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HTTP

HyperText Transfer Protocol (HTTP) is the most common protocol used to transfer data over the web. It defines how messages are formatted and transmitted, and what actions web servers and browsers should take in response to various commands.

HTTP Request

In it's most basic form, an HTTP request is just the following text:

GET / HTTP/1.1
Host: www.example.com

Which means, that the client wants to make a request to the server, to get the root document (/) of the website www.example.com.

Request Headers

Everything else below this line is called the HTTP Header. It contains additional information about the request, like the host, the user agent, the accepted languages, etc. For example:

GET / HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0
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
Connection: keep-alive

Request Body

The request headers may be followed with an empty line, then extra data which is called the request body. For example, when you submit a form, the data you enter is sent in the request body. For example:

POST / HTTP/1.1
Host: www.example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 13

say=Hi&to=Mom

This data may include form data, or files, or any other data you want to send to the server.

Request Methods

Notice that in the previous example, we stopped using the GET method, and started using the POST method. This is because the GET method is used to retrieve data from the server, while the POST method is used to send data to the server. There are other methods as well, like:

Method Description
GET Retrieve data from the server
POST Send data to the server
PUT Send data to the server, and replace the existing data
DELETE Delete data from the server
HEAD Retrieve only the headers of a resource

None of them are used as much as GET and POST though, and any server can select how it responds to each method.

HTTP Response

After a request is made, the server responds with an HTTP response. It's structure is similar to the request, but it contains different information.

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Content-Length: 13

Hello, World!

Status Codes

The number 200 in the previous example is the status code. It indicates that the request was successful. The word "OK" after it is a text description of it. There are many other status codes, the most common ones are:

Code Description
200 OK
301 Moved Permanently
400 Bad Request
401 Unauthorized
403 Forbidden
404 Not Found
500 Internal Server Error
503 Service Unavailable

These can be roughly divided into 5 categories:

Category Description
1xx Informational
2xx Success
3xx Redirection / Temporary Error
4xx Client Error
5xx Server Error

Response Headers & Body

Like the request, the response may contain headers as well. For example:

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Content-Length: 13
Date: Sat, 26 May 2012 16:04:12 GMT
Expires: -1
Cache-Control: private, max-age=0

Hello, World!

In this case, everything after the first line, up until the empty line, is the response headers. The response body is everything after that empty line. The body can be HTML, JSON, XML, a file, or anything else, and the headers are always key-value pairs.