On Tue, Dec 8, 2015 at 8:21 AM, <villasc...@gmail.com> wrote: > Per https://wiki.python.org/moin/PythonVsPhp, "The vast majority of Python > Web applications are run in a separate process. This has some important > implications." > > From a PHP background, guess I just don't understand the concept of "separate > processes". What does this mean, how is it implemented, and what are the > "important implications"?
This is talking about how PHP code is generally run inside the server's process. I'll use Apache as my example, as that's what I have handy. There are a couple of ways that Apache can run PHP code. One is spawning a new process for every page request, and using CGI mode; and the other is a thing called mod_php, which runs PHP as a module inside the Apache process. The first one gives great isolation between pages, but it's horribly slow (every page request has to start a process - cheap on Unix, expensive on Windows - and fully initialize PHP, which is expensive everywhere); the second fixes the performance problem, at the cost of maintaining hidden state in the Apache process. Most PHP web sites out there are using (the equivalent of) mod_php. Coupled with features like persistent database connections (another way of improving performance at the cost of isolation), or changes to PHP error handling configs, this can create *very* debug-hard problems that depend on the exact distribution of work among Apache processes. This becomes even worse if you have multiple PHP web sites running in the same Apache instance; any one of them could interfere with any other. In contrast, Python code is run in a separate process from Apache. It openly and honestly does NOT reset its state between page requests (which you do need to be aware of; for instance, always commit/roll back your database transactions before returning), but there's no way for Python to affect Apache or vice versa. If you have two Python web sites on the same server, they are completely isolated from each other. As well as the massively-beneficial implications of this isolation, there are some less positive effects. One is that you need to have a complete Python process for each site you have active - or else pay a hefty startup penalty on first page load. This can cost a lot of memory, especially if you have a lot of isolated little web sites running. But the benefits of isolation are usually worth that cost; it's possible to run the web server as one user (say, www-data) and the application server as a different user (say, python-app), and then grant file system permissions differently to the different users. You could have two apps running in different versions of Python, or with different versions of third-party packages. You can put memory or CPU usage limits on the Python process, without worrying about the web server itself going down. It's a very different structure from the mod_php model, and worth understanding. Personally, I think the costs are worth it; but if you have a mostly-static web site with just a tiny amount of scripting in it, it might be better to follow a PHP model. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
