Creating Swagger (Reference documentation)

AutoRest is a tool for generating HTTP client libraries from Swagger files. This document explains the conventions and extensions used by AutoRest in processing Swagger to produce client libraries. The Swagger specification can be found at Swagger RESTful API Documentation Specification.

This page is intended to serve as reference documentation. If you have any questions about a specific part of a Swagger file or how AutoRest uses it to generate code, this is the correct place.

Swagger
Info Object
Host
Schemes
Consumes / Produces
Paths
Path Item
Operation and OperationId
Parameters 1. Path Parameters 2. Query Parameters 3. Body Parameters 4. Header Parameters 5. FormData Parameters
Responses 1. Default Response 2. Negative Responses
Defining Model Types
Model Inheritance
Polymorphic Response Models
ResourceFlattening
Conditions
x-ms-azure-resource
Enums with x-ms-enum
Structure
Understanding modelAsString
Paging with x-ms-pageable
Structure
Pageable Operation
Pageable Model
URL Encoding
Long Running operation
Global parameters

Swagger

In this document, references to the 'spec' or to the 'swagger' refer to an instance of a Swagger file. AutoRest supports Swagger version 2.0. The version of Swagger being used must be included in the spec.

{
  "swagger": "2.0"
  ...
}

Info Object

Each spec includes an "info object." The title field is used as the name of the generated client.

"info": {
  "title": "MyClientName",
}

var client = new MyClientName();

Override the title client name by passing the -ClientName to AutoRest.

autorest.exe -ClientName MyClientName

The version field specifies the service version of the API described in the spec. This is used as the default value for Azure clients where api-version is passed in the querystring.

"info": {
  "version": "2014-04-01-preview"

https://management.azure.com/...?api-version="2014-04-01-preview"

Host

The host field specifies the baseURI. (everything after the protocol and before the path).

{
  "swagger": "2.0",
  "host": "management.azure.com"
}

Schemes

The schemes field is an array of transfer protocols that can be used by individual operations. AutoRest supports http and https. The generated client uses the first scheme from the array.

{
  "swagger": "2.0",
  "schemes": [
    "https",
    "http"
  ]
  . . .
}

Consumes / Produces

The consumes and produces fields declare the mime-types supported by the API. The root-level definition can be overridden by individual operations. AutoRest supports JSON payloads.

{
  "swagger": "2.0",
  "consumes": [
    "application/json"
  ],
  "produces": [
    "application/json"
  ]
  . . .
}

Paths

The paths object defines the relative paths of API endpoints (appended to the baseURI to invoke operations).

"swagger": "2.0",
"paths": {
  "/tenants": {
  ...
  },
  "/subscriptions": {
  ...
  }
}

https://management.azure.com/tenants?api-version=2014-04-01-preview
OR
https://management.azure.com/subscriptions?api-version=2014-04-01-preview

Path Item

Each path item in the spec defines the operations available at that endpoint. The individual operations are identified by the HTTP operation (sometimes referred to as HTTP verbs). AutoRest supports: head, get, put, post, delete and patch.

{
  "swagger": "2.0",
  "paths": {
    "/stations": {
      "get": {
      ...
      },
      "put": {
      ...

Operation and OperationId

Every operation must have a unique operationId. The operationId is used to determine the generated method name.

"paths": {
  "/users": {
    "get": {
      "operationId": "ListUsers"
    }
  }

var client = new MyClientName();
var listResult = client.ListUsers();
IList<User> users = listResult.Value;

All of the operations specified become methods on the client. The client API model can easily become a long and cumbersome list of methods. AutoRest allows grouping of operations by using a naming convention. Operation groups are identified by splitting the operationId by underscore.

"paths": {
  "/{tenantId}/users": {
    "get": {
      "operationId": "Users_List",
      ...

instead of

var listResult = client.ListUsers();

the method becomes part of an operation group interface

var listResult = client.Users.List();

The operation group interface and the interface implementation is the "core" of the operation. Other signatures of this API ultimately call the "core" implementation. This allows the operations to be easily mocked for testing without needing to mock all signature variations. Other API signatures for the same operation are generated as extension methods. For example, consider this GetUserById operation on the SampleClient.

