ModServer is an experimental mod_python driver, just like the cgi and wsgi drivers, etc., for Mapnik.
Mainly it was developed because Mapnik was missing a mod_python-based WMS server. Internally, it's extremely similar to cgiserver.py, and externally, it is identical - as far as WMS compatibility, etc.
Modserver isn't meant to be a replacement for TileCache - it is quite different in a few ways:
- Modserver, like cgiserver, etc., supports WMS, not WMS-C
- Modserver doesn't have any caching mechanism built in. We're using it with a lightweight PHP cache layer - something Python-based or something that uses memcached is a possibility but right now just plug in what you have.
Currently the code is available on a Git ticket here. A well-polished patch with error handling is in the works.