Executing commands before and after Puppet runs

If you need to have a command executed before each Puppet run, you can do this using the prerun_command configuration setting. Similarly, you can use postrun_command to execute a command after the run has completed. This mechanism gives you a powerful hook to integrate Puppet with other software, or even trigger events on other machines. The prerun and postrun commands must succeed (that is, return a zero exit status), or Puppet will report an error. This enables you to have any command failures reported using Puppet’s reporting mechanism, for example.

How to do it…

Set prerun_command or postrun_command in puppet.conf to the commands you want to run:

prerun_command = /usr/local/bin/before-puppet-run.sh postrun_command = /usr/local/bin/after-puppet-run.sh

There’s more

You can use prerun and postrun commands to integrate Puppet with Ubuntu’s etckeeper system. Etckeeper is a version control system for tracking changes to files in the /etc directory. To do this, define these commands in puppet.conf:

prerun_command=/etc/puppet/etckeeper-commit-pre postrun_command=/etc/puppet/etckeeper-commit-post

Using public modules

“Plagiarize, plagiarize, plagiarize / Only be sure always to call it please ‘research’ “—Tom Lehrer, ‘Lobachevsky’

If in doubt, steal. In many cases when you write a Puppet module to manage some software or service, you don’t have to start from scratch. Community-contributed modules are available at the Puppet Forge site for many popular applications. Sometimes, a community module will be exactly what you need and you can download and start using it straight away. In other cases, you will need to make some modifications to suit your particular needs and environment. If you are new to Puppet, it can be a great help to have some existing code to start with. On the other hand, community modules are often written to be as general and portable as possible, and the extra code required can make them harder to understand. In general I would not recommend treating Puppet Forge as a source of ‘drop-in’ modules which you can deploy without reading or understanding the code. This introduces an external dependency to your Puppet infrastructure, and doesn’t help advance your understanding and experience of Puppet. Rather, I would use it as a source of inspiration, help, and examples. A module taken from Puppet Forge should be a jumping-off point for you to develop and improve your own modules. Be aware that a given module may not work on your Linux distribution. Check the README file which comes with the module to see if your operating system is supported.

Getting ready

1. The easiest way to use Puppet Forge modules is to install the puppet-module tool:

1. Run puppet-module to see the available commands:

How to do it

In this example, we’ll use puppet-module to find and install a module to manage the Tomcat application server.

1. Search for a suitable module as follows:

# puppet-module search tomcat ===================================== Searching http://forge.puppetlabs.com ————————————- 2 found. ——– camptocamp/tomcat (0.0.1) jeffmccune/tomcat (1.0.1)

1. In this example we’ll install the Jeff McCune version:

# cd /etc/puppet/modules # puppet-module install jeffmccune/tomcat Installed “jeffmccune-tomcat-1.0.1” into directory: jeffmccune-tomcat

1. The module is now ready to use in your manifests: looking at the source code will show you how to do this.

How it works…

The puppet-module tool simply automates the process of searching and downloading modules from the Puppet Forge site. You can browse the site to see what’s available at: forge.puppetlabs.com.

There’s more

Not all publically available modules are on Puppet Forge. Some other great places to look are on GitHub: github.com/camptocamp, github.com/example42 and Dean Wilson maintains an excellent repository of Puppet patterns, tips, and recipes, at the Puppet Cookbook website: puppetcookbook.com.