腾讯云API网关跨域预检请求实现示例
腾讯云API网关默认不支持跨域访问CORS功能，如要需要支持跨域访问CORS，请先在API网关管理控制台开启CORS并重新发布API，同时按照预检请求原理的描述创建预检API。
1. 浏览器发送携带Origin的API请求：
OPTIONS / HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.170 Safari/537.36
Host: pinganyun.com
Accept: */*
Origin: http://www.Server-test.com
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: PA-AG-AppId, PA-AG-Timestamp
• Origin：请求的来源。
• Access-Control-Request-Method：浏览器实际的请求方法。
• Access-Control-Request-Headers：实际请求中额外携带的请求头信息。
2. 后端服务器是否允许发送跨域请求的响应信息：
HTTP/1.1 200 OK
Date: Tue, 21 Jan 2019 21:15:13 GMT
Content-Type: application/json
Access-Control-Allow-Origin: http://www.Server-test.com
Access-Control-Allow-Headers: PUT
Access-Control-Allow-Methods: PA-AG-AppId, PA-AG-Timestamp
• Access-Control-Allow-Origin：允许跨域访问的源。取值为*，表示后端服务器接受任意域的请求。
• Access-Control-Allow-Methods：允许浏览器使用的请求方法。
• Access-Control-Allow-Headers：允许浏览器在请求中携带的请求头。
3. API网关是否允许发送跨域请求的响应信息：
HTTP/1.1 200 OK
Date: Tue, 21 Jan 2019 21:15:13 GMT
Content-Type: application/json
Access-Control-Allow-Origin: http://www.Server-test.com
Access-Control-Allow-Headers: PUT
Access-Control-Allow-Methods: PA-AG-AppId, PA-AG-Timestamp
Vary: Origin, Access-Control-Request-Method, Access-Control-Request-Headers
Access-Control-Allow-Origin: http://www.Server-test.com
Access-Control-Max-Age: 1800
• Access-Control-Allow-Origin：允许跨域访问的源。取值为*，表示后端服务器接受任意域的请求。
• Access-Control-Allow-Methods：允许浏览器使用的请求方法。
• Access-Control-Allow-Headers：允许浏览器在请求中携带的请求头。
• Access-Control-Max-Age：预检响应的有效时间，单位：秒。在有效期内，浏览器无需再次发送预检请求。
如果后端服务器返回的结果中没有以上4个字段，或仅包含其中的几个字段，API网关会补齐剩余字段后返回给浏览器。
4. 浏览器发送携带Origin的API请求：
PUT /putUser HTTP/1.1
Host: pinganyun.com
Orgin: http://www.Server-test.com
Content-type: application/x-www-form-urlencoded; charset=utf-8
Accept: application/json; charset=utf-8
Date: Tue, 21 Jan 2019 21:15:13 GMT
5. 后端服务器返回的响应信息：
HTTP/1.1 200 OK
Date: Tue, 21 Jan 2019 21:15:13 GMT
Content-Type: application/json
Content-Length: 25
Access-Control-Allow-Origin: http://www.Server-test.com
{"ret":"0", "msg":"succ"}
6. API网关发送给浏览器的响应信息：
HTTP/1.1 200 OK
Date: Tue, 21 Jan 2019 21:15:13 GMT
Content-Type: application/json
Access-Control-Allow-Origin: http://www.Server-test.com
Access-Control-Allow-Headers: PUT
Access-Control-Allow-Methods: PA-AG-AppId, PA-AG-Timestamp
Vary: Origin, Access-Control-Request-Method, Access-Control-Request-Headers
Access-Control-Allow-Origin: http://www.Server-test.com
Access-Control-Max-Age: 1800
Content-Length: 25
{"ret":"0", "msg":"succ"}
