Puppet implementation patterns: Safe Default

description.md

Safe Default

When

You have a defined type and want to use a default value for a parameter from a params class.

How

Set the default value of the variable as undef, then put a conditional after including the params class in the class body in which you set another internal variable with the value of the params class variable or the user-defined value.

Notes

The most elegant way to do this would be to directly reference the variable as the default value of the parameter, such as in:

  define test ( $myparam = $test::params::myparam ) {
    include test::params
    ...

This, however, won't work unless you include the test::params class in a higher scope.

Drawbacks

Code for the defined type tend to grow quite a bit with the number of parameters. Also, since parameters cannot be reassigned, one is forced to introduce a new local variable; this worsen code readability and introduce a problem of clashing names.

example.pp

class test::params {
  $message = "Default message from params class"
}

# This will work with a parameterized class as well
# but inheritance would do a cleaner job
define test (
  $message = undef,
) {
  include test::params

  $real_message = $message ? {
    undef =>   $test::params::message,
    default => $message
  }

  notify { $real_message: }
}

test { "simple notification": }