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RE: ACTION-187: extensibility and parsing

From: Davy Van Deursen <davy.vandeursen@ugent.be>
Date: Fri, 24 Sep 2010 06:56:33 +0200
To: "'Silvia Pfeiffer'" <silviapfeiffer1@gmail.com>, 'Philip Jägenstedt' <philipj@opera.com>
Cc: "'Media Fragment'" <public-media-fragment@w3.org>
Message-ID: <001501cb5ba4$dc478570$94d69050$@ugent.be>
Citeren Silvia Pfeiffer <silviapfeiffer1@gmail.com>:
> On Wed, Sep 22, 2010 at 9:12 PM, Philip Jägenstedt <philipj@opera.com>wrote:
>> As request, a short summary of the long standing issue of syntax, parsing
>> and how that relates to extensibility.
>> By extensibility I am not primarily talking about 3rd parties extending MF,
>> but about our own possibilities of updating the spec after MF 1.0. For the
>> purpose of discussion, assume that we want to add a dimension for filtering
>> the audio, e.g., freq=300,3000 to keep only the part of the audio that
>> corresponds (approximately) to human voice (300Hz-3000Hz).
>> How will implementations of MF 1.0 handle t=10,500&freq=300,3000 ? This is
>> the core point of disagreement, and the question is really about how MF 1.0
>> parsers should work. Leaving it undefined is not a good option, as the
>> history clearly shows. Two other options have been on the table:
>> 1. Require that parsing follow a strict ABNF syntax like the one we have.
>> Since freq is not part of the MF 1.0 syntax, parsing t=10,500&freq=300,3000
>> will fail and the whole fragment will be ignored, including t=10,500.
>> 2. Require that parsing follow an algorithm or a more forgiving ABNF
>> syntax. The concrete suggestion I've made is that the algorithm or syntax
>> should match how query strings work. That is, a list or key-value pairs is
>> formed by splitting the string on & and =. As a second step, that list is
>> traversed to match the keys against the dimensions and parsed according to
>> the ABNF syntax of each dimension. Crucially, unrecognized/invalid keys or
>> values are ignored. That means that in the above example, the time dimension
>> will keep working even if an unrecognized (to a MF 1.0 implementation) freq
>> dimension is used.
>> Note: Neither 1 or 2 are requirements on using any specific implementation
>> technique, only to behave *as if* you are, which still leaves plenty of room
>> for different approaches.
>> I strongly favor option number 2, and see these benefits:
>> * It works like query strings, just like one would expect from looking at
>> the syntax. The algorithm I've suggested is actually from testing query
>> string parsing in PHP, ASP, ASP.NET, CGI.pl and JSP, as reported earlier
>> on this list.
>> * It's simpler for implementors, as we won't have to implement everything
>> at once. This is likely what's going to happen, as the time dimension is
>> ready to implement, while the named dimension is still not clear how to
>> apply to e.g. a WebM or Ogg resource.
>> * It's better for extensibility, as adding new dimensions doesn't break all
>> existing implementations. Imagine if adding a new element to HTML would
>> cause pages to render completely blank in all existing browsers. Not even
>> XHTML is that strict.
>> Please comment, we need to reach some kind of consensus on this soon and
>> move on. If we can agree on what we want, we can then discuss how to change
>> the spec accordingly (algorithm or ABNF, etc...)
> I also strongly favor option number 2. I don't think anything else makes
> sense, actually, because we would fail to interoperate with  other schemes
> that use fragments and queries on media resources. Only name-value pairs
> that do not parse according to our ABNF will be ignored from the viewpoint
> of media fragments. They can be used by the browser or server for other
> purposes.

Same opinion here, option 1 doesn't seem to make sense. However, should we allow any unknown constructions in the URI fragment or
just key-value pairs with an unknown key? For example:
- t=10,500&freq=300,3000: should be a valid fragment IMO, as indicated by Philip's arguments;
- t=10,500&foo: is this a valid media fragment? According to Philip's parsing algorithm, I think it is not. From an extension point
of view, disallowing such a construction should be fine since we can rewrite this as t=10,500&foo=true if we want to obtain
key-value pairs. Note that I'm not in favor of allowing other things than key-value pairs, I just wanted to point out this case.

Best regards,


Davy Van Deursen

Ghent University - IBBT
Department of Electronics and Information Systems - Multimedia Lab
URL: http://multimedialab.elis.ugent.be/dvdeurse
Received on Friday, 24 September 2010 04:57:28 UTC

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