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[XHR2] HTTP Trailers

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 21 Oct 2010 12:12:36 +1100
Message-Id: <483046C5-11D9-4F8A-BAC9-2FF5135F78E2@mnot.net>
To: public-webapps@w3.org
I'm sure I've got more feedback for the XHR2 draft, but one big thing is support for HTTP trailers.

HTTP trailers are essentially headers that occur after the body; they are only present when chunking is in use.

Historically, HTTP trailers haven't been used much, but I'm seeing an increasing number of use cases for them recently, along with corresponding interest in them from various communities:

1. It's often necessary to collect debug and trace information about how a request was processed (possibly including information from other, upstream sources), but putting this information in the response body makes it awkward to get to (as it's dependent upon the format of the response), and putting it in headers means buffering the entire response. Trailers allow debug and trace information to be appended to responses in an easily accessible form with low overhead.

For example, a response trailer could contain information about how much CPU it took to service a request, and/or information about database latency, etc. If XHR2 supported trailers, it would be possible to incorporate this information into tools like Firebug, so that they could not only show client-visible performance metrics, but also an end-to-end view.

2. Trailers can also be useful for denoting problems that occur after the response status code is sent, so that the client knows a response is incomplete or otherwise incorrect. E.g., if XHR2 supported trailers, client code could check for any error messages in trailers and take that into account when processing a response.

Trailers are supported by most deployed intermediaries, in the sense that they're passed through. Servers have varying levels of support; there's recently been discussion on the Apache mailing list about adding support, and node.js's HEAD has support for them as well.

In XHR2, supporting trailers probably means just making it possible to read trailers on the response when they're present, rather than discarding them. This could either be done by making them available from getResponseHeader() after the message is complete, or using a separate property or method if it's felt that keeping them distinct from the "normal" headers is desirable. E.g., getResponseTrailer().

Cheers,

--
Mark Nottingham   http://www.mnot.net/
Received on Thursday, 21 October 2010 01:13:08 GMT

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