"/users/{userId}": {
    "get": {
      "operationId": "Users_GetById"
...

The generated method signature is in the IUsers interface

public interface IUsers
{
    Task<HttpOperationResponse<User>> GetByIdWithOperationResponseAsync(string userId, CancellationToken cancellationToken = default(CancellationToken));
}

The core signature returns a generic HttpOperationResponse which includes the HttpRequest and HttpResponse objects as well as the payload object. In addition to the "core" signature, there is a synchronous version and an async version that returns the payload object without the request and response objects.

public static partial class UsersExtensions
{
    public static User GetById(this IUser operations, string userId) {...}

    public static async Task<User> GetByIdAsync( this IUser operations, string userId, CancellationToken cancellationToken = default(CancellationToken)) {...}
}

Parameters

Each operation defines the parameters that must be supplied. An operation may not require any parameters. A parameter defines a name, description, schema, what request element it is in (AutoRest supports parameters in the path, query, header, and body of the request) and whether or not is it required.

Path Parameters

The value of path parameters are replaced in the templated URL path at the position specified by the name. Parameters that are in the path must have a true value for the required field. In this example, the {userId} in this operation is populated with the value of userId provided in the client method.

"paths": {
  "/users/{userId}": {
    "get": {
      "operationId": "users_getById",
      "parameters": [
        {
          "name": "userId",
          "in": "path",
          "required": true,
          "type": "string",
          "description": "Id of the user."
        }
      ]
    }
  }
}

In the generated code, the path parameter is an argument of the method.

string userId = "abcxyz";
var user = client.Users.GetById(userId);

https://{host}/{basePath}/users/abcxyz

Query Parameters

Query parameters are appended to the request URI. They can be specified as required or optional.

"paths": {
  "/users/{userId}": {
    "get": {
      "operationId": "users_getUserById",
      "parameters": [
        {
          "name": "api-version",
          "in": "query",
          "required": true,
          "type": "string",
          "description": "Version of API to invoke."
        }
      ]
    }
  }
}

The user doesn't need to know where the parameter is placed. Doc comments surface the required/optional distinction.

Body Parameters

Body parameters include schema for the payload. In this example, the schema element is a $ref to the type details in the #/definitions section of the spec. More on the #/definitions later.

"paths": {
  "/subscriptions/{subscriptionId}/providers/Microsoft.Storage/checkNameAvailability": {
    "post": {
      "operationId": "StorageAccounts_CheckNameAvailability",
      "parameters": [
        {
          "name": "accountName",
          "in": "body",
          "required": true,
          "schema": {
            "$ref": "#/definitions/StorageAccountCheckNameAvailabilityParameters"
          }
        }
      ],
      ...

Header Parameters

Header parameters are sent as part of the HTTP request header. In general, reserved headers (Content-Length, Content-Type, ...) should not be documented since their values are derivable (e.g. from the request body) and not really part of the protocol specified by the OpenAPI definition. Rather, they are part of the REST standard that the protocol is supposed to adhere to anyway.

However, there are rare scenarios for making the Content-Type customizable as part of a request, e.g. in case of a binary/stream request body. The media type of binary request bodies is not reliably derivable: Maybe the service endpoint accepts PNG, JPEG or BMP images, which is expressed in OpenAPI using

consumes:
  - image/png
  - image/jpeg
  - image/bmp

and give the request body type file. Now, when a request is made, the protocol has to somehow communicate to the server which of the media types the body has. Since there is a range of possibilities (and it's certainly not a protocol's job to parse and classify binary data), we suggest adding a Content-Type header parameter to the operation's definition. Unless one provides an enum restriction for that parameter, AutoRest will automatically make the parameter an enum with values drawn from the consumes declaration. This allows for deduplication and hence prevents potential bugs. More information on how AutoRest treats a Content-Type header parameter can be found here.

FormData Parameters

Note: FormData parameters are not currently supported by AutoRest.

Responses

Each operation defines the possible responses by HTTP status code:

"/users/{userId}": {
  "get": {
    "operationId": "users_getUserById",
    "responses": {
      "200": {
        "schema": {
          "$ref": "#/definitions/user"
        }
      }
    }
  }
}

Default Response

Swagger allows for specifying a default response. AutoRest treats the default response as defining an error response status code unless default is the only status code defined. The reason for imposing this convention is to produce higher quality API signatures. The return type of the generated API is determined by finding a common base type of the success responses. In practice, if the default is considered as a potential success definition, the common ancestor of success responses and error responses ends up being Object.

