Ben Ramsey

Specifying rev="canonical" With HTTP

by Ben Ramsey 10 comments

It looks like there’s a lot of momentum behind rev="canonical" now—and all built up within the span of about forty-eight hours. So, while I disagree with the use of “canonical” for semantic reasons and rev for the potential of mass misunderstanding and improper implementation, I think I’ll bite the bullet on this one for now, but time will tell what the community ultimately decides.

So, I’ve decided to eat my own dog food. Like Simon Willison, I bought my own short URL, and now all blog posts on my site include a link tag that covers all the bases similar to the following:

<link rev="canonical" rel="alternate shorter" href="http://brtny.me/382" />

And all without any special WordPress plugin. For the ID, I’m simply using the WordPress post ID, which means that, if I change to another blogging tool in the future, I will need to maintain my indexes. Note that I’ve implemented it with both the popular rev="canonical" and my preferred rel="alternate shorter".

Chris Shiflett posts about the need for an HTTP header. I think this is also a good idea, for the same reasons he mentions. Chris’s original recommendation is for an X-Rev-Canonical header, but Stephen Paul Weber mentions the Link header. I think the Link header is the right way to go about this, since it offers an HTTP analogue of the HTML link element.

Link is an IETF proposal by Mark Nottingham that is still in the Internet-Draft stage, going through the IETF process for standardization, but it’s current, which is a good thing. If the community chooses to use it, though, it’s interesting to note that that it states:

Applications that don’t merit a registered relation type may use an extension relation type. An extension relation type is a URI that, when dereferenced, SHOULD yield a document describing that relation type.

This means that one should not simply put “canonical” as the value for rev. Instead, it would be more proper to use something like “http://revcanonical.appspot.com/#canonical” until “canonical” is accepted as a registered IANA relation type. I wonder if the same is technically true of the rel and rev attributes in HTML.

For now, I’ve decided to cover all bases in my HTTP headers, as well, and you can see this by making a HEAD request to any blog post on my website, as seen in the following:

HEAD /archives/summarizing-my-revcanonical-argument/ HTTP/1.1
Host: benramsey.com

HTTP/1.1 200 OK
Date: Sun, 12 Apr 2009 05:24:34 GMT
Link: <http://brtny.me/382>; rev="http://revcanonical.appspot.com/#canonical"; rel="alternate http://revcanonical.appspot.com/#shorter"
X-Rev-Canonical: http://brtny.me/382
Content-Type: text/html; charset=utf-8

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