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Re: Question: how should addTrack work in relation to createOffer et. al.?

From: Harald Alvestrand <harald@alvestrand.no>
Date: Tue, 05 May 2015 19:31:00 +0200
Message-ID: <5548FE54.6070402@alvestrand.no>
To: Jan-Ivar Bruaroey <jib@mozilla.com>, Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>, "public-webrtc@w3.org" <public-webrtc@w3.org>
On 05/04/2015 10:29 PM, Jan-Ivar Bruaroey wrote:
> On 5/4/15 2:31 PM, Harald Alvestrand wrote:
>> On 05/04/2015 07:47 PM, Jan-Ivar Bruaroey wrote:
>>> User perception was not part of my analysis. I focused on two 
>>> inherently non-deterministic properties of the createOffer() 
>>> function (and its siblings):
>>>
>>>  1. Its observable behavior varies depending on whether something is
>>>     on the internal queue or not (action at a distance).
>>>
>>>  2. Iff nothing is on the queue (or in absence of a queue FWIW),
>>>     then a parallel thread will be running when the call returns,
>>>     making it impossible to reason about the effects of even
>>>     subsequent synchronous JS actions on things already underway.
>>>
>>
>> Does the JS programming model allow truly parallel threads?
>>
>> I have been told many times that the JS event loop requires that 
>> things must happen as if only one piece of JS is active at any one 
>> time, and that therefore most of the locks and critical section 
>> mechanisms we're used to in other languages are not needed in JS.
>
> This matches my understanding.
>
>> (I've always taken this to mean that any effect of true parallelism 
>> has to be hidden from JS, rather than imagining that it's truly 
>> single-threaded.....)
>
> You're right! Though, as I understand it, this is a matter of good 
> spec design rather than some implicit guarantee or assumption (e.g. 
> why we have "queue a task" everywhere).

I think it is actually a hard requirement.

Otherwise, stuff as simple as

    a = new ObjectWithHandlerForAsynchronousEvent():
    a.handler = function() {}

would never function reliably. People used to other languages who come 
to Javascript always recoil in horror and ask "but where's the critical 
section?" when they see code like this.
Received on Tuesday, 5 May 2015 17:31:31 UTC

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