Python web hosting: Why are server restarts necess

问题:

We currently run a small shared hosting service for a couple of hundred small PHP sites on our servers. We'd like to offer Python support too, but from our initial research at least, a server restart seems to be required after each source code change.

Is this really the case? If so, we're just not going to be able to offer Python hosting support. Giving our clients the ability to upload files is easy, but we can't have them restart the (shared) server process!

PHP is easy -- you upload a new version of a file, the new version is run.

I've a lot of respect for the Python language and community, so find it hard to believe that it really requires such a crazy process to update a site's code. Please tell me I'm wrong! :-)

回答1:

Python is a compiled language; the compiled byte code is cached by the Python process for later use, to improve performance. PHP, by default, is interpreted. It's a tradeoff between usability and speed.

If you're using a standard WSGI module, such as Apache's mod_wsgi, then you don't have to restart the server -- just touch the .wsgi file and the code will be reloaded. If you're using some weird server which doesn't support WSGI, you're sort of on your own usability-wise.

回答2:

Depends on how you deploy the Python application. If it is as a pure Python CGI script, no restarts are necessary (not advised at all though, because it will be super slow). If you are using modwsgi in Apache, there are valid ways of reloading the source. modpython apparently has some support and accompanying issues for module reloading.

There are ways other than Apache to host Python application, including the CherryPy server, Paste Server, Zope, Twisted, and Tornado.

However, unless you have a specific reason not to use it (an since you are coming from presumably an Apache/PHP shop), I would highly recommed mod_wsgi on Apache. I know that Django recommends modwsgi on Apache and most of the other major Python frameworks will work on modwsgi.

回答3:

Is this really the case?

It Depends. Code reloading is highly specific to the hosting solution. Most servers provide some way to automatically reload the WSGI script itself, but there's no standardisation; indeed, the question of how a WSGI Application object is connected to a web server at all differs widely across varying hosting environments. (You can just about make a single script file that works as deployment glue for CGI, mod_wsgi, passenger and ISAPI_WSGI, but it's not wholly trivial.)

What Python really struggles with, though, is module reloading. Which is problematic for WSGI applications because any non-trivial webapp will be encapsulating its functionality into modules and packages rather than simple standalone scripts. It turns out reloading modules is quite tricky, because if you reload() them one by one they can easily end up with bad references to old versions. Ideally the way forward would be to reload the whole Python interpreter when any file is updated, but in practice it seems some C extensions seem not to like this so it isn't generally done.

There are workarounds to reload a group of modules at once which can reliably update an application when one of its modules is touched. I use a deployment module that does this (which I haven't got around to publishing, but can chuck you a copy if you're interested) and it works great for my own webapps. But you do need a little discipline to make sure you don't accidentally start leaving references to your old modules' objects in other modules you aren't reloading; if you're talking loads of sites written by third parties whose code may be leaky, this might not be ideal.

In that case you might want to look at something like running mod_wsgi in daemon mode with an application group for each party and process-level reloading, and touch the WSGI script file when you've updated any of the modules.

You're right to complain; this (and many other WSGI deployment issues) could do with some standardisation help.