10 Jan 09 Experimenting with mod_wsgi
I’m getting a bit closer to wanting to deploy some of the work I have done on the new site in order to get feedback and find bugs. I’ve already got The Madeira’s site running under mod_python, and it seems like a bit of a pain to get another site running under mod_python. Or, I should say, I don’t fully understand how to do it. 
First a bit of background on the server I am renting. It is running Fedora Core 6 and has Plesk installed. Plesk is a web-based application that manages the server for you. I click around in it to add domains, sub-domains, configure the mail service, etc. Plesk auto-generates the apache configuration files based on your settings.
I’ve been reading good things about an alternate way to deply Python web applications: mod_wsgi. So a couple of days ago I decided to try it out. Here is a quick sketch of what happened.
It was easy enough to grab the source and to do the usual “make” and “make install” to build it. However, I did notice right away that (via Graham Dumpleton’s excellent documentation) that the mod_wsgi.so module was linking against the static version of my Python library instead of the dynamic one. Of course if I am going to have multiple processes running a Python interpreter, I am going to want them to share the code to avoid bloat. I did some poking around on my server and found the correct Python shared library, and I put a symlink to it inside my python/config directory. Completely rebuilding mod_wsgi at that point seemed to resolve that issue.
I then configured a subdirectory on the virtual host running this site to be served by the sample wsgi application. This worked straight away when mod_wsgi was configured to run in embedded mode. However the real goal is to run in daemon mode, in which separate processes are launched to run the application, and apache defers to them. You can also run these processes as a different user than apache for added security.
When I configured mod_wsgi for daemon mode I ran into problems. Fortunately I received excellent help from Graham Dumpleton on the mod_wsgi user’s list. First of all I was still running mod_python, and this was probably still using the static Python library. I temporarily disabled it and Graham had me change the apache log level and examine the logs. For whatever reason, the WSGI daemon processes were not inheriting the apache user and group, instead they were using -1/-1 for uid/gid. When I added configuration settings to explicitly set the user and group for these WSGI processes to run under, everything was fine (if mod_python was out of the picture that is).
After a bit of closer examination, it turns out that in my main http.conf file I have this:
Include conf.d/*.conf User apache Group apache
So for whatever reason, user and group ID’s were not set when my conf.d/mod_wsgi.conf file was being processed.
So now I am poised to cut The Madeira site over to mod_wsgi and retire mod_python. I can then more easily create a new process to run a test site for SG101 2.0.
While I have no real runtime experience with mod_wsgi yet, I am still very impressed with the project and Graham Dumpleton’s extensive documentation and knowledge of apache.
