What's with "long running processes" ?

Bruno Desthuilliers bdesth.quelquechose at free.quelquepart.fr
Tue Sep 18 00:41:16 CEST 2007


walterbyrd a écrit :
> I understand that Python has them, but PHP doesn't.

Really ?

> I think that is because mod_php is built into apache, but mod_python
> is not usually in apache.

Language etc aside, what the difference between mod_php and mod_python 
(or mod_whatever) from apache's POV ?

> If  mod_python was built into apache, would
> python still have long running processes (LRP)?

mod_php is not more 'built into' apache than mod_python. Both are 
extension modules for apache. FWIW, when apache is configured to use 
mod_python, it starts a first python interpreter process at startup, 
then some more python "sub-interpreters" when needed, according to your 
configuration.

> Do LRPs have to do with a Python interpreter running all the time? Or
> is it something else?

The notion of "long running process" is nothing Python-specific. The 
most basic way to deliver dynamic content from a web server like apache 
is to use CGI - which means that on each http request, the web server 
starts a new (Python | Perl | PHP | Whatever) process. A more elaborate 
solution is to have another process running independently, and some way 
for both processes (web server and application) to communicate. This was 
is usually known as a "long running process".

> I also understand that LRPs are the reason that shared hosting is less
> common, and more expensive for python than php.

Yes and no. You can use Python with apache without a LRP - either with 
CGI or mod_python -  but the first solution is very inefficient, and the 
second is not really appropriate for shared hosting given how mod_python 
works. Also, while LRPs have a big advantage (you don't need to rebuild 
the whole world for each and every request), they have a couple 
drawbacks when it comes to shared hosting. The first one is that your 
host must provide some way to manage (run/stop/restart/monitor) the 
process. The second is that, while CGI scripts are (more or less) under 
control of the web server (IOW : they won't hang out forever), it's not 
true for LRPs. Which means that a program error (ie: an endless loop 
allocating memory on each iteration...) can potentially bring the while 
server down. Not that there are no ways to limit the risks and 
consequences, but this requires much more work for the host (and believe 
me, shared hosting administration is not simple...).

> The LRP have a major
> effect on how many users can packed onto a single server.

Not necessarily. From a purely technical POV, they are usually less 
wasteful wrt/ resources than CGI or mod_php.

> As to LRPs: does it matter if you're using mod_python, fastcgi, wsgi,
> scgi, cgi, or whatever?
 >
> Obviously, I am a little foggy about LRPs, can somebody help me out?

You can help yourself out, by installing apache on your own machine and 
learning the different ways to deploy web applications with it. While 
you won't have the same constraints as a shared hosting admin, you'll at 
least get a better understanding of the whole damn thing.






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