Automatic Ansible Inventory with Vagrant

Yesterday, I posted about using Vagrant to learn Ansible, in which I showed you one way to combine these two tools to make it easier to learn Ansible. This is a combination I’m currently using as I continue to explore Ansible. Today, I’m going to expand on yesterday’s post by showing you how to make Vagrant automatically build an Ansible inventory for a particular Vagrant environment.

As you may already know, the Vagrantfile that Vagrant uses to instantiate and configure the VMs in a particular Vagrant environment is just Ruby. As such, it can be extended in a lot of different ways to do a lot of different things. In my case, I’ve settled on a design pattern that involves a separate YAML file with all the VM-specific data, which is read by the Vagrantfile when the user runs vagrant up. The data in the YAML file determines how many VMs are instantiated, what box is used for each VM, and the resources that are allocated to each VM. This is a design pattern I’ve used repeatedly in my GitHub “learning-tools” repository, and it seems to work pretty well (for me, at least).

Using this arrangement, since I already have all the VM-specific data in the external YAML file—this could include IP addresses to be assigned to additional network adapters—it’s relatively straightforward to extend the Vagrantfile to dynamically build an Ansible inventory file when the user runs vagrant up.

Here’s the particular snippet of Ruby code to add to the Vagrantfile:

require "fileutils"
f = File.open("hosts","w")
servers.each do |servers|
  f.puts servers["ip_addr"]
end # servers.each
f.close

This snippet of code opens a file named hosts (which is the inventory file name specified in the ansible.cfg in this directory, as I described in yesterday’s post). If the file already exists, it is overwritten—the preferred behavior, actually, since it allows us to write the contents fresh every time the user runs vagrant up. It then iterates through the entries from the external YAML file, and writes the IP address to the file, closing the file after the loop is complete.

I’m not a Ruby programmer, so I freely admit there may be a more efficient or effective way of accomplishing this task. That being said, the code above does work. (I leave it as an exercise for the reader to incorporate this into a YAML-equipped Vagrantfile. Numerous examples of such are found in my “learning-tools” repository.)

With this approach, your workflow looks like this:

• You edit the external YAML file to include a key (named “ip_addr” in the code snippet above) that includes the IP address Ansible should use for this particular Vagrant environment.
• You ensure there is an Ansible configuration file, as per yesterday’s post, in the same directory where your Vagrantfile is located.
• You run vagrant up. Vagrant instantiates the VMs, configures them according to the external YAML file, and writes the IP addresses to the Ansible inventory file.
• When the Vagrant environment is turned up, you run whatever Ansible ad-hoc commands you need, or run playbooks using ansible-playbook. The Ansible configuration file will tailor Ansible to work seamlessly in a Vagrant environment, and the inventory file will tell Ansible to work only against the VMs in that particular Vagrant environment.
• When you’re done, you run vagrant destroy.
• When you’re ready to start again, you run vagrant up and start all over again with a fresh environment. The Ansible inventory file gets automatically re-created with the correct IP addresses.

Pretty useful, yes?

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