lab-10.1-Inherited-classes

Lab 10.1: Inherited classes

Many applications share a common core configuration. One way to limit the amount of code duplication when designing for this is to create a base class and inherit its functionality in more specific classes. Let's assume for the purposes of this class that we have a series of web applications that all share the same LAMP stack. A main webapp class that reflects this might look like:

class webapp {
  include mysql::server

  class { 'mysql::bindings':
    php_enable => true,
  }

  include apache
  include apache::mod::php

  apache::vhost { $facts['fqdn']:
    priority   => '10',
    vhost_name => $facts['fqdn'],
    port       => '80',
    docroot    => '/var/www/html',
  }

  @@haproxy::balancermember { $facts['fqdn']:
    listening_service => 'webapp',
    ports             => '80',
  }
}

If we wanted to manage a WordPress web application, we could do that with a webapp::wordpress subclass that inherits the webapp functionality and also declares the ::wordpress module. This example also overrides the load balancer configuration and the docroot.

class webapp::wordpress inherits webapp {
  include wordpress

  # override load balancer to 'wordpress' listening service
  Haproxy::Balancermember[$facts['fqdn']] {
    listening_service => 'wordpress',
  }
}

Older versions of Puppet resolved class names like variables, so wordpress would resolve to local scope first, or webapp::wordpress. Since that class exists, it essentially tried to include itself. This is no longer an issue in Puppet 4.x. This is why you'll see the common pattern of including classes explicitly as top-scope, e.g. include ::wordpress.

Your job is to refactor this example to use composition rather than inheritance. To do this, you'll need to pass in parameters to the base class.

You may encounter a duplicate resource error if your node group is still classified with the ordering class. Remove that classification to proceed.

Steps

Obtain the component modules

1. Install the modules used for this lab:

   • Edit [control-repo]/Puppetfile.

   • Uncomment the following lines in the Puppetfile.

     mod 'puppetlabs/apache'
     mod 'puppetlabs/haproxy'
     mod 'hunner/wordpress'
     

2. Read the documentation for these modules as needed.

   • You may use the README file in the module directory or the Puppet Forge module page as you prefer.

Create a new module webapp with pdk

1. Change directory to your [modulepath]

   cd $(puppet agent --configprint environmentpath)/production/modules

2. Create the module structure

   pdk new module

3. Press Enter. You will see several questions requiring an answer. Enter the answers as you see below:

   Replace the N in studentN with your student number, e.g. student8

   Question	Answer
   Module Name	webapp
   Forge Name	studentN
   Credit author	Student N
   License	Apache-2.0
   Operating systems	RedHat

4. Create the base webapp class.

   pdk new class webapp

5. Edit manifests/init.pp

   • Accept a String $docroot, defaulting to /var/www/html
     • Pass $docroot to the apache::vhost declaration
   • Accept a String $app_name, defaulting to webapp
     • Pass the $app_name to the haproxy::balancermember declaration.

6. Create a webapp::wordpress class.

   pdk new class wordpress

7. Edit manifests/wordpress.pp

   • Declare the webapp class and pass parameters as needed.

Test your module

1. Validate and test your classes.

   pdk validate

2. Edit examples/wordpress.pp

   include webapp::wordpress

3. To test locally you will need the following modules installed locally.

   puppet module install puppetlabs/apache

   puppet module install puppetlabs/haproxy

   puppet module install hunner/wordpress

4. Run a local puppet apply

   puppet apply examples/wordpress.pp --noop

   The output should be similar to this:

   [root@training modules]# puppet apply webapp/examples/wordpress.pp
   notice: /Stage[main]/Mysql::Server/Package[mysql-server]/ensure: created
   notice: /Stage[main]/Mysql::Server/Service[mysqld]/ensure: ensure changed
   'stopped' to 'running'
   notice: /Stage[main]/Website/Apache::Vhost[training.puppetlabs.vm]/File[/var
   /log/httpd]/ensure: created
   notice: /Stage[main]/Apache/Package[httpd]/ensure: created
   info: /Stage[main]/Apache/Package[httpd]: Scheduling refresh of Service[httpd]
   [...]
   notice: /Stage[main]/Wordpress::Db/Database[wordpress]/ensure: created
   notice: /Stage[main]/Wordpress::Db/Database_user[wordpress@localhost]/ensure:
   created
   notice: /Stage[main]/Wordpress::Db/Database_grant[wordpress@localhost/wordpress]
   /privileges: privileges changed '' to 'all'
   notice: /Stage[main]/Apache/Service[httpd]/ensure: ensure changed 'stopped' to
   'running'
   notice: /Stage[main]/Apache/Service[httpd]: Triggered 'refresh' from 53 events
   notice: Finished catalog run in 29.09 seconds
   

Extra Credit

While we're at it, you can earn some brownie points by extending the functionality of the module to support multiple Linux distributions using the params pattern. Calculate the default path of the $docroot in params.pp and inherit that class in webapp to make the variables available.

1. Create a webapp::params class that calculates the default $docroot based on the $facts['os']['family'] fact

   pdk new class params

2. Edit manifests/params.pp

3. Refactor webapp to inherit from the webapp::params class and use it to calculate appropriate platform defaults

   • Edit manifests/init.pp

Solution

Your module structure should resemble

[root@training modules]# tree webapp/
webapp/
├── examples
│   ├── init.pp
└── manifests
    ├── init.pp
    ├── params.pp
    └── wordpress.pp

Example file: webapp/manifests/wordpress.pp

class webapp::wordpress {
  include wordpress

  class { 'webapp':
    docroot  => '/opt/wordpress',
    app_name => 'wordpress',
  }
}

Example file: webapp/manifests/init.pp

class webapp (
  String $docroot = '/var/www/html',
  String $app_name = 'webapp',
) {
  include mysql::server
  class { 'mysql::bindings':
    php_enable => true,
  }

  include apache
  include apache::mod::php
  apache::vhost { $facts['fqdn']:
    priority   => '10',
    vhost_name => $facts['fqdn'],
    port       => '80',
    docroot    =>  $docroot,
  }

  @@haproxy::balancermember { $fqdn:
    listening_service => $app_name,
    ports             => '80',
  }
}

Extra Credit Solution Example file: webapp/manifests/init.pp

class webapp (
  String $docroot,
  String $app_name = 'webapp'
) inherits webapp::params {
  include mysql::server
  class { 'mysql::bindings':
    php_enable => true,
  }

  include apache
  include apache::mod::php
  apache::vhost { $facts['fqdn']:
    priority   => '10',
    vhost_name => $facts['fqdn'],
    port       => '80',
    docroot    =>  $docroot,
  }

  @@haproxy::balancermember { $fqdn:
    listening_service => $app_name,
    ports             => '80',
  }
}

Extra Credit Solution Example file: webapp/manifests/params.pp

class webapp::params {
  case $facts['os']['family'] {
    'RedHat': {
      $docroot = '/var/www/html'
    }
    'Debian': {
      $docroot = '/var/www'
    }
    default: {
      fail("Module ${module_name} is not supported on ${facts['os']['family']}")
    }
  }
}

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