[http-post] When do you use POST and when do you use GET?

From what I can gather, there are three categories:

1. Never use GET and use POST
2. Never use POST and use GET
3. It doesn't matter which one you use.

Am I correct in assuming those three cases? If so, what are some examples from each case?

There is nothing you can't do per-se. The point is that you're not supposed to modify the server state on an HTTP GET. HTTP proxies assume that since HTTP GET does not modify the state then whether a user invokes HTTP GET one time or 1000 times makes no difference. Using this information they assume it is safe to return a cached version of the first HTTP GET. If you break the HTTP specification you risk breaking HTTP client and proxies in the wild. Don't do it :)

Use GET when you want the URL to reflect the state of the page. This is useful for viewing dynamically generated pages, such as those seen here. A POST should be used in a form to submit data, like when I click the "Post Your Answer" button. It also produces a cleaner URL since it doesn't generate a parameter string after the path.

i dont see a problem using get though, i use it for simple things where it makes sense to keep things on the query string.

Using it to update state - like a GET of delete.php?id=5 to delete a page - is very risky. People found that out when Google's web accelerator started prefetching URLs on pages - it hit all the 'delete' links and wiped out peoples' data. Same thing can happen with search engine spiders.

From w3schools.com:

What is HTTP?

The Hypertext Transfer Protocol (HTTP) is designed to enable communications between clients and servers.

HTTP works as a request-response protocol between a client and server.

A web browser may be the client, and an application on a computer that hosts a web site may be the server.

Example: A client (browser) submits an HTTP request to the server; then the server returns a response to the client. The response contains status information about the request and may also contain the requested content.

Two HTTP Request Methods: GET and POST

Two commonly used methods for a request-response between a client and server are: GET and POST.

GET – Requests data from a specified resource POST – Submits data to be processed to a specified resource

Here we distinguish the major differences:

In brief

• Use GET for safe andidempotent requests
• Use POST for neither safe nor idempotent requests

In details There is a proper place for each. Even if you don't follow RESTful principles, a lot can be gained from learning about REST and how a resource oriented approach works.

A RESTful application will use GETs for operations which are both safe and idempotent.

A safe operation is an operation which does not change the data requested.

An idempotent operation is one in which the result will be the same no matter how many times you request it.

It stands to reason that, as GETs are used for safe operations they are automatically also idempotent. Typically a GET is used for retrieving a resource (a question and its associated answers on stack overflow for example) or collection of resources.

A RESTful app will use PUTs for operations which are not safe but idempotent.

I know the question was about GET and POST, but I'll return to POST in a second.

Typically a PUT is used for editing a resource (editing a question or an answer on stack overflow for example).

A POST would be used for any operation which is neither safe or idempotent.

Typically a POST would be used to create a new resource for example creating a NEW SO question (though in some designs a PUT would be used for this also).

If you run the POST twice you would end up creating TWO new questions.

There's also a DELETE operation, but I'm guessing I can leave that there :)

Discussion

In practical terms modern web browsers typically only support GET and POST reliably (you can perform all of these operations via javascript calls, but in terms of entering data in forms and pressing submit you've generally got the two options). In a RESTful application the POST will often be overriden to provide the PUT and DELETE calls also.

But, even if you are not following RESTful principles, it can be useful to think in terms of using GET for retrieving / viewing information and POST for creating / editing information.

You should never use GET for an operation which alters data. If a search engine crawls a link to your evil op, or the client bookmarks it could spell big trouble.

In PHP, POST data limit is usually set by your php.ini. GET is limited by server/browser settings I believe - usually around 255 bytes.

Another difference is that POST generally requires two HTTP operations, whereas GET only requires one.

Edit: I should clarify--for common programming patterns. Generally responding to a POST with a straight up HTML web page is a questionable design for a variety of reasons, one of which is the annoying "you must resubmit this form, do you wish to do so?" on pressing the back button.

The original intent was that GET was used for getting data back and POST was to be anything. The rule of thumb that I use is that if I'm sending anything back to the server, I use POST. If I'm just calling an URL to get back data, I use GET.

