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Re: Fwd: Re: Can we use TCP backpressure instead of paging?

From: Stian Soiland-Reyes <soiland-reyes@cs.manchester.ac.uk>
Date: Wed, 11 Jun 2014 07:41:40 +0100
Message-ID: <CAPRnXtnor2iPta8w-uN-MWt=6Po7L97Ewt=EjzwL0ACpGAOwvg@mail.gmail.com>
To: Sandro Hawke <sandro@w3.org>
Cc: Andreas Kuckartz <a.kuckartz@ping.de>, LDP <public-ldp@w3.org>
Simple principles that seems reasonable, using existing infrastructure.

However it would come at a large cost for the server to maintain a large
number of open TCP sockets. Paging has the advantage of allowing a series
of smaller, independent requests for different subsets of the resource (and
as we've discussed at the cost of ambiguous results on collection
modifications), and so could be served by multiple backend servers
(including caches and proxies) without the client noticing.

A TCP stream would also fall apart as a mobile client moves from 3G to
WiFi, with no good way to "continue". (Or would you combine this with byte
level HTTP ranges?)

In fact, this technique is now used for streaming video, by basically
having a playlist of a series of static, smaller video snippets which are
distributed on Content Delivery Networks using regular HTTP GET (and so can
be cached, proxied, etc) and joined together in the client.

http://en.m.wikipedia.org/wiki/HTTP_Live_Streaming
http://en.wikipedia.org/wiki/MPEG-DASH

If you were to take this analogy into LDP, then one could request for a
paging content type, and get back a list of IRIs for pages which can then
be requested at the client's pace and choice.

If a server insists on paging (collection is massive), but the client can't
Accept it, then it is a simple case of HTTP 406 Not Acceptable, which
should be more reasonable then giving only half the collection,  without
the client understanding that there is paging.
On 10 Jun 2014 19:05, "Sandro Hawke" <sandro@w3.org> wrote:

> On 06/10/2014 01:34 PM, Andreas Kuckartz wrote:
>
>> -------- Original Message --------
>> To: Sandro Hawke <sandro@w3.org>
>> CC: Linked Data Platform WG <public-ldp-wg@w3.org>
>>
>> Thanks a lot for thinking outside the current box of paging. I have
>> looked at several paging approaches and do not really like any of them.
>> They contaminate the real data and/or seem to be unnecessarily complex
>> to implement.
>>
>
> You're welcome.     I did a little more investigation today, including
> writing a tiny node.js server that streams data and lets me see what what
> happens when I do things on the client.    This little bit of testing was
> encouraging.
>
> I also spoke to a few people at lunch here at the W3C AC meeting and again
> no one saw any serious problem.
>
> No clue yet how likely it is that browser vendors might implement an
> extension to xhr to allowed WebApps to use this properly.
>
>         -- Sandro
>
>  Cheers,
>> Andreas
>> ---
>>
>> Sandro Hawke:
>>
>>> Thinking about paging a little, I'm really wondering if one isn't better
>>> off using TCP backpressure instead of explicit paging.  It would have
>>> the huge advantage of requiring little or no special code in the client
>>> or the server, if they already implement high-performance streaming.
>>> (I started thinking about this because as far as I can tell, if we want
>>> to allow LDP servers to initiate paging, we have to require every LDP
>>> client to understand paging.   That's a high bar.   If you want to
>>> respond to that particular point, please change the subject line!)
>>>
>>> The key point here is that TCP already provides an elegant way to handle
>>> arbitrarily large data flows to arbitrary small devices on poor
>>> connections.    If someone knows of a good simple explanation of this,
>>> please send along a pointer.   My knowledge is largely pre-web.
>>>
>>> In web software we often to think of HTTP operations as atomic steps
>>> that take an arbitrary long time.   With that model, doing a GET on a
>>> 100G resource is pretty much always a mistake.  But nothing in the web
>>> protocols requires thinking about it that way.   Instead, one can think
>>> of HTTP operations as opening streams which start data flowing.
>>>
>>> In some situations, those streams will complete in a small number of
>>> milliseconds, and there was no advantage to thinking of it as a stream.
>>>    But once you hit human response time, it starts to make sense to be
>>> aware that there's a stream flowing.     If you're a client doing a GET,
>>> and it's taking more than maybe 0.5s, you can provide a better UX by
>>> displaying something for the user based on what you've gotten so far.
>>>
>>> What's more, if the app only needs the first N results, it can read the
>>> stream until it gets N results, then .abort() the xhr.   The server may
>>> produce a few more results than were consumed before it knows about the
>>> .abort(), but I doubt that's too bad in most cases.
>>>
>>> The case that's not handled well by current browsers is pausing the
>>> stream.   In theory, as I understand it (and I'm no expert), this can be
>>> done by simply using TCP flow control.   A non-browser app that stops
>>> reading data from its socket will exert backpressure, eventually
>>> resulting in the process writing data finding the stream's not ready for
>>> writing.   My sense is that can and does work rather well in a wide
>>> variety of situations.
>>>
>>> Unfortunately, as I understand it, this doesn't work in WebApps today,
>>> because the browser will just keep reading and buffering until it runs
>>> out of VM.   If instead xhr (and websockets) had a limit on how much it
>>> would buffer, and webapps could set that (and probably it starts around
>>> 10M), then a WebApp that stopped consuming data would produce
>>> backpressure that would result in the server learning it can't send any
>>> more yet.     When the WebApp consumes more data, the server can start
>>> sending again.
>>>
>>> I'm very curious if there's any technical reason this wont work.   I
>>> understand there may be problems with some existing software, including
>>> browsers, not handling this kind of streaming.  But is there anything in
>>> the basic internet protocols and implementations that make this not
>>> work?     For instance, it may be that after blocking for a long time
>>> (minutes, waiting for the user to request more), restarting is too slow,
>>> or something like that.
>>>
>>> One possible workaround for the lack of browser support would be for
>>> servers to be a bit smarter and make some guesses.  For example, a
>>> server might say that requests with User-Agent being any known browser
>>> should be served normally for the first 10s, then drop to a much slower
>>> speed, consuming resources in the server, the net, and the client.
>>> WebApps that want to sidestep this could do so with a Prefer header,
>>> like Prefer initial-full-speed-duration=1s or 1000s.    At some point,
>>> when browsers allow webapp backpressure, those browser User-Agent
>>> strings could be exempted from this slowdown.
>>>
>>>       -- Sandro
>>>
>>
>>
>>
>>
>
>
Received on Wednesday, 11 June 2014 06:42:09 UTC

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