Postmodern Sysadmin

A blog about servers and junk

Writing Purgable Puppet Code

Whenever possible, I try to write Puppet code that is purgable and “Comment Safe”. That is not a very good description. What I mean is, Puppet code that removes resources from a system when the corresponding Puppet code is “Commented” out of a manifest. Lets look at a few examples.

Example: Managed Sudo

Lets say you used this popular sudo module with the following params:

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class { 'sudo':
  purge => true,
}

Great start. All future sudo::conf blocks you write will automatically disappear from the host:

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sudo::conf { 'web':
   source => 'puppet:///files/etc/sudoers.d/web',
 }

# Commenting out for now. Automatically is purged from the server
# sudo::conf { 'admins':
#   priority => 10,
#   content  => "%admins ALL=(ALL) NOPASSWD: ALL",
# }

Good stuff. Do this.

Example: Managed Firewall

How about another example with the Puppetlabs Firewall module?

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# Automatically remove rules that are not declared
resources { "firewall":
  purge => true
}

# Production needs 111 open
firewall { '111 open port 111':
  dport => 111
}
# Tried this but didn't work. Commenting out for now
# Automatically removed from the server when I commented it out
# firewall { '112 open port 112':
#   dport => 112
# }

The Point?

The point here is that we should encourage a culture of purging. Having resources get automatically purged when you comment them out from puppet is great.

Of course, this is obsoleted in the short-lived world of docker or possibly Amazon EC2. But for those engineers who work on long lived servers, this prevents cruft.

Going Further: Purging Packages

I want to purge packages. If someone installs a package not controlled by Puppet, I want puppet to purge it. Crazy I know.

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package { 'apache': ensure => installed }

# No longer using php
# But puppet leaves this behind!
# package { 'php5': ensure => installed }

Of course puppet will leave the package behind. I should be doing ensure => purged right?

But what if the package is deep within nested classes or simply manually installed?

Some day I would like to get to the point where I at least get notified when puppet detects packages that don’t need to be there. I’m open to suggestions on how to do this.

Going Further: Purging /etc/

Most of the time stale configuration leftover in /etc/ causes no harm.

But what about cron jobs in /etc/cron.d? I would love to purge them, but there are non-puppet controlled things installed by system packages. If everything was a puppet module this could eventually be achieved, but it would be too hard to keep in sync with upstream package changes.

Purgin on a per-app basis with things like sensu, apache, and sudo are a great start.

Crossing the Line: Purging /var/lib/mysql

Seems like if you asked puppet to install mysql databases, and then commented them out, you would not want puppet to purge them.

The subtle difference here might be the difference between configuration and data.

Conclusion

Whenever possible I try to purge => true on whatever I can. I would like to see this as the default in new puppet modules.

Someday I would like us to purge more than just files and iptables rules.