Negative Responses

You can describe all the possible HTTP Response status codes in the responses section of an operation. AutoRest generated code will deserialize the response for all the described response status codes as per their definition in the swagger. If a response status code is not defined then generated code will align to the default response behavior as described above.

If a schema is provided for the negative response codes then this will have an impact on the return type of the generated method.
- For example: if a schema was provided for 200, and 400 was also described with a schema then,
  - the return type would be the Common Ancestor of both the schemas. In most cases there is nothing common between a positive and a negative response code. Hence the return type will be an Object. Note:This may not be very helpful to the customer
  - an exception ** will NOT be thrown for 400** and the generated method will deserialize the response body as per the schema of "400".
  - any other negative response code will be treated as per the "default" response status code defined in the swagger for that operation.
If a schema is NOT provided for the negative response codes then this will NOT have an impact on the return type of the generated method.
- For example: if a schema was provided for 200 and 404 was described as one of the responses. However, 404 does not have a schema. In this scenario,
  - the return type of the generated method will be based upon the schema defined in "200".
  - an exception will NOT be thrown for 404 response status code
  - any other negative response code will be treated as per the "default" response status code defined in the swagger for that operation.

"/users/{userId}": {
  "get": {
    "operationId": "users_getUserById",
    "responses": {
      "200": {
       "description": "Provides User Information.",
        "schema": {
          "$ref": "#/definitions/user"
        }
      },
      "400": {
        "description": "Bad Request. ResponseBody will be deserialized as per the Error definition 
                        mentioned in schema. Exception will not be thrown.",
        "schema": {
          "$ref": "#/definitions/Error"
        }
      },
      "404": {
        "description": "Resource Not Found, ResponseBody will be null as there is no schema definition.
                        Exception will not be thrown.",
      },
      "default": {
        "description": "Default Response. It will be deserialized as per the Error definition 
                        specified in the schema. Exception will be thrown.",
        "schema": {
          "$ref": "#/definitions/Error"
        }
      }
    }
  }
}

Custom Paths

Swagger 2.0 has a built-in limitation on paths. Only one operation can be mapped to a path and http method. There are some APIs, however, where multiple distinct operations are mapped to the same path and same http method. For example GET /mypath/query-drive?op=file and GET /mypath/query-drive?op=folder may return two different model types (stream in the first example and JSON model representing Folder in the second). Since Swagger does not treat query parameters as part of the path the above 2 operations may not co-exist in the standard "paths" element.

To overcome this limitation an "x-ms-paths" extension was introduced parallel to "paths". Urls under "x-ms-paths" are allowed to have query parameters for disambiguation, however they are removed during model parsing.

"paths":{
   "/pets": {
        "get": {
            "parameters": [
                {
                     "name": "name",
                     "required": true
                }
            ]
        }
   }
},
"x-ms-paths":{   
   "/pets?color={color}": {
        "get": {}
   },
}

Please note, that the use of "x-ms-paths" should be minimized to the above scenarios. Since any metadata inside the extension is not part of the default swagger specification, it will not be available to non-AutoRest tools.

Defining Model Types

The request body and response definitions become simple model types in the generated code. The models include basic validation methods, but are generally stateless serialization definitions.

Understanding the importance of "type" keyword while defining model types.

"Type-specific" keywords such as properties, items, minLength, etc. do not enforce a type on the schema. It works the other way around – when an instance is validated against a schema, these keywords only apply when the instance is of the corresponding type, otherwise they are ignored. Here's the relevant part of the JSON Schema Validation spec:

4.1. Keywords and instance primitive types Some validation keywords only apply to one or more primitive types. When the primitive type of the instance cannot be validated by a given keyword, validation for this keyword and instance SHOULD succeed.