1.3 Quick Checklist for Choosing HTTP GET or POST

Use GET if:

    The interaction is more like a question (i.e., it is a safe operation such as a query, read operation, or lookup).

Use POST if:

    The interaction is more like an order, or
    The interaction changes the state of the resource in a way that the user would perceive (e.g., a subscription to a service), or
    The user be held accountable for the results of the interaction.

Source.

As answered by others, there's a limit on url size with get, and files can be submitted with post only.

I'd like to add that one can add things to a database with a get and perform actions with a post. When a script receives a post or a get, it can do whatever the author wants it to do. I believe the lack of understanding comes from the wording the book chose or how you read it.

A script author should use posts to change the database and use get only for retrieval of information.

Scripting languages provided many means with which to access the request. For example, PHP allows the use of $_REQUEST to retrieve either a post or a get. One should avoid this in favor of the more specific $_GET or $_POST.

In web programming, there's a lot more room for interpretation. There's what one should and what one can do, but which one is better is often up for debate. Luckily, in this case, there is no ambiguity. You should use posts to change data, and you should use get to retrieve information.

Short Version

GET: Usually used for submitted search requests, or any request where you want the user to be able to pull up the exact page again.

Advantages of GET:

• URLs can be bookmarked safely.
• Pages can be reloaded safely.

Disadvantages of GET:

• Variables are passed through url as name-value pairs. (Security risk)
• Limited number of variables that can be passed. (Based upon browser. For example, Internet Explorer is limited to 2,048 characters.)

POST: Used for higher security requests where data may be used to alter a database, or a page that you don't want someone to bookmark.

Advantages of POST:

• Name-value pairs are not displayed in url. (Security += 1)
• Unlimited number of name-value pairs can be passed via POST. Reference.

Disadvantages of POST:

• Page that used POST data cannot be bookmark. (If you so desired.)

Longer Version

Directly from the Hypertext Transfer Protocol -- HTTP/1.1:

9.3 GET

The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.

The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.

The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in section 14.35. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.

The response to a GET request is cacheable if and only if it meets the requirements for HTTP caching described in section 13.

See section 15.1.3 for security considerations when used for forms.

9.5 POST

The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions:

• Annotation of existing resources;

• Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;

• Providing a block of data, such as the result of submitting a form, to a data-handling process;

• Extending a database through an append operation.

The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.

Read the article about HTTP in the Wikipedia. It will explain what the protocol is and what it does:

GET

Requests a representation of the specified resource. Note that GET should not be used for operations that cause side-effects, such as using it for taking actions in web applications. One reason for this is that GET may be used arbitrarily by robots or crawlers, which should not need to consider the side effects that a request should cause.

and

POST Submits data to be processed (e.g., from an HTML form) to the identified resource. The data is included in the body of the request. This may result in the creation of a new resource or the updates of existing resources or both.

The W3C has a document named URIs, Addressability, and the use of HTTP GET and POST that explains when to use what. Citing

1.3 Quick Checklist for Choosing HTTP GET or POST

• Use GET if:
  • The interaction is more like a question (i.e., it is a safe operation such as a query, read operation, or lookup).

and

• Use POST if:
  • The interaction is more like an order, or
  • The interaction changes the state of the resource in a way that the user would perceive (e.g., a subscription to a service), or o The user be held accountable for the results of the interaction.

However, before the final decision to use HTTP GET or POST, please also consider considerations for sensitive data and practical considerations.

A practial example would be whenever you submit an HTML form. You specify either post or get for the form action. PHP will populate $_GET and $_POST accordingly.

Because GETs are purely URLs, they can be cached by the web browser and may be better used for things like consistently generated images. (Set an Expiry time)

One example from the gravatar page: http://www.gravatar.com/avatar/4c3be63a4c2f539b013787725dfce802?d=monsterid

GET may yeild marginally better performance, some webservers write POST contents to a temporary file before invoking the handler.

Another thing to consider is the size limit. GETs are capped by the size of the URL, 1024 bytes by the standard, though browsers may support more.

Transferring more data than that should use a POST to get better browser compatibility.

