Book Image

Puppet 5 Essentials Third Edition - Third Edition

By : Felix Frank, Martin Alfke
Book Image

Puppet 5 Essentials Third Edition - Third Edition

By: Felix Frank, Martin Alfke

Overview of this book

Puppet is a configuration management tool that allows you to automate all your IT configurations, giving you control over what you do to each Puppet Agent in a network, and when and how you do it. In this age of digital delivery and ubiquitous Internet presence, it's becoming increasingly important to implement scaleable and portable solutions, not only in terms of software, but also the system that runs it. This book gets you started quickly with Puppet and its tools in the right way. It highlights improvements in Puppet and provides solutions for upgrading. It starts with a quick introduction to Puppet in order to quickly get your IT automation platform in place. Then you learn about the Puppet Agent and its installation and configuration along with Puppet Server and its scaling options. The book adopts an innovative structure and approach, and Puppet is explained with flexible use cases that empower you to manage complex infrastructures easily. Finally, the book will take readers through Puppet and its companion tools such as Facter, Hiera, and R10k and how to make use of tool chains.
Table of Contents (10 chapters)

Adding control structures in manifests

So far, you have written three simple manifests while following the instructions in this chapter. Each comprised only one resource, and one of them was given on the command line using the -e option. Of course, you would not want to write distinct manifests for each possible circumstance. Instead, just as how Ruby or Perl scripts branch out into different code paths, there are structures that make your Puppet code flexible and reusable for different circumstances.

The most common control element is the if/else block. It is quite similar to its equivalents in many programming languages:

if 'mail_lda' in $needed_services {
service { 'dovecot': enable => true }
} else {
service { 'dovecot': enable => false }
}

The Puppet DSL also has a case statement, which is reminiscent of its counterparts in other languages as well:

case $role {
‘imap_server’: {
package { ‘dovecot’: ensure => installed, }
service { ‘dovecot’: ensure => running, }
}
/_webservers$/: {
service { [‘apache’, ‘ssh’]: ensure => running, }
}
default: {
service { ‘ssh’: ensure => running, }
}
}

At the second matcher, you can see how it is possible to use regular expressions.

The case statement can also be used to switch to specific code based on variable data types:

case $role {
Array: {
include $role[0]
}
String: {
include $role
}
default: {
notify { 'This nodes $role variable is neither an
Array nor a String':}
}
}

A variation of the case statement is the selector. It's an expression, not a statement, and can be used in a fashion similar to the ternary if/else operator found in C-like languages:

package { 'dovecot':
ensure => $role ? {
'imap_server' => 'installed',
/desktop$/ => 'purged',
default => 'removed',
},
}

Similar to the case statement, the selector can also be used to return results, depending on the data types:

package { 'dovecot':
ensure => $role ? {
Boolean => 'installed',
String => 'purged',
default => 'removed',
},
}

The selector should be used with caution, because in more complex manifests, this syntax will impede readability.