AsyncAPI Design Skill

When to Use This Skill

Use this skill when:

• Asyncapi Design tasks - Working on event-driven api specification with asyncapi 3.0 for message-based architectures
• Planning or design - Need guidance on Asyncapi Design approaches
• Best practices - Want to follow established patterns and standards

Overview

Event-driven API specification using AsyncAPI 3.0 for message-based and streaming architectures.

MANDATORY: Documentation-First Approach

Before creating AsyncAPI specifications:

1. Invoke docs-management skill for event-driven patterns
2. Verify AsyncAPI 3.0 syntax via MCP servers (context7 for latest spec)
3. Base all guidance on AsyncAPI 3.0 specification

AsyncAPI vs OpenAPI

AsyncAPI 3.0 Structure

Basic Template

asyncapi: 3.0.0
info:
  title: Order Events API
  version: 1.0.0
  description: |
    Event-driven API for order lifecycle events.

    ## Overview
    This API publishes events when orders change state, enabling
    downstream systems to react to order lifecycle changes.
  contact:
    name: Events Team
    email: events@example.com
  license:
    name: MIT
    url: https://opensource.org/licenses/MIT

servers:
  production:
    host: kafka.example.com:9092
    protocol: kafka
    description: Production Kafka cluster
    security:
      - $ref: '#/components/securitySchemes/sasl'

  development:
    host: localhost:9092
    protocol: kafka
    description: Local development Kafka

defaultContentType: application/json

channels:
  orderCreated:
    address: orders.created
    messages:
      orderCreatedMessage:
        $ref: '#/components/messages/OrderCreated'
    description: Published when a new order is created

  orderSubmitted:
    address: orders.submitted
    messages:
      orderSubmittedMessage:
        $ref: '#/components/messages/OrderSubmitted'

  orderStatusChanged:
    address: orders.status.changed
    messages:
      orderStatusChangedMessage:
        $ref: '#/components/messages/OrderStatusChanged'

operations:
  publishOrderCreated:
    action: send
    channel:
      $ref: '#/channels/orderCreated'
    summary: Publish order created event
    description: |
      Published when a customer creates a new order.
      Consumers should use this to initialize order tracking.

  consumeOrderCreated:
    action: receive
    channel:
      $ref: '#/channels/orderCreated'
    summary: Consume order created events
    description: Subscribe to receive new order notifications

  publishOrderSubmitted:
    action: send
    channel:
      $ref: '#/channels/orderSubmitted'

  publishOrderStatusChanged:
    action: send
    channel:
      $ref: '#/channels/orderStatusChanged'

components:
  messages:
    OrderCreated:
      name: OrderCreated
      title: Order Created Event
      summary: Event published when an order is created
      contentType: application/json
      headers:
        $ref: '#/components/schemas/EventHeaders'
      payload:
        $ref: '#/components/schemas/OrderCreatedPayload'
      examples:
        - name: basicOrder
          summary: Basic order creation
          headers:
            correlationId: "550e8400-e29b-41d4-a716-446655440000"
            eventType: "OrderCreated"
            eventVersion: "1.0"
            timestamp: "2025-01-15T10:30:00Z"
          payload:
            orderId: "ord-123456"
            customerId: "cust-789"
            items:
              - productId: "prod-001"
                quantity: 2
                unitPrice: 29.99
            createdAt: "2025-01-15T10:30:00Z"

    OrderSubmitted:
      name: OrderSubmitted
      title: Order Submitted Event
      contentType: application/json
      headers:
        $ref: '#/components/schemas/EventHeaders'
      payload:
        $ref: '#/components/schemas/OrderSubmittedPayload'

    OrderStatusChanged:
      name: OrderStatusChanged
      title: Order Status Changed Event
      contentType: application/json
      headers:
        $ref: '#/components/schemas/EventHeaders'
      payload:
        $ref: '#/components/schemas/OrderStatusChangedPayload'

  schemas:
    EventHeaders:
      type: object
      required:
        - correlationId
        - eventType
        - timestamp
      properties:
        correlationId:
          type: string
          format: uuid
          description: Unique identifier for request tracing
        eventType:
          type: string
          description: Type of the event
        eventVersion:
          type: string
          description: Schema version of the event
        timestamp:
          type: string
          format: date-time
          description: When the event occurred

