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Taking the effort out of server configuration using Ansible

This article was originally published in the NorDevCon 2016 conference programme.

Ansible is an IT automation tool that is growing in popularity. It is ideally suited for configuration management, i.e. automating the configuration of your development and production infrastructure.

Ansible is a relatively new addition to the “DevOps” arena (first released in 2012) and it has quite a different philosophy to some of the more well established players in the field. Most notably, it is “agent-less”; i.e. there is no need to have an “agent” pulling updates from a “master” configuration manager.

Ansible has been designed to be easy to use and it achieves this through two aspects of its architecture:

  • It uses a push based method to interact with the hosts (the machines to be configured)
  • It uses OpenSSH as its authentication method

What this means in practise is that you can install Ansible on your laptop and as long as you have setup password-less ssh to the machines you are wanting to interact with you are ready to go. In other words no master, no databases, no services; no fighting the system that is meant to be making your life easier!

Listing your inventory

Ansible has the concept of an inventory where you list all of the hosts that you want to be able to interact with through Ansible. The inventory is a plain text file using an INI-like format. The default path to the inventory file is /etc/ansible/hosts. However, you can provide an alternative path as a command line argument using the -i option.

Below is an example hosts file that groups three web servers into a webservers group.

[webservers]
web1.example.com
web2.example.com
web3.example.com

It is also possible to create aliases and specify host specific variables. Below is an example of an alias to enable Ansible to communicate with a Vagrant generated virtual machine.

testserver ansible_ssh_host=127.0.0.1 ansible_ssh_port=2222 ansible_ssh_user=vagrant ansible_ssh_private_key_file=.vagrant/machines/default/virtualbox/private_key ansible_sudo=yes

Interacting with Ansible

There are two different ways of interacting with ansible: ad-hoc commands and playbooks. Ad-hoc commands are useful for quick tasks that you don’t want to save for later. Whereas playbooks provides a means to specify reproducible configuration recipes.

Ad-hoc commands are accessed using the ansible program and can be useful if you want to do something quickly. For example, suppose that there was a critical security patch for bash that you needed to apply to all your webservers rapidly.

ansible webservers -m apt -a "name=bash state=latest update_cache=yes"

There are a few things to note in the command above. The first argument (webservers) is the host group defined in the inventory. The -m apt option states that we want to use Ansible’s apt module, in other words our servers are running on a Debian based OS such as Ubuntu. Finally, the -a ... option specifies the arguments to supply, i.e. we want to install the latest version of bash.

It is worth noting that Ansible does not try to abstract away the layer of package management. As such there are separate modules for apt, yum, homebrew, etc. This is useful in that the functionality of Ansible modules are not limited by the constraint of a set of common features. For example in this case we make use of the update_cache option to run the equivalent of apt-get update before the we try to install the latest version of bash.

Batteries included as Ansible modules

Ansible comes with a large number of built-in modules for configuring your systems. These include modules for package management, performing systems administration tasks, working with files, interacting with source control programs, and much more.

Reproducible configuration scripts using playbooks

Ansible playbooks allow you to create reproducible recipes for configuring your servers. Playbooks are written in YAML and are meant to be human readable. As YAML is simply a data serialisation language, a playbook can be thought of as “infrastructure as data”.

Below is a basic playbook for configuring a firewall using firewalld. In real life a playbook would be written to configuration the entire system.

---

- name: configure a web server firewall using firewalld
  hosts: testserver
  
  tasks:
    - name: install firewalld 
      apt: name=firewalld state=present

    - name: ensure that firewalld is started and enable at boot
      service: name=firewalld enabled=yes state=started

    - name: open up port 80 for tcp
      firewalld: port=80/tcp permanent=yes state=enabled
      notify: restart firewalld

  handlers:
    - name: restart firewalld
      service: name=firewalld state=restarted

There are a few things to note in the playbook above. The host(s) that the playbook is defined to be applied to are defined in the hosts entry. In this case it is only run against the Vagrant testserver alias we defined in the inventory earlier. The apt module is used to install firewalld and the service module is used to ensure that firewalld is started and enabled at boot. Finally, we use Ansible’s firewalld module to open up port 80. Note that this task makes use of the notify action to trigger the restart firewalld handler, which we define at the end of the playbook.

What now?

There is much more to Ansible than what has been outlined here. The key is to build things up bit by bit. You don’t need to use every feature of Ansible to get a job done.

For more information about Ansible have a look at the Ansible documentaiton. If you liked this article you may also be interested in the other Ansible tutorials on this site, which illustrate the use of Ansible as a tool to install scientific software.