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Difference Between Methods GET and POST
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Methods GET and POST in HTML forms - what's the difference?
In HTML, one can specify two different submission methods for a form. The method
is specified inside a FORM element, using the METHOD attribute. The difference
between METHOD="GET" (the default) and METHOD="POST" is primarily defined in
terms of form data encoding. The official recommendations say that "GET" should
be used if and only if the form processing is idempotent, which typically means
a pure query form. Generally it is advisable to do so. There are, however,
problems related to long URLs and non-ASCII character repertoires which can make
it necessary to use "POST" even for idempotent processing.
Content:
* The fundamental differences between "GET" and "POST"
* Why the distinction matters
* Differences in form submission
* Differences in server-side processing
* Possible reasons to use "POST" for idempotent queries
The fundamental differences between "GET" and "POST"
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The HTML specifications technically define the difference between "GET" and
"POST" so that former means that form data is to be encoded (by a browser) into
a URL while the latter means that the form data is to appear within a message
body. But the specifications also give the usage recommendation that the "GET"
method should be used when the form processing is "idempotent", and in those
cases only. As a simplification, we might say that "GET" is basically for just
getting (retrieving) data whereas "POST" may involve anything, like storing or
updating data, or ordering a product, or sending E-mail.
The HTML 2.0 specification says, in section Form Submission (and the HTML 4.0
specification repeats this with minor stylistic changes):
If the processing of a form is idempotent (i.e. it has no lasting observable
effect on the state of the world), then the form method should be GET. Many
database searches have no visible side-effects and make ideal applications
of query forms.
- -
If the service associated with the processing of a form has side effects
(for example, modification of a database or subscription to a service), the
method should be POST.
In the HTTP specifications (specifically RFC 2616) the word idempotent is
defined as follows:
Methods can also have the property of "idempotence" in that (aside from
error or expiration issues) the side-effects of N > 0 identical requests is
the same as for a single request.
The word idempotent, as used in this context in the specifications, is
(pseudo)mathematical jargon (see definition of "idempotent" in FOLDOC) and
should not be taken too seriously or literally here. The phrase "no lasting
observable effect on the state of the world" isn't of course very exact either,
and isn't really the same thing. Idempotent processing, as defined above, does
not exclude fundamental changes, only that processing the same data twice has
the same effect as processing it once. But here, in fact, idempotent processing
means that a form submission causes no changes anywhere except on the user's
screen (or, more generally speaking, in the user agent's state). Thus, it is
basically for retrieving data. If such a form is resubmitted, it might get
different data (if the data had been changed meanwhile), but the submission
would not cause any update of data or other events. The concept of changes
should not be taken too pedantically; for instance, it can hardly be regarded as
a change that a form submission is logged into the server's log file. On the
other hand, sending E-mail should normally be regarded as "an effect on the
state of the world".
The HTTP specifications aren't crystal clear on this, and section Safe Methods
in the HTTP/1.1 specification describes the principles in yet another way. It
opens a different perspective by says that users "cannot be held accountable"
for side effects, which presumably means any effect than mere retrieval:
In particular, the convention has been established that the GET and HEAD
methods SHOULD NOT have the significance of taking an action other than
retrieval. These methods ought to be considered "safe". This allows user
agents to represent other methods, such as POST, PUT and DELETE, in a
special way, so that the user is made aware of the fact that a possibly
unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not generate
side-effects as a result of performing a GET request; in fact, some dynamic
resources consider that a feature. The important distinction here is that
the user did not request the side-effects, so therefore cannot be held
accountable for them.
The concept and its background is explained in section Allowing input in Tim
Berners-Lee's Style Guide for online hypertext. It refers, for more information,
to User agent watch points, which emphatically says that GET should be used if
and only if there are no side effects. But this line of thought, however logical,
is not always practical at present, as we shall see.
See also answer to question "What is the difference between GET and POST?" in
CGI Programming FAQ by Nick Kew.
Why the distinction matters
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In the same context where the pragmatic difference is stated, the HTML 2.0
specification describes the corresponding submission methods technically.
However, it does not explain why those methods are recommended for idempotent
vs. non-idempotent submissions. Neither does it explain what practical
difference there might be between the methods.
Alan Flavell has explained, in an article in a thread titled Limit on URL length
in the newsgroup comp.infosystems.www.authoring.html, that "the distinction
[between GET and POST] is a real one, and has been observed by some browser
makers, which can result in undesirable consequences if the inappropriate one is
used". He gives the following reason for conforming to the advice:
When users revisit a page that resulted from a form submission, they might
be presented with the page from their history stack (which they had probably
intended), or they might be told that the page has now expired. Typical user
response to the latter is to hit Reload.
This is harmless if the request is idempotent, which the form author signals
to the browser by specifying the GET method.
Browsers typically will (indeed "should") caution their users if they are
about to resubmit a POST request, in the belief that this is going to cause
a further "permanent change in the state of the universe", e.g. ordering
another Mercedes-Benz against their credit card or whatever. If users get
so accustomed to this happening when they try to reload a harmless idempotent
request, then sooner or later it's going to bite them when they casually
[OK] the request and do, indeed, order a second pizza, or invalidate their
previous competition entry by apparently trying to enter twice, or whatever.
Thus, some browsers can act more cleverly if the author uses "GET" or "POST"
consistently, i.e. using "GET" for pure queries and "POST" for other form
submissions. It needs to be noted, though, that using "GET" gives no protection
against causing changes. A script which processes a form submission sent with
the "GET" could cause a pizza ordering. It's just that authors are expected to
take care that such things don't happen.
