[Chicago] Status of wsgi

Jordan Bettis jordanb at hafd.org
Fri Oct 12 01:23:10 CEST 2012


On 10/11/2012 03:00 PM, Tal Liron wrote:

> My life mission is to dispel this myth (especially because I used to
> believe it myself).

I'm not entirely sure what what myth you're dispelling.

I suppose it's entirely possible that a carefully built synchronous
thread model could be more efficient than an asynchronous epoll worker
process model. I'd think the details would depend on the relative
amounts of bookkeeping and overhead.

Seems irrelevant when discussing Python though, because you'll either be
spinning up and down a ton of interpreter instances or you'll be eaten
by a g(il)rue.

The criticism that tornado is poor at serving files to slow clients also
kinda silly given that nobody does that and I think it's stated pretty
early in the tornado documentation that it's not a good use case.

I was never particularly interested in tornado until my latest project.
I chose it because my old way of doing things fell down, not because
reddit said it was the bee's knees or something.

Anyway, my project has a map tile server builtin to it:
http://urbanfresco.com

Originally I just did the tile generation in the worker threads. But the
thing fell apart with only one user if there were a lot of tiles to be
generated, because all the workers would devote themselves to creating
tiles and the site would go unresponsive.

I reworked it so that the mapserver was a tornado requesthandler and the
rest of the site was stuck inside a tornado wsgi handler. I then
separated out the tilemaking to a set of daemons that the mapserver
could send jobs to.

If the tile is in the cache, the mapserver serves it directly, if it's
not, then the map server requests the tilemaker generate it, with a
callback, and then goes back to serving requests until it's done. I have
nginx in front of the whole lot, like so:

client <--> nginx --> static files
              |
              \-> tornado daemons --> mapserver --> tilemaker daemons
                      /                   \
          wsgi app <-/                     \-> tile cache

I suppose I could have made a faster system by doing the whole thing as
a multi-threaded C++ app... but given the comparative amount of work
involved I'm very happy with the results of using tornado.

The tornado and tile maker instances are started and stopped using a
script and pid files, just like normal unix daemons. Static files
including uploaded photos are served directly by nginx. I am serving the
tile images through python, so I suppose a slow client could tie up my
tornado processes. I was thinking that nginx would buffer the tiles for
me but I suppose I should make sure of that.

At any rate, my interest in tornado had nothing to do with slow clients.
I have nginx for that. My interest was in separating tile generation out
so it was no longer using up all my workers.



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