Ansible: Play it again, Sam

Recently we started using Ansible for the provisioning of some of our servers. Ansible is one of many configuration management / provisioning tools that are popular right now. Puppet and Chef are probably more widely known representatives of their kind, but what attracted us to Ansible was the fact that it’s agentless: the target machines don’t need an agent installed, all you need is remote access via SSH. Well, almost. It turns out that Python is also required on the remote machines, otherwise you’ll be limited to a very basic set of functionality (the raw module). Fortunately, most Linux distributions have Python installed by default.

With Ansible you describe the desired target configuration as a sequence of tasks in a YAML file called Playbook: package installation, copying files, enabling and starting services, etc. The playbook is semi-declarative. Each step usually describes a goal, e.g. package XY should be present. Action is only taken if necessary. On the other hand it’s also very imperative: steps are executed sequentially and you can have conditionals and loops (e.g. “with_items”). You can also define handlers, which are executed once after they have been notified, for example if you want to restart the Apache web server after its configuration has changed.

Before a playbook is applied to a remote machine Ansible will query “facts” about this machine. These facts are available as variables in the playbook. You can also define your own variables.

A playbook is usually applied to a set of machines. Available machines are listed in a separate file, the inventory, where they can be grouped by roles. With one command you can configure or update all the machines of a specific role at once. You can also execute a “dry run”, which simulates a playbook run and tells you what changes would be applied.

So far our experience with Ansible has been good. The concepts are easy to grasp. YAML syntax requires getting used to, but at least it’s not XML. On the website the actual documentation is a bit hidden among promotion for their commercial products, but you can also directly visit docs.ansible.com.

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