POST (HTTP)
Encyclopedia
In computing, POST is one of many request methods supported by the HTTP protocol used by the World Wide Web
. The POST request method is used when the client needs to send data to the server as part of the request, such as when uploading a file or submitting a completed form.
In contrast to the GET request method where only a URL and headers are sent to the server, POST requests also include a message body. This allows for arbitrary length data of any type to be sent to the server.
Headers in the POST request may indicate the message body's Internet media type
to the server.
element, the default Internet media type
is "application/x-www-form-urlencoded". This is a format for encoding key-value pairs
with possibly duplicate keys. Each key-value pair is separated by an '&' character, and each key is separated from its value by an '=' character. Keys and values are both escaped by replacing spaces with the '+' character and then using URL encoding on all other non-alphanumeric
characters.
For example, the key-value pairs
are encoded as
Starting with HTML 4.0, forms can also submit data in multipart/form-data as defined in RFC 2388 (See also RFC 1867 for an earlier experimental version defined as an extension to HTML 2.0 and mentioned in HTML 3.2).
Affecting server state
Per RFC 2616, the POST method should be used for any context in which a request is non-idempotent: that is, it causes a change in server state each time it is performed, such as submitting a comment to a blog post or voting in an online poll. In practice, GET is often reserved, not simply for idempotent actions, but for nullipotent ones, ones with no side-effects (not simply "no side effects on second or future requests"). For this reason, web crawlers
such as search engine indexers normally use the GET method exclusively, to prevent their automated requests from performing such actions.
However, there are reasons to use POST for idempotent requests, notably if the request uses non-ASCII characters or is very long, due to restrictions on URLs – the query string
in the GET method may become very long, especially due to percent-encoding
.
External links
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
. The POST request method is used when the client needs to send data to the server as part of the request, such as when uploading a file or submitting a completed form.
In contrast to the GET request method where only a URL and headers are sent to the server, POST requests also include a message body. This allows for arbitrary length data of any type to be sent to the server.
Headers in the POST request may indicate the message body's Internet media type
Internet media type
An Internet media type, originally called a MIME type after MIME and sometimes a Content-type after the name of a header in several protocols whose value is such a type, is a two-part identifier for file formats on the Internet.The identifiers were originally defined in RFC 2046 for use in email...
to the server.
Use for submitting web forms
When a web browser sends a POST request from a web formForm (web)
A webform on a web page allows a user to enter data that is sent to a server for processing. Webforms resemble paper or database forms because internet users fill out the forms using checkboxes, radio buttons, or text fields...
element, the default Internet media type
Internet media type
An Internet media type, originally called a MIME type after MIME and sometimes a Content-type after the name of a header in several protocols whose value is such a type, is a two-part identifier for file formats on the Internet.The identifiers were originally defined in RFC 2046 for use in email...
is "application/x-www-form-urlencoded". This is a format for encoding key-value pairs
Associative array
In computer science, an associative array is an abstract data type composed of a collection of pairs, such that each possible key appears at most once in the collection....
with possibly duplicate keys. Each key-value pair is separated by an '&' character, and each key is separated from its value by an '=' character. Keys and values are both escaped by replacing spaces with the '+' character and then using URL encoding on all other non-alphanumeric
Alphanumeric
Alphanumeric is a combination of alphabetic and numeric characters, and is used to describe the collection of Latin letters and Arabic digits or a text constructed from this collection. There are either 36 or 62 alphanumeric characters. The alphanumeric character set consists of the numbers 0 to...
characters.
For example, the key-value pairs
Name: Jonathan Doe
Age: 23
Formula: a + b 13%!
are encoded as
Name=Jonathan+Doe&Age=23&Formula=a+%2B+b+%3D%3D+13%25%21
Starting with HTML 4.0, forms can also submit data in multipart/form-data as defined in RFC 2388 (See also RFC 1867 for an earlier experimental version defined as an extension to HTML 2.0 and mentioned in HTML 3.2).
Affecting server state
Per RFC 2616, the POST method should be used for any context in which a request is non-idempotent: that is, it causes a change in server state each time it is performed, such as submitting a comment to a blog post or voting in an online poll. In practice, GET is often reserved, not simply for idempotent actions, but for nullipotent ones, ones with no side-effects (not simply "no side effects on second or future requests"). For this reason, web crawlers
Web crawler
A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner or in an orderly fashion. Other terms for Web crawlers are ants, automatic indexers, bots, Web spiders, Web robots, or—especially in the FOAF community—Web scutters.This process is called Web...
such as search engine indexers normally use the GET method exclusively, to prevent their automated requests from performing such actions.
However, there are reasons to use POST for idempotent requests, notably if the request uses non-ASCII characters or is very long, due to restrictions on URLs – the query string
Query string
In World Wide Web, a query string is the part of a Uniform Resource Locator that contains data to be passed to web applications such as CGI programs....
in the GET method may become very long, especially due to percent-encoding
Percent-encoding
Percent-encoding, also known as URL encoding, is a mechanism for encoding information in a Uniform Resource Identifier under certain circumstances. Although it is known as URL encoding it is, in fact, used more generally within the main Uniform Resource Identifier set, which includes both Uniform...
.
External links