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AsyncAPI Specification

Disclaimer

Part of this content has been taken from the great work done by the folks at the Open API Initiative. Mainly because it's a great work and we want to keep as much compatibility as possible with the Open API Specification.

Version 1.0.0

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The AsyncAPI Specification is licensed under The Apache License, Version 2.0.

Introduction

The AsyncAPI Specification is a project used to describe and document Asynchronous APIs.

The AsyncAPI Specification defines a set of files required to describe such an API. These files can then be used to create utilities, such as documentation, integration and/or testing tools.

The AsyncAPI Specification is often used to describe the inter-process communication (IPC) in distributed systems built using a broker-centric architecture. In such cases, it's very easy to get confused with what the AsyncAPI files must describe. It's RECOMMENDED to create a single file describing the whole system instead of creating a file for each process. Otherwise, you will end up having lots of interdependent files.

The file(s) MUST describe the operations a new process can perform. For instance:

event.user.signup:
  subscribe:
    $ref: "#/components/messages/userSignUp"

It means processes can subscribe to event.user.signup topic. However, it does NOT mean every process must subscribe to this topic.

Table of Contents

Definitions

Message Broker

A message broker is a system in charge of message exchange. It MAY provide additional features, such as message queueing, storage or processing.

Message

A message is a piece of information a process will send to a message broker. It MUST contain headers and payload.

Topic

A topic is a routing key used by the message broker to deliver messages to the subscribed processes. Depending on the protocol used, a message MAY include the topic in its headers.

Process

A process is any kind of computer program connected to a message broker. It MUST be a producer, a consumer or both.

Producer

A producer is a process publishing messages to a message broker.

Consumer

A consumer is a process subscribed to a message broker and consumes messages from it.

Topic Templating

Topic templating refers to the usage of curly braces ({}) to mark a section of a topic as replaceable.

Specification

Format

The files describing the Asynchronous API in accordance with the AsyncAPI Specification are represented as JSON objects and conform to the JSON standards. YAML, being a superset of JSON, can be used as well to represent a A2S (AsyncAPI Specification) file.

For example, if a field is said to have an array value, the JSON array representation will be used:

{
   "field" : [...]
}

While the API is described using JSON it does not impose a JSON input/output to the API itself.

All field names in the specification are case sensitive.

The schema exposes two types of fields. Fixed fields, which have a declared name, and Patterned fields, which declare a regex pattern for the field name. Patterned fields can have multiple occurrences as long as each has a unique name.

In order to preserve the ability to round-trip between YAML and JSON formats, YAML version 1.2 is recommended along with some additional constraints:

  • Tags MUST be limited to those allowed by the JSON Schema ruleset
  • Keys used in YAML maps MUST be limited to a scalar string, as defined by the YAML Failsafe schema ruleset

File Structure

The A2S representation of the API is made of a single file. However, parts of the definitions can be split into separate files, at the discretion of the user. This is applicable for $ref fields in the specification as follows from the JSON Schema definitions.

By convention, the AsyncAPI Specification (A2S) file is named asyncapi.json or asyncapi.yaml.

Schema

AsyncAPI Object

This is the root document object for the API specification. It combines resource listing and API declaration together into one document.

Fixed Fields
Field Name Type Description
asyncapi AsyncAPI Version String Required. Specifies the AsyncAPI Specification version being used. It can be used by tooling Specifications and clients to interpret the version. The structure shall be major.minor.patch, where patch versions must be compatible with the existing major.minor tooling. Typically patch versions will be introduced to address errors in the documentation, and tooling should typically be compatible with the corresponding major.minor (1.0.*). Patch versions will correspond to patches of this document.
info Info Object Required. Provides metadata about the API. The metadata can be used by the clients if needed.
baseTopic BaseTopic String The base topic to the API.
servers Server Object An array of Server Objects, which provide connectivity information to a target server.
topics Topics Object Required. The available topics and messages for the API.
components Components Object An element to hold various schemas for the specification.
security [Security Requirement Object] A declaration of which security mechanisms can be used across the API. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a connection or operation.
tags [Tag Object] A list of tags used by the specification with additional metadata. Each tag name in the list MUST be unique.
externalDocs External Documentation Object Additional external documentation.

This object can be extended with Specification Extensions.

