Linux – use a puppet parameterized class

linuxpuppet

Generally when working with complex puppet modules, I will set variables at the node level or inside a class. e.g.,

node 'foo.com' {
  $file_owner = "larry" 
  include bar 
}

class bar { 
  $file_name = "larry.txt"
  include do_stuff
}

class do_stuff {
  file { $file_name:
    ensure => file,
    owner  => $file_owner,
  }
}

How/when/why does parametrized classes help when this situation? How are you using parameterized classes to structure your puppet modules?

Best Answer

Parameterized classes are a language construct to help you structure your code better. It prevents you from excessively using global variables (like in your example).

Imagine you included 20 more classes in your node description and all would need some variables being set in the manifest's global or node scope. Also parameterized classes allow you to have default parameters easily, so you could use a default value for the $file_owner instead of having to provide the same value (e. g. larry) in several different places.

Your example snippet (with two additional nodes) could be written as follows:

node 'example.com' { 
  class { bar: }
}

node 'example.net' {
  class { bar: owner = "harry" }
}

node 'example.net' {
  class { bar: file_name = "barry.txt" }
}

class bar($owner = "larry", $file_name = "larry.txt") { 
  class { do_stuff: owner => $owner, file_name => $file_name }
}

class do_stuff($owner, $file_name) {
  file { $file_name:
    ensure => file,
    owner  => $owner,
  }
}

With your usage of global variables, you'd need to declare a variable named $owner in each node and you would not be able to overwrite the $file_name variable/parameter per node. Instead you'd need to declare another bar class for each node.

The document on Puppet's language evolution and of course the language guide provide some good examples on how to use parameterized classes and the rationale behind this language construct:

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