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larsks
Asked: 2012-05-26 12:11:29 +0800 CST

How to make a variable in a parameterized Puppet class default to the value of another parameter?

  • 772

I have a Puppet class that sets up a number of services and configuration files. In most cases, there is a default server that can be used, but it is also possible to configure explicit per-service servers. I find myself doing this all the time:

class myclass (
  $default_server    = 'server.example.com',
  $server_for_thing1 = undef,
  $server_for_thing2 = undef,
  $server_for_thing3 = undef
) {

  if $server_for_thing1 {
    $real_server_for_thing1 = $server_for_thing1
  } else {
    $real_server_for_thing1 = $default_server
  }

  # ...and so forth...
}

As the number of possible services grows large, this syntax becomes unwieldy. Is there a better way of doing this? I really want something along the lines of:

$server_for_thing1 = $server_for_thing1 || $default_server

...but variables cannot be re-assigned in Puppet. Are there better ways of doing this?

puppet

2 Answers

  1. While not exactly what you are asking for, have you considered using an External Node Classifier to set and override the default value for specific servers? I believe the ECN is the "puppet way" of doing things in a situation like yours.

    EDIT: (based on the first comment)

    Second idea: you can use a custom function to at least make the multi-line repeated logic a bit more readable. Something like this, which returns the first "defined" argument, though with puppet, I am never sure what "defined" is (in this case, "undef" gets passed as an empty string to the function, which is still good enough).

    module Puppet::Parser::Functions
    newfunction(:get_default, :type => :rvalue) do |args|
        value = nil
        args.each { |x|
            if ! x.nil? and x.length > 0
                value = x
                break
            end
        }
        return value
    end
end

    You can then call it as many times as you want:

    $real_server_for_thing1 = get_default($server_for_thing1, $default_server)
$real_server_for_thing2 = get_default($server_for_thing2, $default_server)
$real_server_for_thing3 = get_default($server_for_thing3, $default_server)
    • 2

  2. Since I posted this question oh those many years ago, Puppet's stdlib module has learned about the pick function, which will return the first non-undef or non-empty value from a list of values. So one can write:

    $real_server_for_thing1 = pick($server_for_thing1, $default_server)

    There is also a pick_default function empty values are valid (you only want to ignore undef).

    • 0