AsyncAPI Version String

The version string signifies the version of the AsyncAPI Specification that the document complies to. The format for this string must be major.minor.patch. The patch may be suffixed by a hyphen and extra alphanumeric characters.

A major.minor shall be used to designate the AsyncAPI Specification version, and will be considered compatible with the AsyncAPI Specification specified by that major.minor version. The patch version will not be considered by tooling, making no distinction between 1.0.0 and 1.0.1.

In subsequent versions of the AsyncAPI Specification, care will be given such that increments of the minor version should not interfere with operations of tooling developed to a lower minor version. Thus a hypothetical 1.1.0 specification should be usable with tooling designed for 1.0.0.

Info Object

The object provides metadata about the API. The metadata can be used by the clients if needed.

Fixed Fields
Field Name Type Description
title string Required. The title of the application.
version string Required Provides the version of the application API (not to be confused with the specification version).
description string A short description of the application. CommonMark syntax can be used for rich text representation.
termsOfService string A URL to the Terms of Service for the API. MUST be in the format of a URL.
contact Contact Object The contact information for the exposed API.
license License Object The license information for the exposed API.

This object can be extended with Specification Extensions.

Info Object Example:
{
  "title": "AsyncAPI Sample App",
  "description": "This is a sample server.",
  "termsOfService": "http://asyncapi.org/terms/",
  "contact": {
    "name": "API Support",
    "url": "http://www.asyncapi.org/support",
    "email": "[email protected]"
  },
  "license": {
    "name": "Apache 2.0",
    "url": "http://www.apache.org/licenses/LICENSE-2.0.html"
  },
  "version": "1.0.1"
}
title: AsyncAPI Sample App
description: This is a sample server.
termsOfService: http://asyncapi.org/terms/
contact:
  name: API Support
  url: http://www.asyncapi.org/support
  email: [email protected]
license:
  name: Apache 2.0
  url: http://www.apache.org/licenses/LICENSE-2.0.html
version: 1.0.1

Contact Object

Contact information for the exposed API.

Fixed Fields
Field Name Type Description
name string The identifying name of the contact person/organization.
url string The URL pointing to the contact information. MUST be in the format of a URL.
email string The email address of the contact person/organization. MUST be in the format of an email address.

This object can be extended with Specification Extensions.

Contact Object Example:
{
  "name": "API Support",
  "url": "http://www.example.com/support",
  "email": "[email protected]"
}
name: API Support
url: http://www.example.com/support
email: [email protected]

License Object

License information for the exposed API.

Fixed Fields
Field Name Type Description
name string Required. The license name used for the API.
url string A URL to the license used for the API. MUST be in the format of a URL.

This object can be extended with Specification Extensions.

License Object Example:
{
  "name": "Apache 2.0",
  "url": "http://www.apache.org/licenses/LICENSE-2.0.html"
}
name: Apache 2.0
url: http://www.apache.org/licenses/LICENSE-2.0.html

Base Topic String

The base topic to the API. You MAY use this field to avoid repeating the beginning of the topics.

Base Topic String Example:
{
  "baseTopic": "hitch.accounts"
}
baseTopic: hitch.accounts

Server Object

An object representing a Server.

Fixed Fields
Field Name Type Description
url string REQUIRED. A URL to the target host. This URL supports Server Variables and MAY be relative, to indicate that the host location is relative to the location where the AsyncAPI document is being served. Variable substitutions will be made when a variable is named in {brackets}.
scheme string REQUIRED. The scheme this URL supports for connection. The value MUST be one of the following: amqp, amqps, mqtt, mqtts, ws, wss, stomp, stomps.
description string An optional string describing the host designated by the URL. CommonMark syntax MAY be used for rich text representation.
variables Map[string, Server Variable Object] A map between a variable name and its value. The value is used for substitution in the server's URL template.

This object MAY be extended with Specification Extensions.

