[whatwg] Issues with Web Sockets API

[Drew Wilson]

On Mon, Jul 27, 2009 at 2:02 PM, Jeremy Orlow <jorlow at chromium.org> wrote:

> On Mon, Jul 27, 2009 at 1:44 PM, Drew Wilson <atwilson at google.com> wrote:
>
>>
>>
>> On Mon, Jul 27, 2009 at 1:36 PM, Alexey Proskuryakov <ap at webkit.org>wrote:
>>
>>>
>>> 27.07.2009, в 13:20, Jeremy Orlow написал(а):
>>>
>>>  I agree that this will help if the application sends data in burst mode,
>>>> but what if it just constantly sends more than the network can transmit? It
>>>> will never learn that it's misbehaving, and will just take more and more
>>>> memory.
>>>>
>>>> An example where adapting to network bandwidth is needed is of course
>>>> file uploading, but even if we dismiss it as a special case that can be
>>>> served with custom code, there's also e.g. captured video or audio that can
>>>> be downgraded in quality for slow connections.
>>>>
>>>> Maybe the right behavior is to buffer in user-space (like Maciej
>>>> explained) up until a limit (left up to the UA) and then anything beyond
>>>> that results in an exception.  This seems like it'd handle bursty
>>>> communication and would keep the failure model simple.
>>>>
>>>
>>>
>>> This sounds like the best approach to me.
>>>
>>>
>>> 27.07.2009, в 13:27, Drew Wilson написал(а):
>>>
>>>  I would suggest that the solution to this situation is an appropriate
>>>> application-level protocol (i.e. acks) to allow the application to have no
>>>> more than (say) 1MB of data outstanding.
>>>>
>>>> I'm just afraid that we're burdening the API to handle degenerative
>>>> cases that the vast majority of users won't encounter. Specifying in the API
>>>> that any arbitrary send() invocation could throw some kind of "retry
>>>> exception" or return some kind of error code is really really cumbersome.
>>>>
>>>
>>> Having a send() that doesn't return anything and doesn't raise exceptions
>>> would be a clear signal that send() just blocks until it's possible to send
>>> data to me, and I'm sure to many others, as well. There is no reason to
>>> silently drop data sent over a TCP connection - after all, we could as well
>>> base the protocol on UDP if we did, and lose nothing.
>>>
>>
>> There's another option besides blocking, raising an exception, and
>> dropping data: unlimited buffering in user space. So I'm saying we should
>> not put any limits on the amount of user-space buffering we're willing to
>> do, any more than we put any limits on the amount of other types of
>> user-space memory allocation a page can perform.
>>
>
> I agree with Alexey that applications need feedback when they're
> consistentiently exceeding what your net connection can handle.  I think an
> application getting an exception rather than filling up its buffer until it
> OOMs is a much better experience for the user and the web developer.
>

I'm assuming that no actual limits would be specified in the specification,
so it would be entirely up to a given UserAgent to decide how much buffering
it is willing to provide. Doesn't that imply that a well-behaved web
application would be forced to check for exceptions from all send()
invocations, since there's no way to know a priori whether limits imposed by
an application via its app-level protocol would be sufficient to stay under
a given user-agent's internal limits?

Even worse, to be broadly deployable the app-level protocol would have to
enforce the lowest-common-denominator buffering limit, which would inhibit
throughput on platforms that support higher buffers. In practice, I suspect
most implementations would adopt a "just blast out as much data as possible
until the system throws an exception, then set a timer to retry the send in
100ms" approach. But perhaps that's your intention? If so, then I'd suggest
changing the API to just have a "canWrite" notification like other async
socket APIs provide (or something similar) to avoid the clunky
catch-and-retry idiom.

Personally, I think that's overkill for the vast majority of use cases which
would be more than happy with a simple send(), and I'm not sure why we're
obsessing over limiting memory usage in this case when we allow pages to use
arbitrary amounts of memory elsewhere.


>
> If you have application level ACKs (which you probably should--especially
> in high-throughput uses), you really shouldn't even hit the buffer limits
> that a UA might have in place.  I don't really think that having a limit on
> the buffer size is a problem and that, if anything, it'll promote better
> application level flow control.
>
> J
>
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