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Re: Blob as URN was Re: [FileAPI] Latest Revision of Editor's Draft

From: Eric Uhrhane <ericu@google.com>
Date: Wed, 18 Nov 2009 11:36:37 -0800
Message-ID: <44b058fe0911181136r132f9268j92b3b265fdbc2eef@mail.gmail.com>
To: arun@mozilla.com
Cc: Web Applications Working Group WG <public-webapps@w3.org>
On Tue, Nov 17, 2009 at 6:25 PM, Arun Ranganathan <arun@mozilla.com> wrote:
> Eric,
>
>
>>    I recall you saying at TPAC that you wanted to keep the Blob
>> interface as small as possible, since it seemed likely to get used in
>> a lot of places.  I think that's an excellent goal, but of course,
>> having said that, I am immediately going to suggest that you add
>> something to it.
>>
>
>
> I'm definitely not averse to additions :)  Actually, I'm also following the
> discussion initiated by Maciej about BinaryData [2] with
> public-script-coord@w3.org.  Eventually, I believe ECMAScript will provide a
> Binary primitive (perhaps ByteArray), and I think Blob should expose that
> primitive.  This would be a natural extension of what I envision Blob to be
> used for.  I also envision natural streaming extensions to this API.
>
>>   How would you feel about exposing a way to produce a URN from a
>> Blob, instead of just getting one from a File?
>
> I'm not averse to it.  In fact, it was originally in the Blob interface
> (which at that time, was dubbed "FileData").  We moved it to the "File"
> interface since the understanding of use cases at the time was that " all
> URL consumers expect a full file." [3]   You've provided use cases that show
> that this isn't the case, and so we should revisit our earlier
> understanding.  More on your use cases below:
>
>> This seems likely to
>> have wide-ranging uses.  Pretty much anywhere you have a blob of data,
>> you might want to hand it off to the browser, even if it didn't come
>> from, or wasn't, a single user-supplied file.  Here are a few use
>> cases, but I'm sure more won't be hard to come up with:
>>
>> * Viewing a single chapter of a book in a frame.
>> * Slicing one episode out of a DVD and handing it to the video tag, so
>> that the player controls start and end at the episode boundaries.
>> * Analogous to the game-asset archive I mentioned at [1], one might
>> pack a number of small files together to speed download [using only
>> HTTP compression], then parse them apart on the client.  Picture a
>> Picasa client written in the web browser; it's got to handle maybe
>> 10000+ thumbnails, and putting each in a separate file would be
>> terribly inefficient.  Pulling down a tarfile would be a lot quicker.
>>
>>
>
> I can understand why you'd want *partial* data exposed through a URL, and
> why your API may "force" a type on the partial data.  Question: would a
> fragment identifier scheme [4] address any of these use cases, or is this
> completely orthogonal to the use cases you envision?  I ask because you
> envision a "chapter within a frame" but I'm not sure what the frame data
> structure is.

Ah--I think I wasn't clear here.  I just pictured taking a file,
chopping out a chapter by, say, byte offset, and writing it into an
iframe for display.  I don't think a fragment identifier is powerful
enough, even if we spec out how to overload it.  For example, say you
wanted to chop a chapter out of an HTML document *and* scroll your
iframe to a specific page of that chapter?

One can also imagine use cases in which the Blob is constructed
completely from scratch by JavaScript, in which case there's no File
at all.

>> it
>> would seem natural to me do to something like this:
>>
>>    interface Blob {
>>      ...
>>      DOMString getURN(in DOMString mediaType,
>>           [Optional] in DOMString contentDisposition,
>>           [Optional] in DOMString name);
>>    };
>>
>> Given that a File that one gets from the user will still tell you its
>> name and detected mediaType, and can have a constant urn, there seems
>> to be no conflict in leaving the File interface as-is and adding
>> something like getURN to Blob.  On the off chance that you want to
>> override the detected mediaType for a file, force a contentDisposition
>> of attachment, or change the name, you might still use getURN there as
>> well.
>>
>
> To be clear: you want the File object's URN capabilities to inherit from
> Blob, and not be separate, correct?  Thus, each Blob has an affiliated URN,
> and when a Blob is a File, it uses the Blob's getURN method?

No, I picture both being there, although I'm open to discussion about
it.  The getURN method forces the user to specify how the data is to
be presented to the UA via the URN created.  In contrast, the urn
property of File is inherently based on what the UA knows about the
file, and the user need not specify anything in order to use it.

Hmm...I guess there's no reason that we couldn't make the mediaType
parameter optional as well, so that in the case of a File getURN could
just return what the UA thought was appropriate.  That makes me
nervous for Blob data, though...perhaps we should spec it to throw in
that case?

> Can you explain what a "contentDisposition" is a bit better?  Can you write
> some psuedo-code showing how contentDisposition is used, perhaps to flesh
> out the above use cases?

Sure.  Let's say that you're writing an offline version of GMail.
You've got a File referring to an image attachment, and you want to
offer links through which the user can either view or download it.

var imageFile = ... // grab the File from somewhere
var viewingLink = imageFile.urn;
var downloadLink = imageFile.getURN(imageFile.type, "attachment",
imageFile.name);

     Eric

>
> -- A*
>>
>> [1]
>> http://lists.w3.org/Archives/Public/public-webapps/2009OctDec/0424.html\
>>
>
> [2]
> http://lists.w3.org/Archives/Public/public-script-coord/2009OctDec/0093.html
>
> [3] http://lists.w3.org/Archives/Public/public-webapps/2009JulSep/0609.html
>
> [4] http://lists.w3.org/Archives/Public/public-webapps/2009JulSep/0587.html
>
>
Received on Wednesday, 18 November 2009 19:37:36 GMT

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