Server Object Example

A single server would be described as:

{
  "url": "development.gigantic-server.com",
  "description": "Development server",
  "scheme": "mqtts"
}
url: development.gigantic-server.com
description: Development server
scheme: mqtts

The following shows how multiple servers can be described, for example, at the AsyncAPI Object's servers:

{
  "servers": [
    {
      "url": "development.gigantic-server.com",
      "description": "Development server",
      "scheme": "mqtts"
    },
    {
      "url": "staging.gigantic-server.com",
      "description": "Staging server",
      "scheme": "mqtts"
    },
    {
      "url": "api.gigantic-server.com",
      "description": "Production server",
      "scheme": "mqtts"
    }
  ]
}
servers:
- url: development.gigantic-server.com
  description: Development server
  scheme: mqtts
- url: staging.gigantic-server.com
  description: Staging server
  scheme: mqtts
- url: api.gigantic-server.com
  description: Production server
  scheme: mqtts

The following shows how variables can be used for a server configuration:

{
  "servers": [
    {
      "url": "{username}.gigantic-server.com:{port}/{basePath}",
      "description": "The production API server",
      "variables": {
        "username": {
          "default": "demo",
          "description": "This value is assigned by the service provider, in this example `gigantic-server.com`"
        },
        "port": {
          "enum": [
            "8883",
            "8884"
          ],
          "default": "8883"
        },
        "basePath": {
          "default": "v2"
        }
      }
    }
  ]
}
servers:
- url: {username}.gigantic-server.com:{port}/{basePath}
  description: The production API server
  variables:
    username:
      # note! no enum here means it is an open value
      default: demo
      description: This value is assigned by the service provider, in this example `gigantic-server.com`
    port:
      enum:
        - '8883'
        - '8884'
      default: '8883'
    basePath:
      # open meaning there is the opportunity to use special base paths as assigned by the provider, default is `v2`
      default: v2

Server Variable Object

An object representing a Server Variable for server URL template substitution.

Fixed Fields
Field Name Type Description
enum [string] An enumeration of string values to be used if the substitution options are from a limited set.
default string The default value to use for substitution, and to send, if an alternate value is not supplied.
description string An optional description for the server variable. CommonMark syntax MAY be used for rich text representation.

At least one of the fields MUST be provided.

This object MAY be extended with Specification Extensions.

Topics Object

Holds the relative paths to the individual topic and their operations. The topic is appended to the Base Topic in order to construct the full one.

Patterned Fields
Field Pattern Type Description
^[^.]{topic} Topic Item Object A relative path to an individual topic. The field name MUST NOT begin with a dot. Topic templating is allowed.

This object can be extended with Specification Extensions.

Topics Object Example
{
  "accounts.1.0.event.user.signup": {
    "subscribe": {
      "$ref": "#/components/messages/userSignedUp"
    }
  }
}
accounts.1.0.event.user.signup:
  subscribe:
    $ref: "#/components/messages/userSignedUp"

Topic Item Object

Describes the operations available on a single topic.

Fixed Fields
Field Name Type Description
$ref string Allows for an external definition of this topic item. The referenced structure MUST be in the format of a Topic Item Object. If there are conflicts between the referenced definition and this Topic Item's definition, the behavior is undefined.
subscribe Message Object A definition of the message a SUBSCRIBE operation will receive on this topic.
publish Message Object A definition of the message a PUBLISH operation will receive on this topic.

This object can be extended with Specification Extensions.

Topic Item Object Example
{
  "subscribe": {
    "summary": "A user signed up.",
    "description": "A longer description of the message",
    "payload": {
      "type": "object",
      "properties": {
        "user": {
          "$ref": "#/components/schemas/user"
        },
        "signup": {
          "$ref": "#/components/schemas/signup"
        }
      }
    }
  }
}
subscribe:
  summary: A user signed up.
  description: A longer description of the message
  payload:
    type: object
    properties:
      user:
        $ref: "#/components/schemas/user"
      signup:
        $ref: "#/components/schemas/signup"

Message Object

Describes a message received on a given topic and operation.

Fixed Fields
Field Name Type Description
headers Schema Object Definition of the message headers. It MAY or MAY NOT define the protocol headers.
payload Schema Object Definition of the message payload.
summary string A short summary of what the message is about.
description string A verbose explanation of the message. CommonMark syntax can be used for rich text representation.
tags [Tag Object] A list of tags for API documentation control. Tags can be used for logical grouping of messages.
externalDocs External Documentation Object Additional external documentation for this message.

This object can be extended with Specification Extensions.

