Stupid Puppet Trick: Poor Man’s Undo

If I apply a set of classes to a puppet client, I may need to roll back those classes’ changes later. Granted, I could just edit out those classes, reformat the system, and rebuild it from scratch, but there may be times when I want to undo my changes in a more granular fashion. So here’s my latest revelation (which is undoubtedly documented elsewhere already, but I didn’t find anything sufficiently simple last time I looked).

Along the lines of Puppet Best Practice 2.0, I’m trying to compartmentalize all my building blocks into separate modules, even the really simple ones that just install a package or two. So here’s the contents of a trivial hello module for Debian:

# /etc/puppet/modules/hello/manifests/init.pp
class hello {
  package { "hello":
    ensure => latest;
  }
}

Nothing complicated here: install this package via the default package provider. Add an include hello to my node’s manifest, and the hello package gets installed.

Now for the “undo” class:

# /etc/puppet/modules/hello/manifests/disabled.pp
class hello::disabled inherits hello {
  Package["hello"] {
    ensure => purged
  }
}

And this is neat. Properly written, the disabling class can undo all the changes made by the parent class. Modify my node’s manifest from include hello to include hello::disabled and I can switch that class on and off like, well, a switch.