On 2011-01-07 08:10:43 -0800, P.J. Eby said:
At 12:39 AM 1/7/2011 -0800, Alice BevanMcGregor wrote:
:: Image scaling would benefit from multi-processing (spreading
the>load across cores). Also, only one sacle is immediately
required>before returning the post-upload page: the thumbnail. The
other>scales can be executed without halting the WSGI application's
return.
:: Asset content extraction and indexing would benefit from>threading,
and would also not require pausing the WSGI application.
In all these cases, ISTM the benefit is the same if you future theWSGI
apps themselves (which is essentially what most current asyncWSGI
servers do, AFAIK).
Image scaling and asset content extraction should not block the
response to a HTTP request; these need to be 'forked' from the main
request. Only template generation (where the app needs to effectively
block pending completion) is solved easily by threading the whole
application call.
:: Long-duration calls to non-async-aware libraries such as DB access.
The WSGI application could queue up a number of long DB queries,>pass
the futures instances to the template, and the template could>then
.result() (block) across them or yield them to be suspended and>resumed
when the result is available.
:: True async is useful for WebSockets, which seem a far
superior>solution to JSON/AJAX polling in addition to allowing real
web-based>socket access, of course.
The point as it relates to WSGI, though, is that there are plenty
ofmature async APIs that offer these benefits, and some of them
(e.g.Eventlet and Gevent) do so while allowing blocking-style code to
bewritten. That is, you just make what looks like a blocking call,
butthe underlying framework silently suspends your code, without
tyingup the thread.
Or, if you can't use a greenlet-based framework, you can use a
yield-based framework. Or, if for some reason you really wanted to
write continuation-passing style code, you could just use the raw
Twisted API.
But is there really any problem with providing a unified method for
indication a suspend point? What the server does when it gets the
yielded value is entirely up to the implementation of the server; if it
(the server) wants to use greenlets, it can. If it has other
methedologies, it can go nuts.
Even if you've already written a bunch of code using raw sockets and
want to make it asynchronous, Eventlet and Gevent actually let youload
a compatibility module that makes it all work, by replacing the socket
API with an exact duplicate that secretly suspends your code whenever a
socket operation would block.
I generally frown upon magic, and each of these implementations is
completely specific. :/
- Alice.
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