Puppet supports functions as any other programming language since the base development language of Puppet is Ruby. It supports two types of functions known with the name of statement and rvalue functions.

Statements stand on their own and they do not have any return type. They are used for performing standalone tasks like importing other Puppet modules in the new manifest file.
Rvalue returns values and can only be used when the statement requires a value, such as an assignment or a case statement.

The key behind the execution of function in Puppet is, it only executes on Puppet master and they do not execute on the client or the Puppet agent. Therefore, they only have access to the commands and data available on the Puppet master. There are different kind of functions which are already present and even the user has the privilege to create custom functions as per requirement. Few inbuilt functions are listed below.

File Function

File function of the file resource is to load a module in Puppet and return the desired output in the form of a string. The arguments that it looks for is, the <module name>/<file> reference, which helps in loading the module from Puppet module’s file directory.

Like script/tesingscript.sh will load the files from <module name>/script/files/testingscript.sh. Function has the capability to read and accept an absolute path, which helps in loading the file from anywhere on the disk.

Include Function

In Puppet, the include function is very much similar to the include function in any other programming language. It is used for declaration of one or more classes, which results in evaluating all the resources present inside those classes and finally add them to a catalog. The way it works is, include function accepts a class name, list of classes or a comma separated list of class names.

One thing to keep in mind while using an include statement is, it can be used multiple times in a class but has the limitation of including a single class only once. If the included class accepts a parameter, the include function will automatically look up values for them using <class name>::<parameter name> as the lookup key.

Include function does not cause a class to be contained in the class when they are declared, for that we need to use a contained function. It even does not create a dependency in the declared class and classes surrounding it.

In include function, only the full name of a class is allowed, relative names are not allowed.

Defined Function

In Puppet, the defined function helps in determining where a given class or resource type is defined and returns a Boolean value or not. One can also use define to determine whether a specific resource is defined or the variable defined has a value. Key point to keep in mind while using the defined function is, this function takes at least one string argument, which can be a class name, type name, resource reference, or variable reference of the form “$name”.

Define function checks for both native and defined function type, including types provided by modules. Type and class are matched by their names. The function matches the resource deceleration by using the resource reference.

Define Function Matches

# Matching resource types

defined(“file”)

defined(“customtype”)

# Matching defines and classes

defined(“testing”)

defined(“testing::java”)

# Matching variables

defined(‘$name’)

# Matching declared resources

defined(File[‘/tmp/file’])

Puppet – Custom Functions

As described in the previous chapter, function provides the user with a privilege of developing custom functions. Puppet can extend its interpretation power by using custom functions. Custom function helps in increasing and extending the power of Puppet modules and manifest files.

Writing Custom Function

There are few things which one needs to keep in mind before writing a function.

In Puppet, functions are executed by compilers which means all the functions run on Puppet master and they don’t need to deal with any of the Puppet client for the same. Functions can only interact with agents, provided information is in the form of facts.
The Puppet master catches custom functions which means that one needs to restart the Puppet master, if one does some changes in Puppet function.
Function will be executed on the server which means any file that the function needs should be present on the server, and one can’t do anything if the function requires direct access to the client machine.
There are completely two different type of functions available, one is the Rvalue function which returns the value and the statement function which does not return anything.
The name of the file containing function should be the same as the name of the function in the file. Otherwise, it will not get loaded automatically.

Location to Put Custom Function

All the custom functions are implemented as separate .rb files and are distributed among modules. One needs to put custom functions in lib/puppet/parser/function. Functions can be loaded from .rb file from the following locations.

$libdir/puppet/parser/functions
puppet/parser/functions sub-directories in your Ruby $LOAD_PATH

Creating a New Function

New functions are created or defined using the newfunction method inside the puppet::parser::Functions module. One needs to pass the function name as a symbol to newfunction method and the code to run as a block. The following example is a function, which is used to write a string to the file inside the /user directory.

module Puppet::Parser::Functions

newfunction(:write_line_to_file) do |args|

filename = args[0]

str = args[1]

File.open(filename, ‘a’) {|fd| fd.puts str }

end

Once the user has the function declared, it can be used in the manifest file as shown below.

write_line_to_file(‘/user/vipin.txt, “Hello vipin!”)

So, this brings us to the end of blog. This Tecklearn ‘How to define Functions and Custom functions in Puppet’ blog helps you with commonly asked questions if you are looking out for a job in DevOps. If you wish to learn Puppet and build a career in DevOps domain, then check out our interactive, Continuous Deployment: Configuration Management using Puppet Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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