Managing servers with Puppet

By: on May 27, 2008

So, I’ve been working on this project recently. In this project there’s no use of version control – in fact, we don’t even have staging or development environments. All changes were just made to the live server, in an ad hoc way, by a variety of people. And inevitably we’ve ended up in a situation where we don’t fully understand what we have, and we’re scared to change anything in case something else breaks.

Sounds awful, right? Chances are that you’ve been working on this project too. Of course I’m referring to system administration. (In fact I’m referring to Unix system administration since we don’t have many Windows servers; the rest of this post will be rather Unix-centric.)

Clearly this situation is not right. Over the years we’ve made a few attempts to improve things – the first attempt consisted of putting /etc in version control. This is clearly a step forward – you can see who’s changed what, when – but there are still several problems. For a start, anything that happens outside /etc is not recorded – so we don’t know which packages are installed or if any changes have been made to the rest of the system. For another thing, there’s no mechanism for abstraction – if you need to make one change to all your servers there’s no sensible way to express that.

So the second attempt consists of writing scripts to build servers, and putting them in version control instead. This is another step forward – look ma, we can do package management! We can do anything! There are still problems though. We can’t just write a script which runs once to build the server; things change and we need to use the same mechanism for maintenance. So although it’s a good feeling to be programming again, we don’t really want to use an imperative language (or even really a functional one) because it’s a pain to make our scripts idempotent (and worse still, to test that they are). And while abstraction is possible in this environment, it’s not exactly convenient.

So we arrive at our third attempt to fix things, and I have a good feeling about this one. I’ve recently been starting to use Puppet, and I think it solves all the problems mentioned so far, as well as some others. It allows all your system administration to be done in one place and automatically replicated out to all your servers. It uses a declarative language that’s designed for idempotence – you describe where you want to be, not how to get there. Abstraction between servers is really easy and quite powerful.

I’ve been going around the office with a messianic glint in my eye for the last couple of days, so I’ll try really hard to think about the negatives. The language has some magic quoting rules I’m not entirely happy with. The project is new-ish and the documentation could be better (although it could certainly be a lot worse too). But that’s about it.

I will say this though: after only a couple of days of work, I’ve got to a state where we can roll out some common foundations to all our servers (use LDAP authentication, use our Debian mirror, automatically apply security updates), and where a couple of our servers are entirely Puppetised – it’s a one liner to recreate them from our Mercurial repository.

That’s a really nice place to be. And you know what? It wasn’t even that hard.


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