1. Intro of HTTP messages#

There are two types of messages: requests sent by the client to trigger an action on the server, and responses, the answer from the server.

HTTP requests, and responses, share similar structure and are composed of:

  1. A start-line describing the requests to be implemented, or its status of whether successful or a failure. This start-line is always a single line.
  2. An optional set of HTTP headers specifying the request, or describing the body included in the message.
  3. A blank line indicating all meta-information for the request has been sent.
  4. An optional body containing data associated with the request (like content of an HTML form), or the document associated with a response. The presence of the body and its size is specified by the start-line and HTTP headers.

The start-line and HTTP headers of the HTTP message are collectively known as the head of the requests, whereas its payload is known as the body.

2. HTTP Requests#

2.1. Start line#

A start line contains three parts:

  • Their start-line contain three elements:
    • An HTTP method, a verb (like GET, PUT or POST) or a noun (like HEAD or OPTIONS), that describes the action to be performed.

GET 请求的结果可被浏览器和其他中间件(代理服务器)缓存，并且可能会被重复执行（例如，用户刷新页面时）。 如果你是在用户点击某个按钮后通过 GET 请求来处理点赞功能，那么理论上，仅当用户再次点击这个按钮时，该 GET 请求才会被重新发送。如果你的前端逻辑确保了每次点击都对应一个点赞操作，那么不会因为浏览器页面刷新而导致重复提交。然而，即使是这种情况，使用 GET 请求来处理点赞动作仍然不是最佳实践，原因包括： 非幂等性：点赞操作是非幂等的，因为每次操作都改变了服务器上的状态（即增加了点赞计数）。而 GET 请求应当是幂等的，即多次执行相同的请求应该具有相同的副作用。在这种情况下，多次点击按钮会导致多次点赞，违反了 GET 请求的幂等性原则。

  • The request target

    • Usually a URL
    • Sometimes url followed by a '?' and query string: http://example.com/?bar1=a&bar2=b

  • The HTTP version

POST www.example.com/test.html?username=alibaba HTTP/1.1

2.2. Headers#

An HTTP header consists of its case-insensitive name followed by a colon (:), then by its value. Whitespace before the value is ignored. This is an HTTP header:

Content-Type: text/html; charset=utf-8

Usually, a request or response consist of many headers:

GET /home.html HTTP/1.1
Host: developer.mozilla.org
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Referer: https://developer.mozilla.org/testpage.html
Connection: keep-alive

As you can see above, there are 7 headers in this request, a header like a key-value string, different header usually has different syntax and its own type. For example:

The type of Accept header is a Request header, and its syntax is below:

Accept: <MIME_type>/<MIME_subtype>
Accept: <MIME_type>/*
Accept: */*

// Multiple types, weighted with the quality value syntax:
Accept: text/html, application/xhtml+xml, application/xml;q=0.9, image/webp, */*;q=0.8

The type of Content-Type header is Representation header, and its syntax:

Content-Type: text/html; charset=utf-8
Content-Type: multipart/form-data; boundary=something

The offcial way to indicate aobve is:

Many different headers can appear in an HTTP request. They can be divided in several groups:

2.3. Body#

The final part of the request is its body. Not all requests have one: requests fetching resources, like GET, HEAD, DELETE, or OPTIONS, usually don’t need one. Some requests send data to the server in order to update it: as often the case with POST requests (containing HTML form data).

Bodies can be broadly divided into two categories:

3. HTTP Responses#

HTTP Reponse also contains three parts:

  • Status line

    • Three part: protocol version, a status code, a status text (A brief, purely informational, textual description of the status code to help a human understand the HTTP message.)
    • HTTP/1.1 404 Not Found

  • Headers

    • Same as http request.

  • Body

    • Same as http request.

Reference: HTTP Messages - HTTP | MDN