively. Both are Linux Foundation projects, share JSON Schema as a common schema vocabulary, and align on governance and tooling practices. The CloudEvents specification — a CNCF project for standardizing event envelope formats — is complementary to both specifications and is increasingly documented using AsyncAPI.

Conclusion #

AsyncAPI is the event-driven API community’s counterpart to OpenAPI: a machine-readable, JSON Schema-based specification for describing the channels, messages, and operations of asynchronous systems. While OpenAPI excels at documenting REST APIs, AsyncAPI is the right tool for Kafka, AMQP, MQTT, WebSocket, and other messaging protocol interfaces. Together, the two specifications provide a comprehensive description layer for the full spectrum of modern API communication patterns — synchronous and asynchronous, HTTP and messaging.

Last updated on April 30, 2026.