HTTP Message Methods - Part 2 of Chapter 3 from HTTP: The Definitive Guide (1/4)

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HTTP: The Definitive Guide, Chapter 3: HTTP Messages

Methods

[The following is a continuation of our series of excerpts from chapter 3 of the O'Reilly title, HTTP: The Definitive Guide.]

Let's talk in more detail about the some of the basic HTTP methods, listed earlier in Table 3-1. Note that not all methods are implemented by every server. To be compliant with HTTP Version 1.1, a server need implement only the GET and HEAD methods for its resources.

Even when servers do implement all of these methods, the methods most likely have restricted uses. For example, servers that support DELETE or PUT (described later in this section) would not want just anyone to be able to delete or store resources. These restrictions generally are set up in the server's configuration, so they vary from site to site and from server to server.

Safe Methods

HTTP defines a set of methods that are called safe methods. The GET and HEAD methods are said to be safe, meaning that no action should occur as a result of an HTTP request that uses either the GET or HEAD method.

By no action, we mean that nothing will happen on the server as a result of the HTTP request. For example, consider when you are shopping online at Joe's Hardware and you click on the "submit purchase" button. Clicking on the button submits a POST request (discussed later) with your credit card information, and an action is performed on the server on your behalf. In this case, the action is your credit card being charged for your purchase.

There is no guarantee that a safe method won't cause an action to be performed (in practice, that is up to the web developers). Safe methods are meant to allow HTTP application developers to let users know when an unsafe method that may cause some action to be performed is being used. In our Joe's Hardware example, your web browser may pop up a warning message letting you know that you are making a request with an unsafe method and that, as a result, something might happen on the server (e.g., your credit card being charged).

GET

GET is the most common method. It usually is used to ask a server to send a resource. HTTP/1.1 requires servers to implement this method. Figure 3-7 shows an example of a client making an HTTP request with the GET method.

Figure 3-7. GET example

HEAD

The HEAD method behaves exactly like the GET method, but the server returns only the headers in the response. No entity body is ever returned. This allows a client to inspect the headers for a resource without having to actually get the resource. Using HEAD, you can:

• Find out about a resource (e.g., determine its type) without getting it.

• See if an object exists, by looking at the status code of the response.

• Test if the resource has been modified, by looking at the headers.

Server developers must ensure that the headers returned are exactly those that a GET request would return. The HEAD method also is required for HTTP/1.1 compliance. Figure 3-8 shows the HEAD method in action.

Figure 3-8. HEAD example

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Created: January 21, 2003
Revised: January 21, 2003

URL: https://webreference.com/programming/http/chap3/2/