    OrderCreatedPayload:
      type: object
      required:
        - orderId
        - customerId
        - createdAt
      properties:
        orderId:
          type: string
          description: Unique order identifier
        customerId:
          type: string
          description: Customer who created the order
        items:
          type: array
          items:
            $ref: '#/components/schemas/LineItem'
        createdAt:
          type: string
          format: date-time

    OrderSubmittedPayload:
      type: object
      required:
        - orderId
        - customerId
        - total
        - submittedAt
      properties:
        orderId:
          type: string
        customerId:
          type: string
        total:
          $ref: '#/components/schemas/Money'
        submittedAt:
          type: string
          format: date-time

    OrderStatusChangedPayload:
      type: object
      required:
        - orderId
        - previousStatus
        - newStatus
        - changedAt
      properties:
        orderId:
          type: string
        previousStatus:
          $ref: '#/components/schemas/OrderStatus'
        newStatus:
          $ref: '#/components/schemas/OrderStatus'
        reason:
          type: string
          description: Optional reason for status change
        changedAt:
          type: string
          format: date-time

    LineItem:
      type: object
      required:
        - productId
        - quantity
        - unitPrice
      properties:
        productId:
          type: string
        productName:
          type: string
        quantity:
          type: integer
          minimum: 1
        unitPrice:
          type: number
          format: decimal

    Money:
      type: object
      required:
        - amount
        - currency
      properties:
        amount:
          type: number
          format: decimal
        currency:
          type: string
          pattern: '^[A-Z]{3}$'

    OrderStatus:
      type: string
      enum:
        - draft
        - submitted
        - paid
        - shipped
        - delivered
        - cancelled

  securitySchemes:
    sasl:
      type: scramSha256
      description: SASL/SCRAM-SHA-256 authentication

    apiKey:
      type: apiKey
      in: user
      description: API key authentication

Protocol-Specific Patterns

Kafka

servers:
  kafka:
    host: broker1.example.com:9092,broker2.example.com:9092
    protocol: kafka
    protocolVersion: '3.0'
    bindings:
      kafka:
        schemaRegistryUrl: http://schema-registry:8081
        schemaRegistryVendor: confluent

channels:
  orderEvents:
    address: orders.events.v1
    bindings:
      kafka:
        topic: orders.events.v1
        partitions: 12
        replicas: 3
        topicConfiguration:
          cleanup.policy: ['delete']
          retention.ms: 604800000
          segment.bytes: 1073741824

operations:
  publishOrderEvent:
    action: send
    channel:
      $ref: '#/channels/orderEvents'
    bindings:
      kafka:
        groupId:
          type: string
        clientId:
          type: string
        bindingVersion: '0.5.0'

RabbitMQ

servers:
  rabbitmq:
    host: rabbitmq.example.com:5672
    protocol: amqp
    protocolVersion: '0.9.1'

channels:
  orderQueue:
    address: order-processing-queue
    bindings:
      amqp:
        is: queue
        queue:
          name: order-processing
          durable: true
          exclusive: false
          autoDelete: false
        exchange:
          name: orders-exchange
          type: topic
          durable: true
        bindingVersion: '0.3.0'

MQTT (IoT)

servers:
  mqtt:
    host: mqtt.example.com:1883
    protocol: mqtt
    protocolVersion: '5.0'

channels:
  deviceTelemetry:
    address: devices/{deviceId}/telemetry
    parameters:
      deviceId:
        description: Unique device identifier
        schema:
          type: string
    bindings:
      mqtt:
        qos: 1
        retain: false
        bindingVersion: '0.2.0'

WebSocket

servers:
  websocket:
    host: ws.example.com
    protocol: ws
    protocolVersion: '13'

channels:
  orderUpdates:
    address: /orders/updates
    bindings:
      ws:
        method: GET
        headers:
          type: object
          properties:
            Authorization:
              type: string

