Creating a host inventory

Before we even start writing our playbook with Ansible, we need to define an inventory of all hosts that need to be configured, and make it available for Ansible to use. Later, we will start running plays against a selection of hosts from this inventory. If you have an existing inventory, such as cobbler, LDAP, a CMDB software, or wish to pull it from a cloud provider, such as ec2, it can be pulled from Ansible using the concept of a dynamic inventory.

For text-based local inventory, the default location is /etc/ansible/hosts. For our learning environment, however, we will create a custom inventory file customhosts in our working directory, the contents of which are shown as follows. You are free to create your own inventory file:

#customhosts
#inventory configs for my cluster
[db]
192.168.61.11  ansible_ssh_user=vagrant

[www]
www-01.example.com ansible_ssh_user=ubuntu
www-02 ansible_ssh_user=ubuntu

[lb]
lb0.example.com

Now, when our playbook maps a play to the group, the www (hosts: www), hosts in that group will be configured. The all keywords will match to all hosts from the inventory.

The following are the guidelines to for creating inventory files:

Patterns

In the preceding playbook, the following lines decide which hosts to select to run a specific play:

- hosts: all
- hosts: www

The first code will match all hosts, and the second code will match hosts which are part of the www group.

Patterns can be any of the following or their combinations:

Tasks

Plays map hosts to tasks. Tasks are a sequence of actions performed against a group of hosts that match the pattern specified in a play. Each play typically contains multiple tasks that are run serially on each machine that matches the pattern. For example, take a look at the following code snippet:

- group:
 name:devops
 state: present
- name: create devops user with admin privileges
 user:
 name: devops
 comment: "Devops User"
 uid: 2001
 group: devops

In the preceding example, we have two tasks. The first one is to create a group, and second is to create a user and add it to the group created earlier. If you notice, there is an additional line in the second task, which starts with name:. While writing tasks, it's good to provide a name with a human-readable description of what this task is going to achieve. If not, the action string will be printed instead.

Each action in a task list can be declared by specifying the following:

action: apt name=htop state=present update_cache=yes
apt: name=nginx state=latest

Ansible stands out from other configuration management tools, with its batteries-included included approach. These batteries are "modules." It's important to understand what modules are before we proceed.

Running the playbook

Ansible comes with the ansible-playbook command to launch a playbook with. Let's now run the plays we created:

$ ansible-playbook simple_playbook.yml -i customhosts

Here is what happens when you run the preceding command:

Launching the preceding command will start calling plays, orchestrating in the sequence that we described in the playbook. Here is the output of the preceding command:

Let's now analyze what happened: