Re: [xri] Quick overview of descriptor discovery flow

On Mon, Dec 15, 2008 at 12:55 PM, Eran Hammer-Lahav <eran@hueniverse.com> wrote:

The following method describes how to perform R-URI --> D-URI: finding the descriptor URI from the resource URI. The process starts with a resource URI. It doesn't matter how that URI has been obtained. In the examples below, 'D-URI' is used to indicate the placeholder containing the desired descriptor URI.

1. Method selection

The consumer (entity performing discovery) has three options based on information available to it with regard to the resource:

* If the resource is known to be an HTML or ATOM document, and the document has been and will be retrieved via HTTP GET either way, the consumer can look for <Link> elements:

    <Link href=""D-URI"" rel="describedby" type="application/xrd+xml">

* If the resource is going to be retrieved via HTTP GET/HEAD, the consumer can look for Link: headers:

    Link: <D-URI>; rel="describedby"; type="application/xrd+xml"

* If the first two options are not applicable, the consumer obtains the /site-meta document for the resource's domain. The consumer may request an XRD representation but must be able to parse the text representation. It is not clear if there is real value in offering an XRD representation of /site-meta (please discuss).

Text:

    Link-Template: <http://example.com?meta={+uri}>; rel="describedby"; type="application/xrd+xml"; syntax="plain"; vocabulary="uri"; scheme="mailto http"

I'm probably a bit late to the party, but are these really all necessary? Especially the "syntax" and "vocabulary" pieces smell of design by committee. Won't we in practice just all use vocabulary=uri? Also, enumerating the possible schemes seems like a pain in the neck. Why can't the servlet serving the XRD branch on the scheme, instead of putting this in site-meta?

Dirk.

XRD:

    <XRD>
        <Link>
            <Rel>describedby</Rel>
            <MediaType>application/xrd+xml</MediaType>
            <TemplateURI syntax="plain" vocabulary="uri">http://example.com?meta={uri}</TemplateURI>
            <site-meta:scheme>mailto http</site-meta:scheme>
        </Link>
    </XRD>

The 'plain' template syntax means simple substitution of {variable}s. If the variable name is prefixed with a '+', it must be percent-encoded prior to substitution. The 'uri' template vocabulary includes: uri, scheme, authority, domain, port, path, query, fragment, and username.

Using the template, the consumer construct the D-URI from the R-URI.

EHL
Ps. The above syntax for /site-meta is the expected format in the next draft. This flow does not use the regular Link records in /site-meta as those are used for the entire site (an abstract concept) and not for individual resources. There is no such URI identifying the "site".

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