Puppet 3 Cookbook: An essential book if you have responsibility for servers. Real-world examples and code will give you Puppet expertise, allowing more control over servers, cloud computing, and desktops. A time-saving, career-enhancing tutorial , Second Edition

In this chapter you'll learn to write elegant Puppet manifests. By elegant in this context I mean readable, efficient, and consistent code that conforms to community usage.

We'll look at how to organize and structure your code into modules following community conventions, so that other people will find it easy to read and maintain your code. I'll also show you some powerful features of the Puppet language which will let you write concise, yet expressive, manifests.

If other people need to read or maintain your manifests, or if you want to share code with the community, it's a good idea to follow the existing style conventions as closely as possible. These govern such aspects of your code as layout, spacing, quoting, alignment, and variable references, and the official Puppet Labs recommendations on style are available at

http://docs.puppetlabs.com/guides/style_guide

node 'monitoring' inherits 'server' {
  include icinga::server
  include repo::apt
}

package { 'exim4':

package { exim4:

The Puppet Labs official style guide outlines a number of style conventions for Puppet code, some of which we've touched on in the preceding section. For example, according to the style guide, manifests:

Following the style guide will make sure that your Puppet code is easy to read and maintain and, if you're planning to release your code to the public, style compliance is essential. The puppet-lint tool will automatically check your code against the style guide. Here's how to use it:

One of the most important things you can do to make your Puppet manifests clearer and more maintainable is to organize them into modules.

A module is simply a way of grouping related things: for example, a webserver module might include everything necessary for a machine to be a web server: Apache configuration files, virtual host templates, and the Puppet code necessary to deploy these.

Separating things into modules makes it easier to re-use and share code; it's also the most logical way to organize your manifests. In this example we'll create a module to manage memcached, a memory caching system commonly used with web applications.

Choosing appropriate and informative names for your modules and classes will be a big help when it comes to maintaining your code. This is even truer if other people need to read and work on your manifests.

Templates are a powerful way of using embedded Ruby to help build config files dynamically and iterate over arrays, for example. But you can embed Ruby in your manifests directly without having to use a separate file by calling the inline_template function.

cron { 'chkrootkit':
  command => '/usr/sbin/chkrootkit >
    /var/log/chkrootkit.log 2>&1',
  hour    => inline_template('<%= @hostname.sum % 24 %>'),
  minute  => '00',
}

Arrays are a powerful feature in Puppet; wherever you want to perform the same operation on a list of things, an array may be able to help. You can create an array just by putting its contents in square brackets:

$lunch = [ 'franks', 'beans', 'mustard' ]

Puppet's if statement allows you to change the manifest based on the value of a variable or an expression. With it, you can apply different resources or parameter values depending on certain facts about the node, for example, the operating system, or the memory size. You can also set variables within the manifest which can change the behavior of included classes. For example, nodes in data center A might need to use different DNS servers than nodes in data center B, or you might need to include one set of classes for an Ubuntu system, and a different set for other systems.

Another kind of expression you can test in if statements and other conditionals is the regular expression. A regular expression is a powerful way of comparing strings using pattern matching.

if $::lsbdistdescription !~ /LTS/ {

Although you could write any conditional statement using if, Puppet provides a couple of extra forms to help you express conditionals more easily: the selector and the case statement.

The in operator tests whether one string contains another. Here's an example:

if 'spring' in 'springfield'

The preceding expression is true if the string spring is a substring of springfield, which it is. The in operator can also test for membership of arrays as follows:

if $crewmember in ['Frank', 'Dave', 'HAL' ]

When in is used with a hash, it tests whether the string is a key of the hash:

$interfaces = { 'lo' => '127.0.0.1', 
                'eth0' => '192.168.0.1' }
if 'eth0' in $interfaces {
  notify { "eth0 has address ${interfaces['eth0']}": }
}

Puppet's regsubst function provides an easy way to manipulate text, search and replace within strings, or extract patterns from strings. We often need to do this with data obtained from a fact, for example, or from external programs.

In this example, we'll see how to use regsubst to extract the first three octets of an IPv4 address (the network part, assuming it's a class C address).