[Python-ideas] PEP 3156 / Tulip question: write/send callback/future

[Laurens Van Houtven]

Also, ISTR that it's not always possible to consistently have that behavior
everywhere (i.e. have it in the first place or fake it where it's not
directly available), so it's of somewhat limited utility, since a protocol
can't actually rely on it existing. Most behavior that requires it is
generally implemented using IPushProducer/IPullProducer (i.e. the
pause/resume API Guido mentioned earlier).

There's been some attempts at work towards a better producer/consumer API
(e.g. supporting things like buffer changes, and generally just simplifying
things that seem duplicated amongst transports and consumers/producers)
called 'tubes', but I don't think any of that is ready enough to be a
template for tulip :)


On Fri, Jan 18, 2013 at 11:25 PM, Guido van Rossum <guido at python.org> wrote:

> On Fri, Jan 18, 2013 at 1:40 PM, Eli Bendersky <eliben at gmail.com> wrote:
> > On Fri, Jan 18, 2013 at 1:02 PM, Guido van Rossum <guido at python.org>
> wrote:
> >>
> >> On Fri, Jan 18, 2013 at 6:56 AM, Eli Bendersky <eliben at gmail.com>
> wrote:
> >> > I'm looking through PEP 3156 and the Tulip code, and either something
> is
> >> > missing or I'm not looking in the right places.
> >> >
> >> > I can't find any sort of callback / future return for asynchronous
> >> > writes,
> >> > e.g. in transport.
> >>
> >> I guess you should read some Twisted tutorial. :-)
> >
> >
> > Yes, I noticed that Twisted also doesn't have it, so I suspected that
> > influence.
> >
> >>
> >>
> >> > Should there be no "data_sent" parallel to "data_received" somewhere?
> >> > Or,
> >> > alternatively, "write" returning some sort of future that can be
> checked
> >> > later for status? For connections that aren't infinitely fast it's
> >> > useful to
> >> > know when the data was actually sent/written, or alternatively if an
> >> > error
> >> > has occurred. This is also important for when writing would actually
> >> > block
> >> > because of full buffers. boost::asio has such a handler for
> async_write.
> >>
> >> The model is a little different. Glyph has convinced me that it works
> >> well in practice. We just buffer what is written (when it can't all be
> >> sent immediately). This is enough for most apps that don't serve 100MB
> >> files. If the buffer becomes too large, the transport will call
> >> .pause() on the protocol until it is drained, then it calls .resume().
> >> (The names of these are TBD, maybe they will end up .pause_writing()
> >> and .resume_writing().) There are some default behaviors that we can
> >> add here too, e.g. suspending the task.
> >>
> >
> > I agree it can be made to work, but how would even simple "done sending"
> > notification work? Or "send error" for that matter? AFAIR, low-level
> socket
> > async API do provide this information. Are we confident enough it will
> never
> > be needed to simply hide it away?
>
> AFAIK the Twisted folks have found that most of the time (basically
> all of the time) you don't need a positive "done sending"
> notification; when the send eventually *fails*, the transport calls
> the protocol's connection_lost() method with an exception indicating
> what failed.
>
> --
> --Guido van Rossum (python.org/~guido)
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>



-- 
cheers
lvh
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