Message Object Example
{
  "summary": "Action to sign a user up.",
  "description": "A longer description",
  "tags": [
    { "name": "user" },
    { "name": "signup" },
    { "name": "register" }
  ],
  "headers": {
    "type": "object",
    "properties": {
      "qos": {
        "$ref": "#/components/schemas/MQTTQoSHeader"
      },
      "retainFlag": {
        "$ref": "#/components/schemas/MQTTRetainHeader"
      }
    }
  },
  "payload": {
    "type": "object",
    "properties": {
      "user": {
        "$ref": "#/components/schemas/userCreate"
      },
      "signup": {
        "$ref": "#/components/schemas/signup"
      }
    }
  }
}
summary: Action to sign a user up.
description: A longer description
tags:
  - name: user
  - name: signup
  - name: register
headers:
  type: object
  properties:
    qos:
      $ref: "#/components/schemas/MQTTQoSHeader"
    retainFlag:
      $ref: "#/components/schemas/MQTTRetainHeader"
payload:
  type: object
  properties:
    user:
      $ref: "#/components/schemas/userCreate"
    signup:
      $ref: "#/components/schemas/signup"

Tag Object

Allows adding meta data to a single tag.

Fixed Fields
Field Name Type Description
name string Required. The name of the tag.
description string A short description for the tag. CommonMark syntax can be used for rich text representation.
externalDocs External Documentation Object Additional external documentation for this tag.

This object can be extended with Specification Extensions.

Tag Object Example
{
	"name": "user",
	"description": "User-related messages"
}
name: user
description: User-related messages

External Documentation Object

Allows referencing an external resource for extended documentation.

Fixed Fields
Field Name Type Description
description string A short description of the target documentation. CommonMark syntax can be used for rich text representation.
url string Required. The URL for the target documentation. Value MUST be in the format of a URL.

This object can be extended with Specification Extensions.

External Documentation Object Example
{
  "description": "Find more info here",
  "url": "https://example.com"
}
description: Find more info here
url: https://example.com

Reference Object

A simple object to allow referencing other components in the specification, internally and externally.

The Reference Object is defined by JSON Reference and follows the same structure, behavior and rules.

For this specification, reference resolution is done as defined by the JSON Reference specification and not by the JSON Schema specification.

Fixed Fields
Field Name Type Description
$ref string Required. The reference string.

This object cannot be extended with additional properties and any properties added SHALL be ignored.

Reference Object Example
{
  "$ref": "#/components/schemas/Pet"
}
  $ref: '#/components/schemas/Pet'

Components Object

Holds a set of reusable objects for different aspects of the AsyncAPI specification. All objects defined within the components object will have no effect on the API unless they are explicitly referenced from properties outside the components object.

Fixed Fields
Field Name Type Description
schemas Map[string, Schema Object | Reference Object] An object to hold reusable Schema Objects.
messages Map[string, Message Object | Reference Object] An object to hold reusable Message Objects.
securitySchemes Map[string, Security Scheme Object | Reference Object] An object to hold reusable Security Scheme Objects.

This object can be extended with Specification Extensions.

All the fixed fields declared above are objects that MUST use keys that match the regular expression: ^[a-zA-Z0-9\.\-_]+$.

Field Name Examples:

User
User_1
User_Name
user-name
my.org.User
Components Object Example
"components": {
  "schemas": {
    "Category": {
      "type": "object",
      "properties": {
        "id": {
          "type": "integer",
          "format": "int64"
        },
        "name": {
          "type": "string"
        }
      }
    },
    "Tag": {
      "type": "object",
      "properties": {
        "id": {
          "type": "integer",
          "format": "int64"
        },
        "name": {
          "type": "string"
        }
      }
    }
  },
  "messages": {
    "userSignUp": {
      "summary": "Action to sign a user up.",
      "description": "Multiline description of what this action does.\nHere you have another line.\n",
      "tags": [
        {
          "name": "user"
        },
        {
          "name": "signup"
        }
      ],
      "headers": {
        "type": "object",
        "properties": {
          "qos": {
            "$ref": "#/components/schemas/MQTTQoSHeader"
          },
          "retainFlag": {
            "$ref": "#/components/schemas/MQTTRetainHeader"
          }
        }
      },
      "payload": {
        "type": "object",
        "properties": {
          "user": {
            "$ref": "#/components/schemas/userCreate"
          },
          "signup": {
            "$ref": "#/components/schemas/signup"
          }
        }
      }
    }
  }
}
components:
  schemas:
    Category:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
    Tag:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
  messages:
    userSignUp:
      summary: Action to sign a user up.
      description: |
        Multiline description of what this action does.
        Here you have another line.
      tags:
        - name: user
        - name: signup
      headers:
        type: object
        properties:
          qos:
            $ref: "#/components/schemas/MQTTQoSHeader"
          retainFlag:
            $ref: "#/components/schemas/MQTTRetainHeader"
      payload:
        type: object
        properties:
          user:
            $ref: "#/components/schemas/userCreate"
          signup:
            $ref: "#/components/schemas/signup"

Schema Object

The Schema Object allows the definition of input and output data types. These types can be objects, but also primitives and arrays. This object is an extended subset of the JSON Schema Specification Wright Draft 00.

