---------- Forwarded message ---------- From: Richard Jones <rich...@oneoverzero.com> Date: 11 January 2011 07:49 Subject: Re: content negotiating for package formats To: techadvisorypa...@swordapp.org
Hi Folks, Thanks, this is really great stuff. On 10/01/11 16:05, Robert D. Sanderson wrote: >> >> On 7 January 2011 17:36, Richard Jones<rich...@oneoverzero.com> wrote: >>> >>> 2/ Define an extension to the application/zip mimetype which allows us to >>> specify the package format as a parameter. So we could, for example, > > specify >>> >>> a parameter "swordpackage" which can take the URI of a package format, and >>> construct mimetypes like >>> >>> application/zip;swordpackage=uri:METSDSpaceSIP >>> (see http://tools.ietf.org/html/draft-ietf-atompub-typeparam-00) >>> >>> The questions here are: is this a legitimate extension/approach, would > > this >>> >>> break anything else on the web in general, and is it naive to assume that >>> all packages have the top level mimetype of application/zip? > > > First of all, no, it's not legitimate to invent new parameters for > existing mime types. > > RFC 2048, Section 2.2.3 > ... the names, values, and meanings of any parameters must > be fully specified when a media type is registered in the IETF tree ... > > http://www.faqs.org/rfcs/rfc2048.html > > So it's not legal to create a parameter swordpackage and attach it to the > existing application/zip. Ok, it sounds like this option then is simply out altogether. >> More generally, the HTTP specification defines the accept headers as: >> http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html > > Note that the extension parameter here is for the header, not the mimetype. > The BNF allows the accept-extension rule ONLY after the mandatory q value > in accept-params. I have to confess to having overlooked the accept-extension rule, as there wasn't an example of usage in that document. Can you give us an example as to how that is used? >> Which means using Accept-Encoding in this way is potentially problematic, >> but Accept does have provision that would make this use legitimate. > > Like mime types, content encodings also have a registry. > See: http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html section 3.5 > > Basically, there are two routes to avoid breaking the rules, neither easy: > > 1. Register new Mime Types for every packaging format. > > 2. Use an x- header and eventually write an RFC to standardize it. > > We looked at this in both SRU (eg what it would take to have a wrapper > format and an internal format: SRU vs Atom, wrapping Simple DC vs METS) > and in conjunction with the digital format registry for preservation > purposes. > > So my recommendation would be to go with a new x- header, and if/when the > community has implemented it, take it to an RFC. It's looking like a separate header is the way to do this, with the following couple of options immediately standing out: Accept-Features (or X-Accept-Features if it isn't sufficiently official) X-Packaging X-Accept-Packaging (which I just made up for the purposes of this discussion) Some comments on these: Accept-Features Having looked at the document [1] (thanks Graham (K)) it looks like it would give us the leeway that we need to describe requirements while ensuring that Graham (T)'s concerns (which I share) about matching up package format requirements with mimetypes would be dealt with. On the other hand, this document is 12/13 years old and the header has not made it into the HTTP content negotiation documentation and is significantly different in format to all the other Accept- headers. It could also be a substantial effort for servers to implement the full requirements of this header. X-Packaging I'm against using this in this way as it is already used to alert the server during POST as to the package format that is being supplied. The format of the header for content negotiation would have to be totally different to this usage: a list of package formats and q values for example, rather than a single definitive URI. I see scope for confusion. X-Accept-Packaging Given my concerns about X-Packaging and the comments above about Accept-Feature, perhaps there is a middle ground that we can define which does something more minimal with just mimetypes, package formats and q values in a way similar to having a mimetype that has added parameters. For example: Accept: application/zip; q=1.0, application/atom+xml;type=entry;q=0.8 X-Accept-Packaging: application/zip;{package=METSDSpaceSIP};q=1.0, application/atom+xml;type=entry;{package=AtomSIP};q=0.8 Or some other suitably neat and unambiguous serialisation which is in line with how the other Accept- headers work and also gives us the information we want in a totally definitive mimetype<->package format way. This could be supplied alongside the usual Accept header so that clients which can't generate the X-Accept-Packaging header can fall back easily to the usual content negotiation route. Thoughts? Cheers, Richard [1] http://www.ietf.org/rfc/rfc2295.txt ------------------------------------------------------------------------------ Special Offer-- Download ArcSight Logger for FREE (a $49 USD value)! Finally, a world-class log management solution at an even better price-free! Download using promo code Free_Logger_4_Dev2Dev. 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