[Web-SIG] WSGI and asyncio (tulip)?

[Justin Holmes]

Fascinating and exciting.

Up until now, my go-to tactic for contain WSGI inside async has been
to use the WSGI container in twisted.web (this is how hendrix works:
https://github.com/hangarunderground/hendrix).

However, if we're talking about an actual flag in WSGI (like
wsgi.async_input), this is the probably the most significant
game-changer of all the proposals so far.

On Tue, Oct 14, 2014 at 9:47 AM, Guido van Rossum <guido at python.org> wrote:
> I am fascinated by the new WSGI - HTTP/2 discussions. I don't have much to
> contribute, because my own experience with web development is either very
> old (when CGI was new and exciting) or uses corporate frameworks where
> there's a huge set of layers between the app code and the external network
> (e.g. Google, Dropbox).
>
> I have strong emotional responses to some of the discussion topics (e.g. I
> feel that REMOTE_ADDR should represent the public IP address from which the
> request originated, not the internal address of a reverse proxy, since my
> use cases for it are all about blocking or rate-limiting specific clients,
> and I assume the network between the reverse proxy and the app server is
> secure) but I am sure there are already enough voices here and I trust that
> the sig will come up with the right answers (even if they override my gut
> feelings!).
>
> My most recent foray into web stuff was writing a small web crawler as an
> example for asyncio (PEP 3156, a.k.a. tulip). The crawler is written in
> Python 3 and the source code is here:
> https://github.com/aosabook/500lines/blob/master/crawler/crawling.py and it
> supports several advanced HTTP features: TLS, connection reuse, chunked
> transfer encoding, redirects (but not compression -- I think it would be
> straightforward to add, but the code would then exceed the 500 lines limit
> imposed by the book).
>
> Perhaps the main lesson I learned from writing this is how different yet
> similar web code looks when you use an asynchronous framework. Which makes
> me wonder -- can there be a future where you can write your web app as an
> asyncio coroutine?
>
> It looks like the WSGI protocol already supports asynchronously producing
> the response (using yield), and I don't think asyncio would have a problem
> with converting this convention to its own "yield from" convention. However,
> the situation is different for reading the request body. The app can read
> this in dribs and drabs if it wants to, by reading limited amounts of data
> from environ['wsgi.input'], but in an asyncio-driven world reading
> operations should really be marked with "yield from" (or perhaps just yield
> -- again, I'm sure an adaptor won't have a problem with this).
>
> I'm wondering if a small extension to the WSGI protocol might be sufficient
> to support this: the special environ variable "wsgi.async_input" could
> optionally be tied to a standard asyncio stream reader
> (https://docs.python.org/3/library/asyncio-stream.html#streamreader), from
> which bytes can be read using "yield from stream.read([nbytes])" or "yield
> from stream.readline()".
>
> Thinking a little more about this, it might be better if an async app could
> be a regular asyncio coroutine. In this case it might be better if
> start_response() were to return an asyncio stream writer
> (https://docs.python.org/3/library/asyncio-stream.html#streamwriter) and if
> it was expected to produce all its output by writing to this stream.
>
> Anyway, I think I'm getting ahead of myself, but I do think it would be nice
> if the next WSGI standard supported asyncio. For older Python versions it
> could then instead support Trollius (https://pypi.python.org/pypi/trollius),
> a backport of asyncio that supports Python 2.7 and 3.2 (and newer).
>
> --
> --Guido van Rossum (python.org/~guido)
>
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