Moreover, the use of "POST" cannot guarantee that the user does not
inadvertantly submit the same form data twice; the browser might not give a
warning, or the user might fail to understand the warning. Users are known to
become impatient when it seems that "nothing happens" when they click on a
button, so they might click on it again and again. Thus, robust processing of
forms should take precautions against unintended duplicate actions. (As a simple
example, a submission might be processed first by a script which sends back a
page containing a confirmation request, echoing back the data submitted and
asking the user to verify it and then submit the confirmation.)
A "GET" request is often cacheable, whereas a "POST" request can hardly be. For
query systems this may have a considerable efficiency impact, especially if the
query strings are simple, since caches might serve the most frequent queries.
For information about caches, see Caching Tutorial for Web Authors and
Webmasters, especially section Writing Cache-Aware Scripts.
Differences in form submission
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For both METHOD="GET" and METHOD="POST", the processing of a user's submit
request (such as clicking on a submit button) in a browser begins with a
construction of the form data set, which is then encoded in a manner which
depends on the ENCTYPE attribute. That attribute has two possible values
mentioned in the specifications, but multipart/form-data is for "POST"
submissions only, whereas application/x-www-form-urlencoded (the default) can be
used both for "POST" and for "GET".
Then the form data set is transmitted as follows (quotation from the HTML 4.0
specification):
* If the method is "get" - -, the user agent takes the value of action,
appends a ? to it, then appends the form data set, encoded using the
application/x-www-form-urlencoded content type. The user agent then
traverses the link to this URI. In this scenario, form data are restricted
to ASCII codes.
* If the method is "post" --, the user agent conducts an HTTP post transaction
using the value of the action attribute and a message created according to
the content type specified by the enctype attribute.
Thus, for METHOD="GET" the form data is encoded into a URL (or, speaking more
generally, into a URI). This means that an equivalent to a form submission can
be achieved by following a normal link referring to a suitable URL; see the
document Choices in HTML forms for details and examples. On a typical browser,
the user sees the URL of a document somewhere (e.g. on Location line), and if he
is viewing the results of a query sent using METHOD="GET", he will see what the
actual query was (i.e. the part of the URL that follows the ? sign). The user
could then bookmark it or cut&paste it for later use (e.g. to be E-mailed or put
into one's own HTML document after some editing).
Although the HTML specifications don't say it very explicitly, the fundamental
difference between the methods is really that they correspond to different HTTP
requests, as defined in the HTTP specifications. See especially Method
Definitions in RFC 2616. For form submission with METHOD="GET", the browser
constructs a URL as described above, then processes it as if following a link
(or as if the user had typed the URL directly). The browser divides the URL into
parts and recognizes a host, then sends to that host a GET request with the rest
of the URL as argument. The server takes it from there. Submission of a form
with METHOD="POST" causes a POST request to be sent.
Differences in server-side processing
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In principle, processing of a submitted form data depends on whether it is sent
with METHOD="GET" or METHOD="POST". Since the data is encoded in different ways,
different decoding mechanisms are needed. Thus, generally speaking, changing the
METHOD may necessitate a change in the script which processes the submission.
For example, when using the CGI interface, the script receives the data in an
environment variable when METHOD="GET" is used but in the standard input stream
(stdin) when METHOD="POST" is used.
It is, however, possible to construct libraries of subroutines (e.g. Perl
functions) which allow one to write scripts in a manner which works both for
METHOD="GET" and METHOD="POST". This would be based on distinguishing between
the cases within the subroutine code and returning the data to the caller in a
uniform manner.
Possible reasons to use "POST" for idempotent queries
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For reasons explained above, one should normally use METHOD="POST" if and only
if the form submission may cause changes. There are some exceptional situations
where one may consider using METHOD="POST" even for pure queries, too:
* If the form data would contain non-ASCII characters, then METHOD="GET" is
inapplicable in principle, although it may work in practice (mainly for ISO
Latin 1 characters). Thus, for a query where the keywords might contain e.g.
accented letters, you have to select among two evils: using METHOD="GET"
against the rules which restrict the character reportoire to ASCII within it,
or using METHOD="POST" against the rules which says that it should not be
used when the processing is idempotent. The latter alternative is probably
less dangerous.
* If the form data set is large - say, hundreds of characters - then
METHOD="GET" may cause practical problems with implementations which cannot
handle that long URLs. Such usage is mentioned in the the HTML 2.0
specification in an informative note as follows:
Note - The URL encoding may result in very long URIs, which cause some
historical HTTP server implementations to exhibit defective behavior. As a
result, some HTML forms are written using METHOD=POST even though the form
submission has no side-effects.
* The limitations are not only historical. There is an official statement by
Microsoft, originally published 2000-02-23: INFO: Maximum URL Length Is
2,083 Characters in Internet Explorer (Q208427).
* You might wish to avoid METHOD="GET" in order to make it less visible to
users how the form works, especially in order to make "hidden" fields
(INPUT TYPE="HIDDEN") more hidden. Using POST implies that users won't see
the form data in the URL shown by the user; but note that this is not a very
effective method of hiding, since the user can of course view the source
code of your FORM element!
Date of last revision: 2001-11-30. Last modification: 2003-09-28.
This page belongs to division Web authoring and surfing, subdivision Forms in
the free information site IT and communication by Jukka "Yucca" Korpela. 