dimanche 22 février 2009

Augeas : state of art configuration management

If you are like me, you probably find that configuration management under Linux is a real mess. Each software has it's own configuration file syntax and semantic. Since every project has it's own needs and is developed by different teams, a global and standard repository for configuration is unlikely to ever work in the open source model. We saw some initiatives that worked, like LSB and Freedesktop, but such a shift for configuration file would require a huge coordination and agreement, and is unlikely to happen, and since configuration values are usually highly coupled in software, it makes it hard to change it.

There are few ways we can address the situation. You can choose to not manage at all your config files, you can do backups, use revision management software like subversion. You can also parametrize your configuration files with variables and generate them. You can also do some grep'ing and sed'ing on your config files to change values. Everything there tends to be cumbersome and error prone. But, there is better. If it was possible to have a parser for every config file out there and get in memory representation, change whatever you want and save it back, then you have the st-graal of file configuration management. This way, you can manage configuration at a high semantic level, not manipulate file lines but configuration elements.

Guess what, Augeas aims is exactly that! The concept is simple. For each file type, you write a lens that describe the format of the file. Then, the lens can be used to parse an instance of that config file and you get a tree in memory. You can access the tree and modify it. When you're done, the lens is used again to write back the file, including any modification you made to the tree. A lens is bidirectional, you don't have to specify how to read and write a file, only one description describe both directions.

Last year, at the Linux Symposium, I listen to the talk of David Lutterkort about the progress of the project. There is one limitation of Augeas today related to the type of files for which you can write a lens. For example, we can't write lens to process config files that have recursive structures. For example, Apache configuration has this kind of nested structure. Actualy, this is not possible, because the lens semantic doesn't allow to define recursive grammar.

So, I choosed to do that for my master. When this will be done, it will be possible to manage more file types with Augeas. I will post my findings here, thoughts and ideas on the project. Stay tuned!

1 commentaire:

Raphaël a dit…

Hi Francis,

Did you get in touch with the harmony team? They are the guys who do all the theoric reseach that allow Augeas to work.

=> http://www.seas.upenn.edu/~harmony/