C# Implementation Patterns

Event Contracts

// Domain Events
public abstract record DomainEvent
{
    public Guid EventId { get; init; } = Guid.NewGuid();
    public DateTimeOffset OccurredAt { get; init; } = DateTimeOffset.UtcNow;
    public string EventType => GetType().Name;
    public string EventVersion { get; init; } = "1.0";
}

public sealed record OrderCreatedEvent(
    Guid OrderId,
    Guid CustomerId,
    IReadOnlyList<LineItemDto> Items,
    DateTimeOffset CreatedAt) : DomainEvent;

public sealed record OrderSubmittedEvent(
    Guid OrderId,
    Guid CustomerId,
    Money Total,
    DateTimeOffset SubmittedAt) : DomainEvent;

public sealed record OrderStatusChangedEvent(
    Guid OrderId,
    OrderStatus PreviousStatus,
    OrderStatus NewStatus,
    string? Reason,
    DateTimeOffset ChangedAt) : DomainEvent;

// Integration Events (for cross-boundary communication)
public abstract record IntegrationEvent
{
    public Guid Id { get; init; } = Guid.NewGuid();
    public Guid CorrelationId { get; init; }
    public DateTimeOffset Timestamp { get; init; } = DateTimeOffset.UtcNow;
    public string Source { get; init; } = "OrderService";
}

public sealed record OrderCreatedIntegrationEvent(
    Guid OrderId,
    Guid CustomerId,
    decimal TotalAmount,
    string Currency) : IntegrationEvent;

MassTransit Publisher

using MassTransit;

public sealed class OrderService
{
    private readonly IPublishEndpoint _publishEndpoint;
    private readonly ILogger<OrderService> _logger;

    public OrderService(
        IPublishEndpoint publishEndpoint,
        ILogger<OrderService> logger)
    {
        _publishEndpoint = publishEndpoint;
        _logger = logger;
    }

    public async Task CreateOrderAsync(
        CreateOrderCommand command,
        CancellationToken ct = default)
    {
        // Create order logic...
        var order = Order.Create(command.CustomerId, command.Items);

        // Publish event
        await _publishEndpoint.Publish(
            new OrderCreatedEvent(
                order.Id,
                order.CustomerId,
                order.Items.Select(i => i.ToDto()).ToList(),
                order.CreatedAt),
            ct);

        _logger.LogInformation(
            "Published OrderCreatedEvent for order {OrderId}",
            order.Id);
    }
}

// Consumer
public sealed class OrderCreatedConsumer : IConsumer<OrderCreatedEvent>
{
    private readonly ILogger<OrderCreatedConsumer> _logger;

    public OrderCreatedConsumer(ILogger<OrderCreatedConsumer> logger)
    {
        _logger = logger;
    }

    public async Task Consume(ConsumeContext<OrderCreatedEvent> context)
    {
        var @event = context.Message;

        _logger.LogInformation(
            "Processing OrderCreatedEvent: {OrderId}, Customer: {CustomerId}",
            @event.OrderId,
            @event.CustomerId);

        // Handle the event...
    }
}

// Registration
services.AddMassTransit(x =>
{
    x.AddConsumer<OrderCreatedConsumer>();

    x.UsingRabbitMq((context, cfg) =>
    {
        cfg.Host("rabbitmq://localhost", h =>
        {
            h.Username("guest");
            h.Password("guest");
        });

        cfg.ConfigureEndpoints(context);
    });
});

Kafka with Confluent

using Confluent.Kafka;
using System.Text.Json;

public sealed class KafkaOrderPublisher : IAsyncDisposable
{
    private readonly IProducer<string, string> _producer;
    private readonly string _topic;
    private readonly ILogger<KafkaOrderPublisher> _logger;

    public KafkaOrderPublisher(
        IConfiguration config,
        ILogger<KafkaOrderPublisher> logger)
    {
        _logger = logger;
        _topic = config["Kafka:OrdersTopic"] ?? "orders.events.v1";

        var producerConfig = new ProducerConfig
        {
            BootstrapServers = config["Kafka:BootstrapServers"],
            Acks = Acks.All,
            EnableIdempotence = true,
            MessageSendMaxRetries = 3,
            RetryBackoffMs = 1000
        };

        _producer = new ProducerBuilder<string, string>(producerConfig)
            .SetKeySerializer(Serializers.Utf8)
            .SetValueSerializer(Serializers.Utf8)
            .Build();
    }

    public async Task PublishAsync<TEvent>(
        TEvent @event,
        CancellationToken ct = default) where TEvent : DomainEvent
    {
        var key = GetPartitionKey(@event);
        var value = JsonSerializer.Serialize(@event);

        var message = new Message<string, string>
        {
            Key = key,
            Value = value,
            Headers = new Headers
            {
                { "event-type", Encoding.UTF8.GetBytes(@event.EventType) },
                { "event-version", Encoding.UTF8.GetBytes(@event.EventVersion) },
                { "correlation-id", Encoding.UTF8.GetBytes(Guid.NewGuid().ToString()) }
            }
        };

        var result = await _producer.ProduceAsync(_topic, message, ct);

