Set Up Ansible Inventories : Step by Step Procedure

Prerequisites:

In order follow this guide, you’ll need:

• One Ansible control node: an Ubuntu 20.04 machine with Ansible installed and configured to connect to your Ansible hosts using SSH keys. Make sure the control node has a regular user with sudo permissions and a firewall enabled, as explained in our Initial Server Setup guide. To set up Ansible, please follow our guide on How to Install and Configure Ansible on Ubuntu 20.04.
• Two or more Ansible Hosts: two or more remote Ubuntu 20.04 servers

Step 1 — Creating a Custom Inventory File

Upon installation, Ansible creates an inventory file that is typically located at /etc/ansible/hosts. This is the default location used by Ansible when a custom inventory file is not provided with the -i option, during a playbook or command execution.

Even though you can use this file without problems, using per-project inventory files is a good practice to avoid mixing servers when executing commands and playbooks. Having per-project inventory files will also facilitate sharing your provisioning setup with collaborators, given you include the inventory file within the project’s code repository.

To get started, access your home folder and create a new directory to hold your Ansible files:

cd ~
mkdir ansible

Move to that directory and open a new inventory file using your text editor of choice. Here, we’ll use nano:

cd ansible
nano inventory

A list of your nodes, with one server per line, is enough for setting up a functional inventory file. Hostnames and IP addresses are interchangeable:

~/ansible/inventory

203.0.113.111
203.0.113.112
203.0.113.113
server_hostname

Once you have an inventory file set up, you can use the ansible-inventory command to validate and obtain information about your Ansible inventory:

ansible-inventory -i inventory --list

Output{
    "_meta": {
        "hostvars": {}
    }, 
    "all": {
        "children": [
            "ungrouped"
        ]
    }, 
    "ungrouped": {
        "hosts": [
            "203.0.113.111", 
            "203.0.113.112", 
            "203.0.113.113", 
            "server_hostname"
        ]
    }
}

Even though we haven’t set up any groups within our inventory, the output shows 2 distinct groups that are automatically inferred by Ansible: all and ungrouped. As the name suggests, all is used to refer to all servers from your inventory file, no matter how they are organized. The ungrouped group is used to refer to servers that aren’t listed within a group.

Running Commands and Playbooks with Custom Inventories

To run Ansible commands with a custom inventory file, use the -i option as follows:

ansible all -i inventory -m ping

This would execute the ping module on all hosts listed in your custom inventory file.

Similarly, this is how you execute Ansible playbooks with a custom inventory file:

ansible-playbook -i inventory playbook.yml

Note: For more information on how to connect to nodes, please refer to our How to Use Ansible guide, as it demonstrates more connection options.

So far, we’ve seen how to create a basic inventory and how to use it for running commands and playbooks. In the next step, we’ll see how to organize nodes into groups and subgroups.

Step 2 — Organizing Servers Into Groups and Subgroups

Within the inventory file, you can organize your servers into different groups and subgroups. Beyond helping to keep your hosts in order, this practice will enable you to use group variables, a feature that can greatly facilitate managing multiple staging environments with Ansible.

A host can be part of multiple groups. The following inventory file in INI format demonstrates a setup with four groups: webservers, dbservers, development, and production. You’ll notice that the servers are grouped by two different qualities: their purpose (web and database), and how they’re being used (development and production).

~/ansible/inventory

[webservers]
203.0.113.111
203.0.113.112

[dbservers]
203.0.113.113
server_hostname

[development]
203.0.113.111
203.0.113.113

[production]
203.0.113.112
server_hostname

If you were to run the ansible-inventory command again with this inventory file, you would see the following arrangement:

Output{
    "_meta": {
        "hostvars": {}
    }, 
    "all": {
        "children": [
            "dbservers", 
            "development", 
            "production", 
            "ungrouped", 
            "webservers"
        ]
    }, 
    "dbservers": {
        "hosts": [
            "203.0.113.113", 
            "server_hostname"
        ]
    }, 
    "development": {
        "hosts": [
            "203.0.113.111", 
            "203.0.113.113"
        ]
    }, 
    "production": {
        "hosts": [
            "203.0.113.112", 
            "server_hostname"
        ]
    }, 
    "webservers": {
        "hosts": [
            "203.0.113.111", 
            "203.0.113.112"
        ]
    }
}

