Tuesday, 22 February 2011

AJAX content negotiation browser differences

We have been experiencing hard-to-explain problems with the behaviour of ADMIRAL web pages in different browsers. They would work fine for Firefox, but not with IE, Safari or Google Chrome.

Javascript (using jQuery) is used at the client end to retrieve RDF/XML information from the ADMIRAL server using AJAX calls. The server is capable of returning different formats for the requested data, basing its decision on HTTP Accept headers .

The calling Javascript code looks like this:
type:         "GET",
url:          "...",
username:     "...",
password:     "...",
dataType:     "text",
beforeSend:   function (xhr)
xhr.setRequestHeader("Accept", "application/rdf+xml");
success:      function (data, status, xhr)
error:        function (xhr, status)
cache:        false
Using Wireshark to observe the HTTP traffic, we find that Firefox sends the following header:
Accept: application/rdf+xml
But when using Safari we see:
Accept: text/plain,*/*,application/rdf+xml
IE and Chrome also send something different from Firefox, but at the time of writing we've lost the exact trace.

The effect of this has been that even when we write an Ajax call to accept just RDF/XML, the server is seeing the additional Accept header options and in some cases is choosing the wrong response format when using browsers other than Firefox.

We have not yet found a simple work-around that works in all situations. But, generally, servers need to be aware that browsers sometimes add commonly required options to the HTTP Accept header. Prioritizing matches for uncommon content options might go some way to ensuring consistent behaviour across browsers. E.g. in the case illustrated here, servers should favour the less common option application/rdf+xml over the more common text/plain content type.  Also favouring non-wildcard matches that appear later in the Accept header may help in some cases.

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