Further information about the properties can be found in JSON Schema Core and JSON Schema Validation. Unless stated otherwise, the property definitions follow the JSON Schema specification as referenced here.

Properties

The following properties are taken directly from the JSON Schema definition and follow the same specifications:

  • title
  • multipleOf
  • maximum
  • exclusiveMaximum
  • minimum
  • exclusiveMinimum
  • maxLength
  • minLength
  • pattern (This string SHOULD be a valid regular expression, according to the ECMA 262 regular expression dialect)
  • maxItems
  • minItems
  • uniqueItems
  • maxProperties
  • minProperties
  • required
  • enum

The following properties are taken from the JSON Schema definition but their definitions were adjusted to the AsyncAPI Specification.

  • type - Value MUST be a string. Multiple types via an array are not supported.
  • allOf - Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema.
  • oneOf - Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema.
  • anyOf - Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema.
  • not - Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema.
  • items - Value MUST be an object and not an array. Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema. items MUST be present if the type is array.
  • properties - Property definitions MUST be a Schema Object and not a standard JSON Schema (inline or referenced).
  • additionalProperties - Value can be boolean or object. Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema.
  • description - CommonMark syntax can be used for rich text representation.
  • format - See Data Type Formats for further details. While relying on JSON Schema's defined formats, the AsyncAPI Specification offers a few additional predefined formats.
  • default - The default value represents what would be assumed by the consumer of the input as the value of the schema if one is not provided. Unlike JSON Schema, the value MUST conform to the defined type for the Schema Object defined at the same level. For example, of type is string, then default can be "foo" but cannot be 1.

Alternatively, any time a Schema Object can be used, a Reference Object can be used in its place. This allows referencing definitions in place of defining them inline.

Additional properties defined by the JSON Schema specification that are not mentioned here are strictly unsupported.

Other than the JSON Schema subset fields, the following fields MAY be used for further schema documentation:

Fixed Fields
Field Name Type Description
nullable boolean Allows sending a null value for the defined schema. Default value is false.
discriminator string Adds support for polymorphism. The discriminator is the schema property name that is used to differentiate between other schema that inherit this schema. The property name used MUST be defined at this schema and it MUST be in the required property list. When used, the value MUST be the name of this schema or any schema that inherits it. See Composition and Inheritance for more details.
readOnly boolean Relevant only for Schema "properties" definitions. Declares the property as "read only". This means that it MAY be sent as part of a response but SHOULD NOT be sent as part of the request. If property is marked as readOnly being true and is in the required list, the required will take effect on the response only. A property MUST NOT be marked as both readOnly and writeOnly being true. Default value is false.
writeOnly boolean Relevant only for Schema "properties" definitions. Declares the property as "write only". This means that it MAY be sent as part of a request but SHOULD NOT be sent as part of the response. If property is marked as writeOnly being true and is in the required list, the required will take effect on the request only. A property MUST NOT be marked as both readOnly and writeOnly being true. Default value is false.
xml XML Object This MAY be used only on properties schemas. It has no effect on root schemas. Adds Additional metadata to describe the XML representation format of this property.
externalDocs External Documentation Object Additional external documentation for this schema.
example Any A free-form property to include an example of an instance for this schema. To represent examples that cannot naturally represented in JSON or YAML, a string value can be used to contain the example with escaping where necessary.
deprecated boolean Specifies that a schema is deprecated and SHOULD be transitioned out of usage. Default value is false.

This object can be extended with Specification Extensions.

Composition and Inheritance (Polymorphism)

The AsyncAPI Specification allows combining and extending model definitions using the allOf property of JSON Schema, in effect offering model composition. allOf takes in an array of object definitions that are validated independently but together compose a single object.

