Passing parameters to classes

Sometimes it's very useful to parameterize some aspect of a class. For example, you might need to manage different versions of a gem package, and rather than making separate classes for each that differ only in the version number, you can pass in the version number as a parameter.

How to do it…

In this example, we'll create a definition that accepts parameters:

1. Declare the parameter as a part of the class definition:

     class eventmachine($version) {
       package { 'eventmachine': provider => gem, ensure   => $version,
       }
     }

2. Use the following syntax to include the class on a node:

     class { 'eventmachine':
       version => '1.0.3',
     }

How it works…

The class definition class eventmachine($version) { is just like a normal class definition except it specifies that the class takes one parameter: $version. Inside the class, we've defined a package resource:

  package { 'eventmachine':
    provider => gem,
    ensure   => $version,
  }

This is a gem package, and we're requesting to install version $version.

Include the class on a node, instead of the usual include syntax:

include eventmachine

On doing so, there will be a class statement:

  class { 'eventmachine':
    version => '1.0.3',
  }

This has the same effect but also sets a value for the parameter as version.

There's more…

You can specify multiple parameters for a class as:

  class mysql($package, $socket, $port) {

Then supply them in the same way:

  class { 'mysql':
    package => 'percona-server-server-5.5',
    socket  => '/var/run/mysqld/mysqld.sock',
    port    => '3306',
  }

Specifying default values

You can also give default values for some of your parameters. When you include the class without setting a parameter, the default value will be used. For instance, if we created a mysql class with three parameters, we could provide default values for any or all of the parameters as shown in the code snippet:

class mysql($package, $socket, $port='3306') {

or all:

  class mysql(
    package = percona-server-server-5.5",
    socket  = '/var/run/mysqld/mysqld.sock',
    port    = '3306') {

Defaults allow you to use a default value and override that default where you need it.

Unlike a definition, only one instance of a parameterized class can exist on a node. So where you need to have several different instances of the resource, use define instead.