What is an AsyncAPI specification?

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asyncapi: 3.0.0
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info:
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  title: User Notification Service
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  version: 1.0.0
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  description: |
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    Service that handles user notifications through various channels
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    including email, SMS, and push notifications.
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channels:
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  user/signup:
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    address: user/signup
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    messages:
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      UserSignedUp:
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        $ref: '#/components/messages/UserSignedUp'
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  notification/send:
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    address: notification/send
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    messages:
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      SendNotification:
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        $ref: '#/components/messages/SendNotification'
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operations:
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  onUserSignup:
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    action: receive
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    channel:
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      $ref: '#/channels/user~1signup'
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    summary: User signup event
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    description: Triggered when a user signs up for the service
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  sendNotification:
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    action: send
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    channel:
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      $ref: '#/channels/notification~1send'
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    summary: Send notification
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    description: Send a notification to a user
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components:
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  messages:
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    UserSignedUp:
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      name: UserSignedUp
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      title: User Signed Up
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      summary: User has signed up for the service
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      contentType: application/json
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      payload:
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        $ref: '#/components/schemas/User'
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    SendNotification:
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      name: SendNotification
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      title: Send Notification
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      summary: Send a notification to a user
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      contentType: application/json
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      payload:
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        $ref: '#/components/schemas/Notification'
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  schemas:
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    User:
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      type: object
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      properties:
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        id:
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          type: string
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          format: uuid
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          description: Unique identifier for the user
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        email:
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          type: string
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          format: email
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          description: User's email address
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        name:
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          type: string
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          description: User's full name
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        createdAt:
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          type: string
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          format: date-time
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          description: When the user was created
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      required:
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        - id
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        - email
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        - name
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    Notification:
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      type: object
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      properties:
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        userId:
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          type: string
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          format: uuid
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          description: ID of the user to notify
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        type:
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          type: string
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          enum: [email, sms, push]
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          description: Type of notification to send
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        message:
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          type: string
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          description: Message content
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        priority:
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          type: string
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          enum: [low, medium, high, urgent]
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          default: medium
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          description: Priority level of the notification
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      required:
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        - userId
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        - type
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        - message