API Gateway provides RESTful APIs.

REST provides APIs to create, query, update, delete, and access service resources.

A REST API request/response pair is divided into the following parts:

Request URI
Request method
Request headers
Request body
Response headers
Response body

Request URI

A request URI consists of the following parts:

{URI-scheme} :// {Endpoint} / {resource-path} ? {query-string}

Although a request URI is a part of a request header, most programming languages or frameworks require the request URI to be separately transmitted, rather than being conveyed in a request message.

**Table 1** URI parameter description
Parameter	Description
URI-scheme	Protocol used to transmit the request.
Endpoint	Domain name or IP address of the server where the RESTful service endpoint is hosted. You can obtain the value from Regions and Endpoints.
resource-path	Path in which the resource requested by the API is located. The path is provided by the URI module of APIs, for example, v3/auth/tokens.
Query string	This is an optional parameter. For example, the value can be the API version or resource selection criteria.

Request Method

HTTP method: the type of requested operation.

**Table 2** HTTP methods supported
Method	Description
GET	Requests a server to provide a specified resource.
PUT	Requests a server to update a specified resource.
POST	Requests a server to add resources or perform special operations.
DELETE	Requests a server to delete a specified resource, for example, an object.
HEAD	Similar to the GET method, the HEAD method requests a server to provide the specified resource, but the server returns only the response header (excluding the response body) to this request.
PATCH	Requests a server to update a part of a specified resource. If the resource does not exist, the PATCH method may create a new resource.

Request Headers

Optional header fields: For example, such fields could be those required by a specified URI and HTTP method. Table 3 describes common HTTP request header fields.

**Table 3** Common request headers
Header	Description	Remarks	Example
Content-Type	Type (or format) of the message body.	Mandatory	application/json
X-Auth-Token	Token authentication information, which can be obtained by following the procedure in Token Authentication.	Mandatory if token authentication is used.	-
X-Sdk-Date	Time at which the request was sent.	Mandatory if AK/SK authentication is used.	20151222T034042Z
Authorization	Signature authentication information, which comes from the request signature result.	Mandatory if AK/SK authentication is used.	-

Request Body (Optional)

A request body conveys information other than the request header and is generally sent in a structured format (for example, JSON or XML) defined by the Content-type field.

Response Headers

A response header consists of an HTTP status code and additional response header fields.

HTTP status code: A status code consists of three digits (2xx to 5xx). 2xx indicates a success response. 4xx and 5xx indicate an error response. The status code returned can also be defined by the service.

Optional header fields: For example, Content-type could be one of such fields. Table 4 describes common response header fields.

**Table 4** Response headers
Header	Description	Example
Date	A standard HTTP header, which indicates the date and time when a message is sent. The format of this header field is defined in RFC 822.	Mon, 12 Nov 2007 15:55:01 GMT
Server	A standard HTTP header, which contains the information about the software that the server uses to process requests.	Apache
Content-Length	A standard HTTP header, which indicates the size of the response body, in decimal number of bytes.	xxx
Content-Type	A standard HTTP header, which specifies the media type of the response body sent to the recipient.	application/json

Response Body (Optional)

A response body conveys information other than the response header and is generally sent in a structured format (for example, JSON or XML) defined by the Content-type field.

Initiating Requests

A request can be initiated by using any of the following methods:

cURL
cURL is a command line tool used to perform URL operations and transmit information. It serves as an HTTP client to send HTTP requests to the server and receive response messages. cURL is suitable for use in API tuning scenarios. For more information about cURL, visit https://curl.haxx.se/.
Code
You can call APIs through code to assemble, send, and process requests.
REST client
Mozilla Firefox and Google Chrome provide a graphical browser plug-in for REST clients to send and process requests. For Mozilla Firefox, see REST Client APIsHub. For Google Chrome, see Postman.