For example, consider this schema:

definitions:
  Something:
    properties:
      id:
        type: integer
    required: [id]
    minLength: 8

It's a valid schema, even though it combines object-specific keywords properties and required and string-specific keyword minLength. This schema means:

If the instance is an object, it must have an integer property named id. For example, {"id": 4} and {"id": -1, "foo": "bar"} are valid, but {} and {"id": "ACB123"} are not.
If the instance is a string, it must contain at least 8 characters. "Hello, world!" is valid, but "" and abc are not.
Any instances of other types are valid - true, false, -1.234, [], [1, 2, 3], [1, "foo", true], etc. (Except null - OpenAPI 2.0 does not have the null type and does not support null except in extension properties. OpenAPI 3.0 supports the null value for schemas with nullable: true.)

If there are tools that infer the type from other keywords (for example, handle schemas with no type but with properties as always objects), then these tools are not exactly following the OpenAPI Specification and JSON Schema.

Bottom line: If a schema must always be an object, add "type": "object" explicitly. Otherwise you might get unexpected results.

Credits - Stack Overflow link.

Model Inheritance

Swagger schema allows for specifying that one type is allOf other types, meaning that the entire specification of the referenced schema applies is included in the new schema. By convention, if a schema has an 'allOf' that references only one other schema, AutoRest treats this reference as an indication that the allOf schema represents a base type being extended. In this example, the generated code would include a Dog model that inherits from the Pet model.

{
  "definitions": {
    "pet": {
      "description": "Defines the Pet model.",
      "properties": {
        "name": {
          "type": "string"
        },
      },
      "required": [
        "name"
      ]
    },
    "dog": {
      "description": "Defines the Dog model.",
      "required": [
        "breed"
      ],
      "properties": {
        "breed": {
          "type": "string"
        }
      },
      "allOf": [
        {
          "$ref": "#/definitions/pet"
        }
      ]
    }
  }
}

Polymorphic Response Models

Besides using allOf to define inheritance, model definitions can indicate that the payload will include a discriminator to disambiguate polymorphic payloads. The discriminator field allows the deserializer to resolve into an instance of a more specific type. Suppose the Dog and Cat type are allOf Pet and an operation will return a Dog or a Cat. If the Pet definition includes a discriminator then payloads can be Dog or Cat. A response can be defined as a Pet model and the API signature will indicate Pet. At runtime, the returned object is an instance of the more specific type.

{
  "definitions": {
    "Pet": {
      "type": "object",
      "discriminator": "petType",
      "properties": {
        "name": {
          "type": "string"
        },
        "petType": {
          "type": "string"
        }
      },
      "required": [
        "name",
        "petType"
      ]
    },
    "Cat": {
      "description": "A representation of a cat",
      "allOf": [
        {
          "$ref": "#/definitions/Pet"
        },
        {
          "type": "object",
          "properties": {
            "huntingSkill": {
              "type": "string",
              "description": "The measured skill for hunting",
              "default": "lazy",
              "enum": [
                "clueless",
                "lazy",
                "adventurous",
                "aggressive"
              ]
            }
          },
          "required": [
            "huntingSkill"
          ]
        }
      ]
    },
    "Dog": {
      "description": "A representation of a dog",
      "allOf": [
        {
          "$ref": "#/definitions/Pet"
        },
        {
          "type": "object",
          "properties": {
            "packSize": {
              "type": "integer",
              "format": "int32",
              "description": "the size of the pack the dog is from",
              "default": 0,
              "minimum": 0
            }
          },
          "required": [
            "packSize"
          ]
        }
      ]
    }
  }
}

Defining Azure Resource Types with x-ms-azure-resource (Resource Flattening)

Azure Resource Manager brings a common pattern that is leveraged to provide a more consistent programming model for users. Resource types all have a common set of Resource properties: id, name, location, tags... In a resource payload, the common properties are at the top-level and the resource-specific properties are nested within properties. The top-level outer properties are sometimes referred to as the 'ARM envelope' and the inner data as the 'Resource properties.'

