HTTP Explained Page 4

HTTP/1.0 allows an open-ended set of methods to be used to indicate the purpose of a request. The three most often used methods are GET, HEAD, and POST.
The GET Method
Information from a form using the GET method is appended onto the end of the action URI being requested. Your CGI program will receive the encoded form input in the environment variable QUERY_STRING.

The GET method is used to ask for a specific document - when you click on a hyperlink, GET is being used. GET should probably be used when a URL access will not change the state of a database (by, for example, adding or deleting information) and POST should be used when an access will cause a change. Many database searches have no visible side-effects and make ideal applications of query forms using GET.

The semantics of the GET method changes to a "conditional GET" if the request message includes an If-Modified-Since header field. A conditional GET method requests that the identified resource be transferred only if it has been modified since the date given by the If-Modified-Since header.

The HEAD method
The HEAD method is used to ask only for information about a document, not for the document itself. HEAD is much faster than GET, as a much smaller amount of data is transferred. It's often used by clients who use caching, to see if the document has changed since it was last accessed. If it was not, then the local copy can be reused, otherwise the updated version must be retrieved with a GET.

The POST Method
This method transmits all form input information immediately after the requested URI. Your CGI program will receive the encoded form input on stdin.

POST /cgi-bin/post-query HTTP/1.0 Accept: text/html Accept: video/mpeg Accept: image/gif Accept: application/postscript User-Agent: Lynx/2.2 libwww/2.14 From: Stars@WDVL.com Content-type: application/x-www-form-urlencoded Content-length: 150 * a blank line * org=CyberWeb%20SoftWare &users=10000 &browsers=lynx
  • This is a "POST" query addressed for the program residing in the file at "/cgi-bin/post-query", that simply echoes the values it receives.

  • The client lists the MIME-types it is capable of accepting, and identifies itself and the version of the WWW library it is using.

  • Finally, it indicates the MIME-type it has used to encode the data it is sending, the number of characters included, and the list of variables and their values it has collected from the user.

  • MIME-type application/x-www-form-urlencoded means that the variable name-value pairs will be encoded the same way a URL is encoded. Any special characters, including puctuation, will be encoded as %nn where nn is the ASCII value for the character in hex.

This article was originally published on Aug 7, 1999

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