        _logger.LogInformation(
            "Published {EventType} to {Topic}:{Partition}@{Offset}",
            @event.EventType,
            result.Topic,
            result.Partition.Value,
            result.Offset.Value);
    }

    private static string GetPartitionKey<TEvent>(TEvent @event) where TEvent : DomainEvent
    {
        return @event switch
        {
            OrderCreatedEvent e => e.OrderId.ToString(),
            OrderSubmittedEvent e => e.OrderId.ToString(),
            OrderStatusChangedEvent e => e.OrderId.ToString(),
            _ => Guid.NewGuid().ToString()
        };
    }

    public async ValueTask DisposeAsync()
    {
        _producer.Flush(TimeSpan.FromSeconds(10));
        _producer.Dispose();
    }
}

Event Design Patterns

Event Envelope Pattern

components:
  schemas:
    CloudEventEnvelope:
      type: object
      required:
        - specversion
        - type
        - source
        - id
        - time
        - data
      properties:
        specversion:
          type: string
          const: "1.0"
        type:
          type: string
          description: Event type (e.g., com.example.order.created)
        source:
          type: string
          format: uri
          description: Event source URI
        id:
          type: string
          format: uuid
        time:
          type: string
          format: date-time
        datacontenttype:
          type: string
          default: application/json
        dataschema:
          type: string
          format: uri
        subject:
          type: string
          description: Subject of event (e.g., order ID)
        data:
          type: object
          description: Event payload

Event Versioning

channels:
  orderEventsV1:
    address: orders.events.v1
    messages:
      orderCreatedV1:
        $ref: '#/components/messages/OrderCreatedV1'

  orderEventsV2:
    address: orders.events.v2
    messages:
      orderCreatedV2:
        $ref: '#/components/messages/OrderCreatedV2'

components:
  messages:
    OrderCreatedV1:
      name: OrderCreatedV1
      schemaFormat: application/vnd.aai.asyncapi+json;version=3.0.0
      payload:
        type: object
        properties:
          orderId:
            type: string
          # V1 schema

    OrderCreatedV2:
      name: OrderCreatedV2
      schemaFormat: application/vnd.aai.asyncapi+json;version=3.0.0
      payload:
        type: object
        properties:
          orderId:
            type: string
            format: uuid
          metadata:
            type: object
          # V2 schema with breaking changes

Best Practices

Channel Naming

# Hierarchy: {domain}.{entity}.{action}.{version}
channels:
  orderCreated:
    address: orders.order.created.v1
  orderItemAdded:
    address: orders.lineitem.added.v1
  paymentProcessed:
    address: payments.payment.processed.v1

Message Design

1. Self-describing: Include type and version in headers
2. Idempotent: Use event ID for deduplication
3. Ordered: Use partition keys for ordering
4. Backward compatible: Add fields, don't remove
5. Complete: Include all data consumers need (avoid chatty patterns)

Security Considerations

components:
  securitySchemes:
    oauth2:
      type: oauth2
      description: OAuth 2.0 authentication
      flows:
        clientCredentials:
          tokenUrl: https://auth.example.com/oauth/token
          scopes:
            orders:read: Read order events
            orders:write: Publish order events

    mtls:
      type: X509
      description: Mutual TLS authentication

Workflow

When designing AsyncAPI specifications:

1. Identify events: What significant occurrences need to be communicated?
2. Define channels: What topics/queues will carry these events?
3. Design messages: What data does each event contain?
4. Choose protocol: Kafka, RabbitMQ, MQTT, etc.?
5. Add bindings: Protocol-specific configuration
6. Document security: Authentication and authorization
7. Version strategy: How will events evolve?
8. Generate code: Use AsyncAPI generator for clients/handlers

MCP Research

For current AsyncAPI patterns and tools:

perplexity: "AsyncAPI 3.0 specification" "event-driven API design patterns"
context7: "asyncapi" (for official documentation)
ref: "AsyncAPI spec examples" "Kafka binding patterns"

Last Updated: 2025-12-26