{
    "id": "/subscriptions/{id}/resourceGroups/{group}/providers/{rpns}/{type}/{name}",
    "name": "Resource Name",
    "type": "Microsoft.ResourceProvider/type",
    "location": "North US",
    "properties": {
       "foo" : {
           "name" : "{fooA|fooB|fooC}",
           "capacity" : {number}
       },
       "bar": "flim"
    }
}

When the ARM payload shape is reflected directly into the client object model, the nesting of the Resource-specific 'properties' within the 'properties' object becomes cumbersome.

var theResource = theClient.GetResourceById(resourceId);
theResource.Properties.Foo.Name = "fooB";
theResource.Properties.Foo.Capacity = 100;
theResource.Properties.Bar = "flam";

To provide a better end-user experience, types that are identified as ARM resources are "flattened" in the generated C# code. The serialization and deserialization of Resource types hides the "properties" nesting from the user.

var theResource = theClient.GetResourceById(resourceId);
theResource.Location = "North US";
theResource.Foo.Name = "fooB";
theResource.Foo.Capacity = 100;

When will AutoRest flatten resources?

If any model or its parent is marked with an extension "x-ms-azure-resource" : true, then AutoRest will flatten the Resource-specific properties by one level for that model.

x-ms-azure-resource

In using Swagger to describe Azure Resource Manager operations, types are identified as Resources by declaring that a type is "allOf" the common Resource type. That common Resource type includes the x-ms-azure-resource Swagger extension.

    "Resource": {
      "x-ms-azure-resource": true,
      "properties": {
        "id": {
          "type": "string",
          "readOnly": true,
          "description": "Resource Id"
        },
        "type": {
          "type": "string",
          "readOnly": true,
          "description": "Resource Type"
        },
        "tags": {
          "type": "object",
          "additionalProperties": {
            "type": "string"
            },
          "description": "Resource Tags"
        },
        "location": {
          "type": "string",
          "description": "Resource Location"
        },
        "name": {
          "type": "string",
          "readOnly": true,
          "description": "Resource Name"
        }
      }
    }

Notice that the type definitions in Swagger schema use the word 'properties' to identify the 'properties' of the type. When looking at an ARM Resource it also has 'properties', the 'properties' term is overloaded and it looks a bit odd.

In practice, the Resource properties may be re-used in the Swagger spec and can be defined separately. If the schema of the resource properties is included inline, AutoRest still needs to generate a type for the properties and does so by appending Properties to the Resource name

"definitions": {
  "SomeResourceProperties": {
    "properties": {
      "bar": {
        "type": "string"
      }
    }
  },
  "SomeResource": {
    "properties": {
      "properties": {
        "$ref": "#/definitions/SomeResourceProperties"
      }
    },
    "allOf": [
      {
        "$ref": "Resource"
      }
    ]
  }
}

Enums with x-ms-enum

Enum definitions in Swagger indicate that only a particular set of values may be used for a property or parameter. When the property is represented on the wire as a string, it would be a natural choice to represent the property type in C# as an enum. However, not all enumeration values should necessarily be represented as C# enums - there are additional considerations, such as how often expected values might change, since adding a new value to a C# enum is a breaking change requiring an updated API version. Additionally, there is some metadata that is required to create a useful C# enum, such as a descriptive name, which is not represented in swagger. For this reason, enums are not automatically turned into enum types in C# - instead they are rendered in the documentation comments for the property or parameter to indicate allowed values. To indicate that an enum will rarely change and that C# enum semantics are desired, use the x-ms-enum extension.

In C#, an enum type is generated and is declared as the type of the related request/response object. The enum is serialized as the string expected by the REST API.

  "accountType": {
    "type": "string",
    "enum": [
      "Standard_LRS",
      "Standard_ZRS",
      "Standard_GRS",
      "Standard_RAGRS",
      "Premium_LRS"
    ],
    "x-ms-enum": {
      "name": "AccountType",
      "modelAsString": false
    }
  }

x-ms-enum extension structure

{
  "x-ms-enum": {
    "name" : "Specify the name for the Enum."
    "modelAsString": "true/false."
  }
}

modelAsString

true
- When set to true the enum will be modeled as a string. No validation will happen.
false
- When set to false, it will be modeled as an enum if that language supports enums. Validation will happen, irrespective of support of enums in that language.

