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Re: What changes to Web Messaging spec are proposed? [Was: Re: Using ArrayBuffer as payload for binary data to/from Web Workers]

From: Jonas Sicking <jonas@sicking.cc>
Date: Tue, 21 Jun 2011 19:42:50 -0700
Message-ID: <BANLkTinzrzdMxNR6m60ov=pz87U-3XeWig@mail.gmail.com>
To: Ian Hickson <ian@hixie.ch>
Cc: Andrew Wilson <atwilson@google.com>, Kenneth Russell <kbr@google.com>, David Levin <levin@chromium.org>, Arthur Barstow <art.barstow@nokia.com>, Glenn Maynard <glenn@zewt.org>, Dmitry Lomov <dslomov@google.com>, ben turner <bent.mozilla@gmail.com>, "public-webapps@w3.org" <public-webapps@w3.org>, Travis Leithead <Travis.Leithead@microsoft.com>
On Tue, Jun 21, 2011 at 11:07 AM, Ian Hickson <ian@hixie.ch> wrote:
> On Tue, 21 Jun 2011, Jonas Sicking wrote:
>> On Tue, Jun 21, 2011 at 12:39 AM, Ian Hickson <ian@hixie.ch> wrote:
>> > On Mon, 20 Jun 2011, Jonas Sicking wrote:
>> >>
>> >> If data appears both in the .ports array and the .data property, then
>> >> people will be tempted to create protocols which only work if the
>> >> array buffer is transferred, i.e. if the receiver only looks in
>> >> .ports. I.e. people will likely end up with equally ugly solutions
>> >> like:
>> >>
>> >> postMessage('vend-reply', [createClone(arraybuffer)]);
>> >
>> > Yeah, that's a very good point.
>> >
>> > It really seems like part of the problem here is that ports and
>> > arrayBuffers are quite different (in that one will always want to be
>> > transferred, but for the other you might want to sometimes transfer
>> > and sometimes not).
>>
>> I don't think we should worry too much about suboptimal syntax when
>> transferring ports. We can always add more syntax sugar later if people
>> think it's annoying. Better keep the initial version simple than to try
>> to guess which use cases will be the most common ones.
>
> Keeping it simple is one of the reasons I don't want to require that
> authors list every port twice in order to send a port. (Sending a port is
> intended to be a common pattern, at least if the API is used as intended.)
>
>
> So the current situation is as follows:
>
>  - we want to be able to send structured data that is cloned.
>  - we want the structured data to be able to contain arraybuffers and ports.
>  - we want it to be possible for the author to opt-in to transferring
>   specific arraybuffers rather than just cloning them.
>  - we want the author to have to explicitly list the ports that are to be
>   transferred because it would be easy to accidentally transfer one
>   otherwise, and that would cause hard-to-find bugs (e.g. if the data
>   being cloned happened to contain debugging information that happened to
>   contain a reference to a port).
>  - we don't want to require that authors list ports twice in the simple
>   case, since that's quite ugly
>  - we don't want authors to design APIs where arraybuffers can only be
>   transferred and not copied; the receiving side should not be able to
>   influence the sending side's decision as to whether to clone or
>   transfer arraybuffers.
>
> How about we just make postMessage() take the object to clone in the first
> argument, an array of objects to transfer in the second; on the other
> side, the author receives the object cloned, with anything listed in the
> array and in the structured data being transferred instead of cloned, and,
> in addition, in the event.ports array, a list of the ports that were given
> in the transfer array.
>
> This has the advantage of being completely backwards-compatible.
>
> So for example, on the sending side:
>
>   postMessage(['copied', copiedArrayBuffer, transferredArrayBuffer,
>                transferredPort1], // could be an object too
>               [transferredArrayBuffer, transferredPort1,
>                transferredPort2]);
>
> ...on the receiving side, the event has
>
>   data == ['copied', newCopiedArrayBuffer, newTransferredArrayBuffer,
>            newTransferredPort1];
>   ports == [newTransferredPort1, newTransferredPort2];
>
> It's not going to win any design awards, but I think that boat has sailed
> for this particular API anyway, given the contraints we're operating under.

I still think it's a bigger API than what I'm proposing without adding
much value. For example I suspect people will not realize that they
can transfer ports in the main data graph when they want to transfer
them along with metadata about the port.

I'm also not sure that backwards compatibility is a big problem given
that we're not asking any of the current implementations to modify any
of their existing API (only remove parts of it whenever they feel that
that can be done without breaking content).

But it is an improvement over previous proposals.

/ Jonas
Received on Wednesday, 22 June 2011 02:43:56 GMT

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