Hi,
Within the Mobile Web Best Practices Working Group, the Content
Transformation Task Force is writing guidelines [1] for content
transformation proxies and content providers to be able to communicate
their intentions regarding a given HTTP message.
It occurred to us that from a proxy's point of view (note it also
applies from a server's point of view, but that's not the point here),
there is no way to tell whether a given HTTP request originates from the
browser itself or from an XHR object.
In the context of content transformation that is a problem because such
HTTP messages should be passed untouched by the content transformation
proxies: an XHR call involves that some client code will be run on
receipt of the response, so any transformation is likely to break the
content delivered by the device.
We initially thought that using dedicated "data" content-types (such as
application/json) would solve the problem, but then it's typically
common and valid to fetch an XHTML page using an XHR object for display
purposes.
Still from a content transformation's point of view, we've solved the
problem by recommending that content providers include a "Cache-Control:
no-transform" directive in the HTTP headers of XHR calls to "switch off"
the proxy when they don't want the message to be transformed in any way.
This still means that content providers have to update their content for
this to work, but that's another story.
We were wondering if that use case would not meet other similar use
cases in other areas that would require an easy way to tell that a
request originates from an XHR object. Possible solutions:
1. amending the User-Agent string to include an "XHR"-like string
somewhere
2. defining an additional header such as "X-Ajax-Engine" [2]
... and hopefully better solutions we haven't thought about.
Is there a reason for not willing to distinguish between a browser
request and an XHR call from within the browser?
Did we get something wrong and is it already possible to know a request
originates from an XHR object?
Is it something that you think could be worth considering?
On behalf of the Content Transformation Task Force,
François Daoust, BPWG Staff Contact.
[1] http://www.w3.org/TR/ct-guidelines
[2]
http://www.dev411.com/blog/2006/06/30/should-there-be-a-xmlhttprequest-user-agent