Paging with x-ms-pageable

The REST API guidelines define a common pattern for paging through lists of data. The operation response is modeled in Swagger as the list of items and the nextLink. Tag the operation as x-ms-pageable and the generated code will include methods for navigating between pages.

x-ms-pageable extension structure

{
  "x-ms-pageable" : {
    "nextLinkName": "Specify the name of the property that provides the nextLink. 
                     If your model does not have the nextLink property then specify null.",
    "itemName": "Specify the name of the property that provides the collection 
                 of pageable items. It is optional. Default value is 'value'.",
    "operationName": "Specify the name of the Next operation. It is optional. Default value is 'XXXNext' where XXX is the name of the operation." 
  }
}

x-ms-pageable operation definition

"paths": {
  "/products": {
    "get": {
      "x-ms-pageable": {
        "nextLinkName": "nextLink"
      },
      "operationId": "products_list",
      "description": "A pageable list of Products.",
      "responses": {
        "200": {
          "schema": {
            "$ref": "#/definitions/ProductListResult"
          }
        }
      }
    }
  }
}

x-ms-pageable model definition

"ProductListResult": {
  "properties": {
    "value": {
      "type": "array",
      "items": {
        "$ref": "#/definitions/Product"
      }
    },
    "nextLink": {
      "type": "string"
    }
  }
}

Control URL encoding with x-ms-skip-url-encoding

By default, path parameters will be URL-encoded automatically. This is a good default choice for user-provided values. This is not a good choice when the parameter is provided from a source where the value is known to be URL-encoded. The URL encoding is NOT an idempotent operation. For example, the percent character "%" is URL-encoded as "%25". If the parameter is URL-encoded again, "%25" becomes "%2525". Mark parameters where the source is KNOWN to be URL-encoded to prevent the automatic encoding behavior.

"parameters": [
  {
    "name": "databaseName",
    "in": "path",
    "type": "string",
    "required": true,
    "x-ms-skip-url-encoding": true
  }
]

Enable Asynchronous Operations with x-ms-long-running-operation

Some requests like creating/deleting a resource cannot be carried out immediately. In such a situation, the server sends a 201 (Created) or 202 (Accepted) and provides a link to monitor the status of the request. When such an operation is marked with extension "x-ms-long-running-operation": true, in Swagger, the generated code will know how to fetch the link to monitor the status. It will keep on polling at regular intervals till the request reaches one of the terminal statesSucceeded|Failed|Canceled.

"paths": {
  "/products/{name}": {
    "put": {
      "operationId": "products_create",
      "x-ms-long-running-operation": true,
      "description": "A pageable list of Products.",
      "parameters": [
        {
          "name": "name",
          "in": "path",
          "required": true,
          "type": "string",
          "description": "The name of the Product."
        },
        {
          "name": "parameters",
          "in": "body",
          "required": true,
          "schema": {
            "$ref": "#/definitions/ProductCreateParameters"
          },
          "description": "The parameters to provide for the created product."
        }
      ],
      "responses": {
        "200": {
          "schema": {
            "$ref": "#/definitions/Product"
          }
        },
        "202": {
          "description": ""
        }
      }
    }
  }
}

Global parameters

Swagger allows for parameters to be defined separately from the operation where they are used. By convention, AutoRest treats global parameter definitions as Client properties. For example, almost all Azure Resource Manager APIs require subscriptionId and api-version. These are defined as global parameters and become properties of the client.

"parameters": [
  {
    "name": "subscriptionId",
    "type": "string"
  }
]

var client = new MyClient();
client.SubscriptionId = "xyz-123";

By convention, when AutoRest sees that an operation defines a parameter as a reference to a global parameter, the generated method does not expose the parameter. Instead, the parameter is populated with the value from the client property.

"paths": {
  "/subscriptions/{subscriptionId}/providers/MyProvider/SomeOperation": {
    "post": {
      "parameters": [{"$ref": "#/parameters/subscriptionId"}]
    }
  }
}

If an operation requires that a parameter is exposed as a method parameter, it is defined without referencing the global definition.

"paths": {
  "/subscriptions/{subscriptionId}/providers/MyProvider/SomeOperation": {
    "post": {
      "parameters": [
        {
          "name": "subscriptionId",
          "in": "path",
          "required": true,
          "type": "string",
        }
      ]
    }
  }
}

x-ms-odata

TODO: x-ms-odata

TODO: naming standards for operations Create, CreateOrUpdate, Update (respect etag), Get, List, Delete, Patch TODO: patch => no validate Swagger-spec2.0

Files

creating-swagger.md

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