Structuring Ansible Inventories

As a part of working on Komponist, working with Ansible Inventories is essential to deploy the logic on multiple devices mentioned within these inventories.

Most common way of creating an Ansible Inventory is using a hosts file in INI format. However, one can also create the inventories in JSON, YAML. For the project I work with YAML just for consistency.

Assuming standard practice, one might be tempted to generated an inventory in one single file either called hosts or hosts.yml file under a dedicated directory. A TIL (Today I Learned) moment I had a while ago was using distinct directories for hosts and groups in order to add maintain information without messing with the main hosts.yml file.

Example Inventory

We can create a specfic inventory that doesn’t have to exist in /etc/ansible directory.

mkdir -p ~/komponist/inventory && touch ~/komponist/inventory/hosts.yml

Lets have 2 hosts and 1 group in our inventory:

---
testgroup1:
  hosts:
    host1:
    host2:

that should do it for the hosts.yml file.

Host Variables

In order to specify the Host Variables and Ansible specific variables for the hosts, we will create a base directory called host_vars in the inventory directory.

mkdir -p ~/komponist/inventory/host_vars

for host1, host2 information we create specific directories under host_vars directory.

mkdir -p ~/komponist/inventory/host_vars/host{1,2}

now lets add vars.yml and vault.yml to each of the hosts directory.

vars.yml will contain host-specifc variables and vault.yml any sensitive values like passwords, passkeys etc.

touch ~/komponist/inventory/host_vars/host{1,2}/{vars,vault}.yml

The final structure will be:

host_vars
├── host1
│   ├── vars.yml
│   └── vault.yml
└── host2
    ├── vars.yml
    └── vault.yml

Adding Ansible Host Variables

assume that host1 has 192.168.10.10 with user h1user, and host2 has 192.168.10.20 with user h2user.

The following content should exist in host1/vars.yml file:

---
ansible_host: 192.168.10.10
ansible_user: h1user

The following content should exist in host2/vars.yml file:

---
ansible_host: 192.168.10.20
ansible_user: h2user

Lets check how the current inventory might look like using:

ansible-inventory -i ~/komponist/inventory --list --yaml
all:
  children:
    testgroup1:
      hosts:
        host1:
          ansible_host: 192.168.10.10
          ansible_user: h1user
        host2:
          ansible_host: 192.168.10.20
          ansible_user: h2user

if you don’t specify -i, ansible-inventory will always try resolving the inventory in /etc/ansible/ directory.

Similar to vars.yml file, any sensitive information for hosts can be placed in vault.yml and it will be resolved with ansible-inventory.

Group Variables

If you wish that both the hosts obtain variables, we can leverage on Group Variables, similarly to Host Variables.

Similar to host_vars, create group_vars directory and the respective group name’s directory under group_vars.

mkdir -p group_vars/testgroup1/ && touch group_vars/testgroup1/{vars,vault}.yml

Group Variables

As an example, lets add a variable called priority with an int value

The content of testgroup1/vars.yml:

---
priority: 1

Now checking it once again ansible-inventory:

ansible-inventory -i ~/komponist/inventory --list --yaml
all:
  children:
    testgroup1:
      hosts:
        host1:
          ansible_host: 192.168.10.10
          ansible_user: h1user
          priority: 1
        host2:
          ansible_host: 192.168.10.20
          ansible_user: h2user
          priority: 1

group variables trickle to each host in the group i.e., priority: 1

Variable Precedence

It is worth noting that same variables may be overwritten if once doesn’t take care of variable precedence.

The predence is easy to remember:

Example

Let’s play with the priority variable. Lets set the value to 10 for host1 in the host_vars/host1/vars.yml.

The content of host_vars/host1/vars.yml:

---
ansible_host: 192.168.10.10
ansible_user: h1user
priority: 10

Lets check the value:

ansible-inventory -i . --list --yaml
all:
  children:
    testgroup1:
      hosts:
        host1:
          ansible_host: 192.168.10.10
          ansible_user: h1user
          priority: 10
        host2:
          ansible_host: 192.168.10.20
          ansible_user: h2user
          priority: 1

as mentioned, the priority variable value is overriden by the host’s variable value than that of the group variable.

If we wanted to set value to all available groups we can add such a variable to group_vars/all/vars.yml which will add it all hosts in all groups.

But remember, the all group will have the LEAST precedence since it is the parent of all other groups. Info