While composition offers model extensibility, it does not imply a hierarchy between the models. To support polymorphism, AsyncAPI Specification adds the support of the discriminator field. When used, the discriminator will be the name of the property used to decide which schema definition is used to validate the structure of the model. As such, the discriminator field MUST be a required field. There are are two ways to define the value of a discriminator for an inheriting instance.

  • Use the schema's name.
  • Override the schema's name by overriding the property with a new value. If exists, this takes precedence over the schema's name. As such, inline schema definitions, which do not have a given id, cannot be used in polymorphism.
XML Modeling

The xml property allows extra definitions when translating the JSON definition to XML. The XML Object contains additional information about the available options.

Schema Object Examples
Primitive Sample
{
  "type": "string",
  "format": "email"
}
type: string
format: email
Simple Model
{
  "type": "object",
  "required": [
    "name"
  ],
  "properties": {
    "name": {
      "type": "string"
    },
    "address": {
      "$ref": "#/components/schemas/Address"
    },
    "age": {
      "type": "integer",
      "format": "int32",
      "minimum": 0
    }
  }
}
type: object
required:
- name
properties:
  name:
    type: string
  address:
    $ref: '#/components/schemas/Address'
  age:
    type: integer
    format: int32
    minimum: 0
Model with Map/Dictionary Properties

For a simple string to string mapping:

{
  "type": "object",
  "additionalProperties": {
    "type": "string"
  }
}
type: object
additionalProperties:
  type: string

For a string to model mapping:

{
  "type": "object",
  "additionalProperties": {
    "$ref": "#/components/schemas/ComplexModel"
  }
}
type: object
additionalProperties:
  $ref: '#/components/schemas/ComplexModel'
Model with Example
{
  "type": "object",
  "properties": {
    "id": {
      "type": "integer",
      "format": "int64"
    },
    "name": {
      "type": "string"
    }
  },
  "required": [
    "name"
  ],
  "example": {
    "name": "Puma",
    "id": 1
  }
}
type: object
properties:
  id:
    type: integer
    format: int64
  name:
    type: string
required:
- name
example:
  name: Puma
  id: 1
Models with Composition
{
  "schemas": {
    "ErrorModel": {
      "type": "object",
      "required": [
        "message",
        "code"
      ],
      "properties": {
        "message": {
          "type": "string"
        },
        "code": {
          "type": "integer",
          "minimum": 100,
          "maximum": 600
        }
      }
    },
    "ExtendedErrorModel": {
      "allOf": [
        {
          "$ref": "#/components/schemas/ErrorModel"
        },
        {
          "type": "object",
          "required": [
            "rootCause"
          ],
          "properties": {
            "rootCause": {
              "type": "string"
            }
          }
        }
      ]
    }
  }
}
schemas:
  ErrorModel:
    type: object
    required:
    - message
    - code
    properties:
      message:
        type: string
      code:
        type: integer
        minimum: 100
        maximum: 600
  ExtendedErrorModel:
    allOf:
    - $ref: '#/components/schemas/ErrorModel'
    - type: object
      required:
      - rootCause
      properties:
        rootCause:
          type: string
Models with Polymorphism Support
{
  "schemas": {
    "Pet": {
      "type": "object",
      "discriminator": "petType",
      "properties": {
        "name": {
          "type": "string"
        },
        "petType": {
          "type": "string"
        }
      },
      "required": [
        "name",
        "petType"
      ]
    },
    "Cat": {
      "description": "A representation of a cat. Note that `Cat` will be used as the discriminator value.",
      "allOf": [
        {
          "$ref": "#/components/schemas/Pet"
        },
        {
          "type": "object",
          "properties": {
            "huntingSkill": {
              "type": "string",
              "description": "The measured skill for hunting",
              "enum": [
                "clueless",
                "lazy",
                "adventurous",
                "aggressive"
              ]
            }
          },
          "required": [
            "huntingSkill"
          ]
        }
      ]
    },
    "Dog": {
      "description": "A representation of a dog. Note that `Dog` will be used as the discriminator value.",
      "allOf": [
        {
          "$ref": "#/components/schemas/Pet"
        },
        {
          "type": "object",
          "properties": {
            "packSize": {
              "type": "integer",
              "format": "int32",
              "description": "the size of the pack the dog is from",
              "minimum": 0
            }
          },
          "required": [
            "packSize"
          ]
        }
      ]
    }
  }
}
schemas:
  Pet:
    type: object
    discriminator: petType
    properties:
      name:
        type: string
      petType:
        type: string
    required:
    - name
    - petType
  Cat:  ## "Cat" will be used as the discriminator value
    description: A representation of a cat
    allOf:
    - $ref: '#/components/schemas/Pet'
    - type: object
      properties:
        huntingSkill:
          type: string
          description: The measured skill for hunting
          enum:
          - clueless
          - lazy
          - adventurous
          - aggressive
      required:
      - huntingSkill
  Dog:  ## "Dog" will be used as the discriminator value
    description: A representation of a dog
    allOf:
    - $ref: '#/components/schemas/Pet'
    - type: object
      properties:
        packSize:
          type: integer
          format: int32
          description: the size of the pack the dog is from
          minimum: 0
      required:
      - packSize

