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Mar 25, 2017 - 4 minute read - Comments - devops puppet testing debugger

Testing DataTypes with the Puppet Debugger

The puppet language comes with a lot of extremely useful syntax and concepts. However, sometimes it is difficult to understand how these work or how to use them. Datatypes are found in almost every programming language so it is no surprise that puppet 4 has a similar feature for validating parameter data. If you have never used a puppet datatype before have a look here first.

The other day I was showing my client the awesome power of puppet datatypes and I was using the puppet-debugger to illustrate how datatypes work. By using the puppet debugger the client was immediately able to understand datatypes without writing a manifest, so I wanted to share how to use datatypes using the debugger to the rest of the world because datatypes can be very complex and using the debugger will help you understand how they work.

Let us first detail how to use a datatype with puppet parameters.

Using a datatype

Below is a snippet of a puppet defined type that has no datatype validations. Obviously the result of running this code would fail.

define foo(
  $bar = 'barbar'
  ) {
      ensure => $bar,
  foo{'test': bar => [1,2,3]}

So to prevent an error, we should be checking the value of bar in the parameter. Previously you might have used validate_string($bar) which was a function from the stdlib module that performed validations. This validation would occur only when the catalog is applied to a system. But that is way too late, we need to fail faster. So if you want an error to popup well before you deploy the code you need to inform Puppet that you expect a certain type of data. Hence the word ‘datatype’.

Adding a datatype would cause the compilation to fail immediately at compile time which you can check by writing a unit test or running puppet apply.

define foo(
  Enum['directory', 'present', 'file'] $bar,
  ) {
      ensure => $bar,
  foo{'test': bar => 'present' }

However, if you are not ready to jump into unit testing, you can utilize the puppet-debugger to bridge the gap. Using the puppet-debugger is the easiest, fastest way to validate that the parameter datatype accurately reflects the type of data your manifest requires.

Installing the Puppet Debugger

To get started you need to install the puppet-debugger on any system with puppet >= 3.8

gem install puppet-debugger

Testing a DataType with the Debugger

In order to test the data against the datatype we are going to use the =~ operator. This operator lets us test out the datatype. The data being tested goes on the LHS (left hand side) while the datatype goes on the RHS (right hand side).


  1. true =~ Boolean
  2. 'true' =~ String
  3. 'https://www.google.com' =~ Stdlib::HttpsUrl

{% img [class names] /images/datatypes.gif ‘Puppet Datatypes Demo’ ‘Puppet Datatypes Demo’ %}

If the value on the LHS matches the requirements on the RHS, puppet will return true, other false is returned and the value does not match the datatype.

Testing a DataType with the Debugger using the new function

Another way to test your datatype is use the [new operator]( https://docs.puppet.com/puppet/latest/function.html#new_. This only works with some datatypes like structures and core types. But if you have a complex datatype like a structure you can use the new operator to test out the custom datatype. In this example puppet shows an error because we did not specify an owner attribute on line 11.

More Info about Abstract Data Types

1:>> type MyType = Struct[{
  2:>>         mode => Enum[read, write, update],
  3:>>         path            => Optional[String[1]],
  4:>>         NotUndef[owner] => Optional[String[1]]}]
5:>> MyType.new({
  6:>> mode => 'read',
  7:>> path => '/dev/null',
  8:>> owner => 'root'
  9:>> }
  10:>> )
 => {
   "mode" => "read",
  "owner" => "root",
   "path" => "/dev/null"
11:>> MyType.new({mode => 'read'})
 => Evaluation Error: Error while evaluating a Method call, Converted value from MyType = Struct[{'mode' => Enum['read', 'update', 'write'], 'path' => Optional[String[1, default]], NotUndef['owner'] => Optional[String[1, default]]}].new() has wrong type, expects a value for key 'owner' at /var/folders/v0/nzsrqr_n40d4v396b2bqjvdw0000gp/T/puppet_debugger_input20170424-91861-6grwv7.pp:1:11

As you can see above the puppet-debugger makes it dead simple to test datatypes. For repeatable datatype testing you will want to write unit tests instead which is not covered in this article. Additionally, the stdlib module also has some nice datatypes that you can use in your modules.

I hope this has been of value to you, please share with others and star the puppet-debugger project if you enjoy using it.

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