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A Practical, Quick Introduction to Puppet

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A Practical, Quick Introduction to Puppet

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Enough about philosophical posts, let’s get started with some practical Puppet.

Manifests

Puppet Labs logoPuppet configuration files are called manifests, written in a ruby-like DSL. Puppet provides types and functions to manage typical resources (files, services, users, groups,…) and new ones can be defined through extensions called modules.

The standard types that can be used are listed in the Puppet reference. There is a cheat sheet available (pdf) with the main ones.

The resources are grouped in classes, that can later be easily reused.

class 'maven' {
  exec { 'maven-untar':
    command => 'tar xf /tmp/x.tgz',
    cwd     => '/opt',
    creates => "/opt/apache-maven-${version}",
    path    => ["/bin"],
  } ->
  file { '/usr/bin/mvn':
    ensure => link,
    target => "/opt/apache-maven-${version}/bin/mvn",
  }
  file { '/usr/local/bin/mvn':
    ensure  => absent,
    require => Exec["maven-untar"],
  }
  file { "${home}/.mavenrc":
    mode    => '0600',
    owner   => $user,
    content => template('maven/mavenrc.erb'),
    require => User[$user],
  }
}

Infrastructure IS code, for example we can specify that we want the openssh-server package installed

package { 'openssh-server':
  ensure => present,
}

Declarative model

Puppet uses a declarative model, where we define state, not process. We define that a service must be running and puppet will start it if not running, or do nothing if it already is.

service { 'ntp':
  name   => 'ntpd',
  ensure => running,
}

There is no scripting, we don’t make the service start, just define whether it should be running. This is key to understand how puppet works. A side effect is that variables can only be assigned once, so they are pretty much like constants.

Architecture

Puppet is arranged in a master – agent architecture.  The master serves the manifests and files, and the agents poll the master at specific intervals of time to get their configuration. The master does not push anything into the client.

Agents identify with the master using SSL, so the first time an agent tries to connect to the master, the agent certificate needs to be approved (in the default configuration), and that’s usually a source of problems.

File structure

Puppet configuration files are usually in /etc/puppet.

The main files in there are manifests/site.pp which defines the configurations, and the manifests/nodes.pp that defines how those configurations apply to the different nodes or agents, based on their hostname, generally, or other properties.

Site

class 'dave' {
  user { 'dave':
    ensure     => present,
    uid        => '507',
    gid        => 'admin',
    shell      => '/bin/zsh',
    home       => '/home/dave',
    managehome => true,
  }
  file {'/tmp/test1':
    ensure  => present,
    content => "Hi.",
  }
}

Nodes

node 'someserver.domain.com' {
  class { 'dave': }
}

More information

More information about types, resources, manifests, variables,… at learning puppet from PuppetLabs.

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