XML Object

A metadata object that allows for more fine-tuned XML model definitions.

When using arrays, XML element names are not inferred (for singular/plural forms) and the name property SHOULD be used to add that information. See examples for expected behavior.

Fixed Fields
Field Name Type Description
name string Replaces the name of the element/attribute used for the described schema property. When defined within items, it will affect the name of the individual XML elements within the list. When defined alongside type being array (outside the items), it will affect the wrapping element and only if wrapped is true. If wrapped is false, it will be ignored.
namespace string The URL of the namespace definition. Value SHOULD be in the form of a URL.
prefix string The prefix to be used for the name.
attribute boolean Declares whether the property definition translates to an attribute instead of an element. Default value is false.
wrapped boolean MAY be used only for an array definition. Signifies whether the array is wrapped (for example, <books><book/><book/></books>) or unwrapped (<book/><book/>). Default value is false. The definition takes effect only when defined alongside type being array (outside the items).

This object can be extended with Specification Extensions.

XML Object Examples

The examples of the XML object definitions are included inside a property definition of a Schema Object with a sample of the XML representation of it.

No XML Element

Basic string property:

{
    "animals": {
        "type": "string"
    }
}
animals:
  type: string
<animals>...</animals>

Basic string array property (wrapped is false by default):

{
    "animals": {
        "type": "array",
        "items": {
            "type": "string"
        }
    }
}
animals:
  type: array
  items:
    type: string
<animals>...</animals>
<animals>...</animals>
<animals>...</animals>
XML Name Replacement
{
  "animals": {
    "type": "string",
    "xml": {
      "name": "animal"
    }
  }
}
animals:
  type: string
  xml:
    name: animal
<animal>...</animal>
XML Attribute, Prefix and Namespace

In this example, a full model definition is shown.

{
  "Person": {
    "type": "object",
    "properties": {
      "id": {
        "type": "integer",
        "format": "int32",
        "xml": {
          "attribute": true
        }
      },
      "name": {
        "type": "string",
        "xml": {
          "namespace": "http://example.com/schema/sample",
          "prefix": "sample"
        }
      }
    }
  }
}
Person:
  type: object
  properties:
    id:
      type: integer
      format: int32
      xml:
        attribute: true
    name:
      type: string
      xml:
        namespace: http://example.com/schema/sample
        prefix: sample
<Person id="123">
    <sample:name xmlns:sample="http://example.com/schema/sample">example</sample:name>
</Person>
XML Arrays

Changing the element names:

{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    }
  }
}
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
<animal>value</animal>
<animal>value</animal>

The external name property has no effect on the XML:

{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    },
    "xml": {
      "name": "aliens"
    }
  }
}
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
  xml:
    name: aliens
<animal>value</animal>
<animal>value</animal>

Even when the array is wrapped, if no name is explicitly defined, the same name will be used both internally and externally:

{
  "animals": {
    "type": "array",
    "items": {
      "type": "string"
    },
    "xml": {
      "wrapped": true
    }
  }
}
animals:
  type: array
  items:
    type: string
  xml:
    wrapped: true
<animals>
  <animals>value</animals>
  <animals>value</animals>
</animals>

To overcome the above example, the following definition can be used:

{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    },
    "xml": {
      "wrapped": true
    }
  }
}
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
  xml:
    wrapped: true
<animals>
  <animal>value</animal>
  <animal>value</animal>
</animals>

Affecting both internal and external names:

{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    },
    "xml": {
      "name": "aliens",
      "wrapped": true
    }
  }
}
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
  xml:
    name: aliens
    wrapped: true
<aliens>
  <animal>value</animal>
  <animal>value</animal>
</aliens>

If we change the external element but not the internal ones:

{
  "animals": {
    "type": "array",
    "items": {
      "type": "string"
    },
    "xml": {
      "name": "aliens",
      "wrapped": true
    }
  }
}
animals:
  type: array
  items:
    type: string
  xml:
    name: aliens
    wrapped: true
<aliens>
  <aliens>value</aliens>
  <aliens>value</aliens>
</aliens>

Security Scheme Object

Defines a security scheme that can be used by the operations. Supported schemes are User/Password, API key (either as user or as password), X.509 certificate, end-to-end encryption (either symmetric or asymmetric), HTTP authentication and HTTP API key.

