The CometD project implements various Comet techniques to implement a web messaging bus.
You can find an introduction to CometD here.

Web applications often need to access resources residing on different servers, making the request to access those resources a cross origin request and therefore subject to the same origin policy.

Fortunately, all modern browsers implement the Cross Origin Resource Sharing (CORS) specification, and with the support of Jetty‘s Cross Origin Filter, it’s a breeze to write applications that allow cross origin resource sharing.

That is, all modern browsers apart Internet Explorer 8 and 9.

Without CORS support, CometD fallbacks to another Comet technique known as JSONP.
While JSONP is much less efficient than a CORS request, it guarantees the CometD functionality, but it’s 2011 and JSONP should be a relic of the past.

Microsoft’s browsers have another JavaScript object that allows to make cross origin request: XDomainRequest.

Unfortunately this object is non-standard, and it is not, in general, supported by the JavaScript toolkits on which CometD relies for the actual communication with the server.
I cannot really blame toolkits authors for this lack of support.

However, I recently found a way to make XDomain request work with CometD 2.4.0 and the Dojo toolkit library.

The solution (see this blog post for reference) is the following:

Add this code to your JavaScript application:

What remains is to configure CometD with the crossOriginHost:

The last glitch is that XDomainRequest does not seem to allow to send the Content-Type HTTP header, so all of the above will only work in CometD 2.4.0.RC1 or greater where this improvement has been made.

I do not particularly recommend this hack, but sometimes it’s the only way to support cross origin requests for the obsolete Internet Explorers.

CometD, Dojo and XDomainRequest
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