Monday, November 23, 2015

Getting Started with @PuppetLabs - Part 4 - Supporting multiple operating systems

Most puppet modules can be designed to support multiple operating systems. Puppet has built in features to allow this through some simple syntax in the .pp file. It is a good practice to do that, whether you are publishing these modules on Puppet Forge or building highly customized modules for a specific purposes in a private enterprise.

1. Scalability - In case an organization chooses to switch operating systems or support other operating systems, your modules are already supporting them or they already have some operating systems and it is a matter of adding a few lines of code at the very top to add support for other operating systems.

2. Visibility and usability - The modules that support more than one operating system have higher usage counts on Puppet Forge because they support more than one type of need.

We will walk through a simple example to demonstrate multi-OS support. I checked in this code in GitHub at https://github.com/adityai/puppetApache.

Module: Install Apache web server and start it as a service.
OS Support: CentOs, Ubuntu and RHEL

Step 1: Folder structure
Create the following folder structure and blank files.

Folder: puppetApache
|-> Folder: apache -> Folder: manifests -> File: init.pp
|->  runner.pp

Step 2: Create runner.pp
We can use runner.pp to make it easy for us to apply the module from command line. Open the puppetApache/runner.pp file and enter the following line of code.

include apache

Step 3: Create the apache class
Open puppetApache/manifests/init.pp and enter the following code. I will explain each line of code in comments here.

#Create a class named apache
class apache {
#Case: when operating system is: case $::operatingsystem { #When operating system is ubuntu, set the package name and service name to apache2
"ubuntu": {
$packagename = "apache2"
$servicename = "apache2"
} #When operating system is centos or rhel, set the package name and service name to httpd
"centos", "rhel": {
$packagename = "httpd"
$sercicename = "httpd"
} #Default condition when the operating system is not supported, fail and stop.
default: {
fail("Ubsupported OS: ${::operatingsystem}")
}
}
#Ensure Apache is installed #Refer to puppet type reference https://docs.puppetlabs.com/references/latest/type.html#package
package { 'apache':
name => $packagename,
ensure => installed,
}
#Ensure Apache is running #Refer to puppet type reference https://docs.puppetlabs.com/references/latest/type.html#service
service { 'apache':
name => $servicename,
ensure => running, #require acts as a dependency. In this case, this service section requires the apache package section to be completed before this service section can run. #This ensures that dependencies are met before a particular section can execute.
require => Package['apache'],
}
}

Step #4: Run the module
Execute the following command from the puppetApache folder in the command line.

sudo puppet apply --modulepath . runner.pp

Step #5: Verify that apache is installed and running as a service
Execute the following command on the command line. You may have to run it with sudo depending on the user permissions.

For CentOs/RHEL:
service httpd status

For Ubuntu:
service apache2 status

Step #6: A little fun.
Now that we have confirmed that the apache web server is installed and running, let us stop the service and re-apply the puppet module.

Stop the Apache service
service apache2 stop #will stop the service on Ubuntu

Check the status of the service
service apache2 status #will show that the service is stopped

Apply the puppet module again.
sudo puppet apply --modulepath . runner.pp

Check the status of the service
service apache2 status #will show that the service is running

Here's what happened: When the puppet client applied the module:
1. It observed that apache is already installed (ensure => installed) and it does nothing with apache package.
2. It observed that the apache service is stopped (ensure => running) and it started the service.

Helpful Tip: If you really want your module to support multiple operating systems, I recommend setting up the case $::operatingsystem part as soon as you create the init.pp. As you add more sections, update the case $::operatingsystem part in parallel.

Have fun!