Module writing basics for *nix

Included in Puppet Enterprise 2017.1. A newer version is available; see the version menu above for details.

Welcome to the Module Writing section of the Quick Start Guide series. This walk-through will help you become more familiar with Puppet modules and module development. Follow along to learn how to:

Before starting this walk-through, complete the previous exercises in the introductory quick start guide. These steps assume that you have installed Puppet and Puppet agents, and have installed the latest version of the puppetlabs-apache module. You should still be logged in as root or administrator on your nodes.

Editing a Forge module

Although many Forge modules are exact solutions that fit your site, many are almost but not quite what you need. Sometimes you will need to edit some of your Forge modules.

Module basics

About module directories

By default, Puppet keeps modules in an environment’s modulepath, which for the production environment defaults to /etc/puppetlabs/code/environments/production/modules. This includes modules that Puppet installs, those that you download from the Forge, and those you write yourself. In a fresh installation, you need to create this modules subdirectory yourself by navigating to /etc/puppetlabs/code/environments/production and running mkdir modules.

Note: Puppet also creates another module directory: /opt/puppetlabs/puppet/modules. Don’t modify or add anything in this directory, including modules of your own.

There are plenty of resources about modules and the creation of modules that you can reference. Check out Module Fundamentals, the Beginner’s Guide to Modules, and the Puppet Forge.

Modules are directory trees. For these exercises you’ll use the following files:

  • apache/ (the module name)
    • manifests/
      • init.pp (contains the apache class)
    • templates/
      • vhost/
        • _file_header.erb (contains the vhost template, managed by Puppet)

Every manifest (.pp file) in a module contains a single class. File names map to class names in a predictable way, described in the Autoloader Behavior documentation. The init.pp file is a special case that contains a class named after the module, apache. Other manifest files contain classes called <MODULE NAME>::<FILE NAME> or <MODULE NAME>::<FOLDER>::<FILE NAME>. Many modules, including Apache, contain directories other than manifests and templates. For simplicity’s sake, we do not cover them in this introductory guide.

Writing a Puppet module

In this simplified exercise, you’ll modify a template from the Puppet Apache module, specifically vhost.conf.erb, to include some simple variables that will be populated by facts (using Puppet’s implementation of Facter) about your node.

  1. On the Puppet master, navigate to the modules directory by running cd /etc/puppetlabs/code/environments/production/modules.
  2. Run ls to view the currently installed modules, and note that apache is present.
  3. Open apache/templates/vhost/_file_header.erb in a text editor. Avoid using Notepad because it can introduce errors. _file_header.erb contains the following header:

     # ************************************
     # Vhost template in module puppetlabs-apache
     # Managed by Puppet
     # ************************************
    
  4. Collect the following facts about your agent:
    • on your Puppet agent, run facter osfamily. This returns your agent’s OS.
    • on your Puppet agent, run facter id. This returns the id of the currently logged in user.
  5. Edit the header of _file_header.erb so that it contains the following variables for Facter lookups:

     # ************************************
     # Vhost template in module puppetlabs-apache
     # Managed by Puppet
     #
     # This file is authorized for deployment by <%= scope.lookupvar('::id') %>.
     #
     # This file is authorized for deployment ONLY on <%= scope.lookupvar('::osfamily') %> <%= scope.lookupvar('::operatingsystemmajrelease')     %>.
     #
     # Deployment by any other user or on any other system is strictly prohibited.
     # ************************************
    
  6. From the command line of your Puppet agent, run puppet agent -t to trigger a Puppet run.

At this point, Puppet configures Apache and starts the httpd service. When this happens, a default Apache virtual host is created based on the contents of _file_header.erb.

  1. On the agent, navigate to one of the following locations based on your operating system:
    • Redhat-based: /etc/httpd/conf.d
    • Debian-based: /etc/apache2/sites-available
  2. View 15-default.conf. Depending on the node’s OS, the header will show some variation of the following contents:

     # ************************************
     # Vhost template in module puppetlabs-apache
     # Managed by Puppet
     #
     # This file is authorized for deployment by root.
     #
     # This file is authorized for deployment ONLY on Redhat 6.
     #
     # Deployment by any other user or on any other system is strictly prohibited.
     # ************************************
    

As you can see, Puppet has used Facter to retrieve some key facts about your node, and then used those facts to populate the header of your vhost template.

But now, let’s see what happens when you write your own Puppet code.

Writing a Puppet module

Puppet modules save time, but at some point you may need to write your own modules.

Writing a class in a module

In this exercise, you will create a class called puppet_quickstart_app that will manage a PHP-based web app running on an Apache virtual host.