Even less than that limit is a problem, as another poster wrote, anything in the URL could end up in other parts of the brower's UI, like history.

The first important thing is the meaning of GET versus POST :

• GET should be used to... get... some information from the server,
• while POST should be used to send some information to the server.

After that, a couple of things that can be noted :

• Using GET, your users can use the "back" button in their browser, and they can bookmark pages
• There is a limit in the size of the parameters you can pass as GET (2KB for some versions of Internet Explorer, if I'm not mistaken) ; the limit is much more for POST, and generally depends on the server's configuration.

Anyway, I don't think we could "live" without GET : think of how many URLs you are using with parameters in the query string, every day -- without GET, all those wouldn't work ;-)

My general rule of thumb is to use Get when you are making requests to the server that aren't going to alter state. Posts are reserved for requests to the server that alter state.

Use GET if you don't mind the request being repeated (That is it doesn't change state).

Use POST if the operation does change the system's state.

I use POST when I don't want people to see the QueryString or when the QueryString gets large. Also, POST is needed for file uploads.

I don't see a problem using GET though, I use it for simple things where it makes sense to keep things on the QueryString.

Using GET will allow linking to a particular page possible too where POST would not work.

This traverses into the concept of REST and how the web was kinda intended on being used. There is an excellent podcast on Software Engineering radio that gives an in depth talk about the use of Get and Post.

Get is used to pull data from the server, where an update action shouldn't be needed. The idea being is that you should be able to use the same GET request over and over and have the same information returned. The URL has the get information in the query string, because it was meant to be able to be easily sent to other systems and people like a address on where to find something.

Post is supposed to be used (at least by the REST architecture which the web is kinda based on) for pushing information to the server/telling the server to perform an action. Examples like: Update this data, Create this record.

One practical difference is that browsers and webservers have a limit on the number of characters that can exist in a URL. It's different from application to application, but it's certainly possible to hit it if you've got textareas in your forms.

Another gotcha with GETs - they get indexed by search engines and other automatic systems. Google once had a product that would pre-fetch links on the page you were viewing, so they'd be faster to load if you clicked those links. It caused major havoc on sites that had links like delete.php?id=1 - people lost their entire sites.

Apart from the length constraints difference in many web browsers, there is also a semantic difference. GETs are supposed to be "safe" in that they are read-only operations that don't change the server state. POSTs will typically change state and will give warnings on resubmission. Search engines' web crawlers may make GETs but should never make POSTs.

Use GET if you want to read data without changing state, and use POST if you want to update state on the server.

Gorgapor, mod_rewrite still often utilizes GET. It just allows to translate a friendlier URL into a URL with a GET query string.

From RFC 2616:

9.3 GET
The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.

9.5 POST
The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions:

• Annotation of existing resources;
• Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;
• Providing a block of data, such as the result of submitting a form, to a data-handling process;
• Extending a database through an append operation.

The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.

Well one major thing is anything you submit over GET is going to be exposed via the URL. Secondly as Ceejayoz says, there is a limit on characters for a URL.

HTTP Post data doesn't have a specified limit on the amount of data, where as different browsers have different limits for GET's. The RFC 2068 states:

Servers should be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations may not properly support these lengths

Specifically you should the right HTTP constructs for what they're used for. HTTP GET's shouldn't have side-effects and can be safely refreshed and stored by HTTP Proxies, etc.

HTTP POST's are used when you want to submit data against a url resource.

A typical example for using HTTP GET is on a Search, i.e. Search?Query=my+query A typical example for using a HTTP POST is submitting feedback to an online form.

POST can move large data while GET cannot.

But generally it's not about a shortcomming of GET, rather a convention if you want your website/webapp to be behaving nicely.

Have a look at http://www.w3.org/2001/tag/doc/whenToUseGet.html

Simple version of POST GET PUT DELETE

• use GET - when you want to get any resource like List of data based on any Id or Name
• use POST - when you want to send any data to server. keep in mind POST is heavy weight operation because for updation we should use PUT instead of POST internally POST will create new resource
• use PUT - when you