It is also possible to aggregate multiple groups as children under a “parent” group. The “parent” is then called a metagroup. The following example demonstrates another way to organize the previous inventory using metagroups to achieve a comparable, yet more granular arrangement:

~/ansible/inventory

[web_dev]
203.0.113.111

[web_prod]
203.0.113.112

[db_dev]
203.0.113.113

[db_prod]
server_hostname

[webservers:children]
web_dev
web_prod

[dbservers:children]
db_dev
db_prod

[development:children]
web_dev
db_dev

[production:children]
web_prod
db_prod

The more servers you have, the more it makes sense to break groups down or create alternative arrangements so that you can target smaller groups of servers as needed.

Step 3 — Setting Up Host Aliases

You can use aliases to name servers in a way that facilitates referencing those servers later, when running commands and playbooks.

To use an alias, include a variable named ansible_host after the alias name, containing the corresponding IP address or hostname of the server that should respond to that alias:

~/ansible/inventory

server1 ansible_host=203.0.113.111
server2 ansible_host=203.0.113.112
server3 ansible_host=203.0.113.113
server4 ansible_host=server_hostname

If you were to run the ansible-inventory command with this inventory file, you would see output similar to this:

Output{
    "_meta": {
        "hostvars": {
            "server1": {
                "ansible_host": "203.0.113.111"
            }, 
            "server2": {
                "ansible_host": "203.0.113.112"
            }, 
            "server3": {
                "ansible_host": "203.0.113.113"
            }, 
            "server4": {
                "ansible_host": "server_hostname"
            }
        }
    }, 
    "all": {
        "children": [
            "ungrouped"
        ]
    }, 
    "ungrouped": {
        "hosts": [
            "server1", 
            "server2", 
            "server3", 
            "server4"
        ]
    }
}

Notice how the servers are now referenced by their aliases instead of their IP addresses or hostnames. This makes it easier for targeting individual servers when running commands and playbooks.

Step 4 — Setting Up Host Variables

It is possible to use the inventory file to set up variables that will change Ansible’s default behavior when connecting and executing commands on your nodes. This is in fact what we did in the previous step, when setting up host aliases. The ansible_host variable tells Ansible where to find the remote nodes, in case an alias is used to refer to that server.

Inventory variables can be set per host or per group. In addition to customizing Ansible’s default settings, these variables are also accessible from your playbooks, which enables further customization for individual hosts and groups.

The following example shows how to define the default remote user when connecting to each of the nodes listed in this inventory file:

~/ansible/inventory

server1 ansible_host=203.0.113.111 ansible_user=sammy
server2 ansible_host=203.0.113.112 ansible_user=sammy
server3 ansible_host=203.0.113.113 ansible_user=myuser
server4 ansible_host=server_hostname ansible_user=myuser

You could also create a group to aggregate the hosts with similar settings, and then set up their variables at the group level:

~/ansible/inventory

[group_a]
server1 ansible_host=203.0.113.111 
server2 ansible_host=203.0.113.112

[group_b]
server3 ansible_host=203.0.113.113 
server4 ansible_host=server_hostname

[group_a:vars]
ansible_user=sammy

[group_b:vars]
ansible_user=myuser

This inventory arrangement would generate the following output with ansible-inventory:

Output{
    "_meta": {
        "hostvars": {
            "server1": {
                "ansible_host": "203.0.113.111", 
                "ansible_user": "sammy"
            }, 
            "server2": {
                "ansible_host": "203.0.113.112", 
                "ansible_user": "sammy"
            }, 
            "server3": {
                "ansible_host": "203.0.113.113", 
                "ansible_user": "myuser"
            }, 
            "server4": {
                "ansible_host": "server_hostname", 
                "ansible_user": "myuser"
            }
        }
    }, 
    "all": {
        "children": [
            "group_a", 
            "group_b", 
            "ungrouped"
        ]
    }, 
    "group_a": {
        "hosts": [
            "server1", 
            "server2"
        ]
    }, 
    "group_b": {
        "hosts": [
            "server3", 
            "server4"
        ]
    }
}

Notice that all inventory variables are listed within the _meta node in the JSON output produced by ansible-inventory.