  1. On the Puppet master, make sure you’re still in the modules directory (cd /etc/puppetlabs/code/environments/production/modules) and then run mkdir -p puppet_quickstart_app/manifests to create the new module directory and its manifests directory.
  2. Use your text editor to create and open the puppet_quickstart_app/manifests/init.pp file.
  3. Edit the init.pp file so it contains the following Puppet code, and then save it and exit the editor:

     class puppet_quickstart_app {
    
       class { 'apache':
         mpm_module => 'prefork',
       }
    
       include apache::mod::php
    
       apache::vhost { 'puppet_quickstart_app':
         port     => '80',
         docroot  => '/var/www/puppet_quickstart_app',
         priority => '10',
       }
    
       file { '/var/www/puppet_quickstart_app/index.php':
         ensure  => file,
         content => "<?php phpinfo() ?>\n",
         mode    => '0644',
       }
    
     }
    

You have written a new module containing a new class that includes two other classes: apache and apache::mod::php.

Note the following about your new class:

  • The class apache has been configured to include the mpm_module attribute. This attribute determines which multi-process module is configured and loaded for the Apache (HTTPD) process. In this case, the value is set to prefork.
  • include apache::mod::php indicates that your new class relies on those classes to function correctly. Puppet understands that your node needs to be classified with these classes and will take care of that work automatically when you classify your node with the puppet_quickstart_app class. In other words, you don’t need to worry about classifying your nodes with Apache and Apache PHP.
  • The priority attribute of 10 ensures that your app has a higher priority on port 80 than the default Apache vhost app.
  • The file /var/puppet_quickstart_app/index.php contains whatever is specified by the content attribute. This is the content you will see when you launch your app. Puppet uses the ensure attribute to create that file the first time the class is applied.

Using your custom module in the main manifest

  1. From the command line on the Puppet master, navigate to the main manifest (cd /etc/puppetlabs/code/environments/production/manifests).
  2. With your text editor, open site.pp and add the following Puppet code to your default node. Remove the apache class you added previously. Your site.pp file should look like this after you make your changes (although you may have portions from earlier in the Quick Start Guide):

      node default {
        class { 'puppet_quickstart_app': }
      }
    

    Note: Since the puppet_quickstart_app includes the apache class, you need to remove the first apache class you added the master node, as Puppet will only allow you to declare a class once.

  3. From the command line on your agent, run puppet agent -t to trigger a Puppet run.

    When the Puppet run is complete, you will see in the agent’s log that a vhost for the app has been created and the Apache service (httpd) has been started.

  4. Use a browser to navigate to port 80 of the IP address for your node, as in http://<yournodeip>:80.

    Tip: Be sure to use http instead of https.

Congratulations! You have created a new class from scratch and used it to launch a Apache PHP-based web app. Needless to say, in the real world, your apps will do a lot more than display PHP info pages. But for the purposes of this exercise, let’s take a closer look at how Puppet is managing your app.

Using Puppet to manage your app

  1. On the Puppet agent, open /var/www/puppet_quickstart_app/index.php, and change the content to something like, “THIS APP IS MANAGED BY PUPPET!”
  2. Refresh your browser, and notice that the PHP info page has been replaced with your new message.
  3. Run puppet agent -t --onetime on your Puppet agent.
  4. Refresh your browser, and notice that Puppet has reset your web app to display the PHP info page. (You can also see that the contents of /var/www/puppet_quickstart_app/index.php has been reset to what was specified in your manifest.)

Using a site module

Many users create a “site” module. Instead of describing smaller units of a configuration, the classes in a site module describe a complete configuration for a given type of machine. For example, a site module might contain:

  • A site::basic class, for nodes that require security management but haven’t been given a specialized role yet.
  • A site::webserver class for nodes that serve web content.
  • A site::dbserver class for nodes that provide a database server to other applications.

Site modules hide complexity so you can more easily divide labor at your site. System architects can create the site classes, and junior admins can create new machines.

  • On the Puppet master, create /etc/puppetlabs/code/environments/production/modules/site/manifests/basic.pp, and edit the file to contain the following:

      class site::basic {
        if $::kernel == 'Linux' {
          include puppet_quickstart_app
        }
        elsif $::kernel == 'windows' {
          include registry::compliance_example
        }
      }
    

This class declares other classes with the include function. Note the “if” conditional that sets different classes for different kernels using the $kernel fact. In this example, if an agent is a Linux machine, Puppet will apply your puppet_quickstart_app class. If it is a Windows machine, Puppet will apply the registry::compliance_example class.

  1. From the command line on the Puppet master, navigate to the main manifest: cd /etc/puppetlabs/code/environments/production/manifests.
  2. Add the following Puppet code to the default node in site.pp, retaining the classes you have already added:

     class { ‘site::basic’: }
    
  3. Save and exit, then run puppet agent -t from the command line of your Puppet agent.

Summary

You have now performed the core workflows of an intermediate Puppet user. In the course of their normal work, intermediate users:

  • Download and modify Forge modules to fit their deployment’s needs.
  • Create new modules and write new classes to manage many types of resources, including files, services, and more.
  • Build and curate a site module to safely empower junior admins and simplify the decisions involved in deploying new machines.

↑ Back to top