AJAX - The Complete Reference

PART IV

Appendix B: HTTP 1.1 Reference 597

Practically speaking, some Ajax libraries may send such headers for use by a back-end

environment to determine how to handle the request or simply for logging purposes to

understand if a request is triggered using Ajax or not.

X-Requested-By: Awesome-Ajax-Lib

You might also see the header being used to pass special request signatures, as

discussed in Chapter 7, to help improve Ajax request security.

X-Signature: QWxhBalTaRb3l1T3pvN

HTTP Responses

After receiving a request, a Web server attempts to process the request. The result of the

request is indicated by the first line of the response, which includes the following data:

HTTP-version status-code reason-string

where

• HTTP-version is a version string, such as HTTP/1.1.


• status-code is a string containing a numeric code, such as 200 or 404 , indicating
  the status of the response.


• reason-string is the text string, such as OK or Not Found, explaining the numeric
  status code.

For a successful query, a status line might read as follows:

In the case of an error, the status line might read:

The status codes that are found in the response line are grouped in five categories, as

summarized in Table B-6.

The specific nature of each response code is detailed in Table B-7. Response codes not

listed in this table but within an allowed range shown in Table B-6 are treated as a general

type of the class of response. For example, a 499 response would be a client error, while 384

would be a redirection of some sort. While the specification may be clear about this

interpretation, what actually happens, particularly in the case of a redirection when such

generic responses are received, is not defined nor obvious.