Fixed Fields
Field Name Type Applies To Description
type string Any REQUIRED. The type of the security scheme. Valid values are "userPassword", "apiKey", "X509", "symmetricEncryption", "asymmetricEncryption", "httpApiKey", "http".
description string Any A short description for security scheme. CommonMark syntax MAY be used for rich text representation.
name string httpApiKey REQUIRED. The name of the header, query or cookie parameter to be used.
in string `apiKey httpApiKey`
scheme string http REQUIRED. The name of the HTTP Authorization scheme to be used in the Authorization header as defined in RFC7235.
bearerFormat string http ("bearer") A hint to the client to identify how the bearer token is formatted. Bearer tokens are usually generated by an authorization server, so this information is primarily for documentation purposes.

This object MAY be extended with Specification Extensions.

Security Scheme Object Example
User/Password Authentication Sample
{
  "type": "userPassword"
}
type: userPassword
API Key Authentication Sample
{
  "type": "apiKey",
  "in": "user"
}
type: apiKey,
in: user
X.509 Authentication Sample
{
  "type": "X509"
}
type: X509
End-to-end Encryption Authentication Sample
{
  "type": "symmetricEncryption"
}
type: symmetricEncryption
Basic Authentication Sample
{
  "type": "http",
  "scheme": "basic"
}
type: http
scheme: basic
API Key Sample
{
  "type": "httpApiKey",
  "name": "api_key",
  "in": "header"
}
type: httpApiKey
name: api_key
in: header
JWT Bearer Sample
{
  "type": "http",
  "scheme": "bearer",
  "bearerFormat": "JWT",
}
type: http
scheme: bearer
bearerFormat: JWT

Security Requirement Object

Lists the required security schemes to execute this operation. The name used for each property MUST correspond to a security scheme declared in the Security Schemes under the Components Object.

When a list of Security Requirement Objects is defined on the AsyncAPI object, only one of Security Requirement Objects in the list needs to be satisfied to authorize the connection or operation.

Patterned Fields
Field Pattern Type Description
{name} [string] Each name MUST correspond to a security scheme which is declared in the Security Schemes under the Components Object. The value MUST be an empty array.
Security Requirement Object Examples
User/Password Security Requirement
{
  "user_pass": []
}
user_pass: []
API Key Security Requirement
{
  "api_key": []
}
api_key: []

Specification Extensions

While the AsyncAPI Specification tries to accommodate most use cases, additional data can be added to extend the specification at certain points.

The extensions properties are implemented as patterned fields that are always prefixed by "x-".

Field Pattern Type Description
^x- Any Allows extensions to the AsyncAPI Schema. The field name MUST begin with x-, for example, x-internal-id. The value can be null, a primitive, an array or an object. Can have any valid JSON format value.

The extensions may or may not be supported by the available tooling, but those may be extended as well to add requested support (if tools are internal or open-sourced).

Data Type Formats

Primitives have an optional modifier property: format. The AsyncAPI specification uses several known formats to more finely define the data type being used. However, the format property is an open string-valued property, and can have any value to support documentation needs. Formats such as "email", "uuid", etc., can be used even though they are not defined by this specification. Types that are not accompanied by a format property follow their definition from the JSON Schema. Tools that do not recognize a specific format MAY default back to the type alone, as if the format was not specified.

The formats defined by the AsyncAPI Specification are:

Common Name type format Comments
integer integer int32 signed 32 bits
long integer int64 signed 64 bits
float number float
double number double
string string
byte string byte base64 encoded characters
binary string binary any sequence of octets
boolean boolean
date string date As defined by full-date - RFC3339
dateTime string date-time As defined by date-time - RFC3339
password string password Used to hint UIs the input needs to be obscured.