What Ansible is and how it works

Why automate?

Setting up one server by hand takes an hour. Setting up fifty takes a week, and no two end up exactly the same: someone forgets a firewall rule here, a package version differs there. These small differences ("configuration drift") cause the hardest outages. Automation writes the steps down once, as code, and applies them the same way everywhere. The code also documents the setup and can be reviewed and versioned in Git.

How Ansible works

• Control node: the machine where Ansible is installed and where you run it (your admin workstation or a jump server). Needs Linux and Python.
• Managed nodes: the servers you configure. They need only SSH and Python, no Ansible agent. That is what "agentless" means.
• Inventory: a list of managed nodes, in groups such as webservers and dbservers.
• Module: a small program for one job (install a package, copy a file, create a user). Ansible copies it to the node over SSH, runs it, and reads back a JSON result.
• Playbook: a YAML file listing which modules to run on which hosts, in order.

Idempotency: the key idea

Shell scripts usually say what to DO: "append this line", "create this user". Run them twice and you get two lines, or an error that the user exists. Ansible modules say what should BE TRUE: "this line is present", "this user exists". The module checks first and only changes something if needed. The result says "ok" (already right) or "changed" (fixed it). So you can run the same playbook every day safely; it fixes drift and leaves correct servers alone.

Ansible products

# dnf install -y ansible-core
$ ansible --version | head -3
ansible [core 2.14.17]
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/home/asha/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']

YAML in five rules

Playbooks and many inventories are written in YAML, a text format for data. Learn these rules now; most beginner errors are YAML errors:

1. Indentation shows structure, and it must be SPACES (usually 2), never tabs.
2. key: value pairs, with a space after the colon.
3. A list item starts with "- " (dash and space).
4. Quote values that start with {{ (Jinja) or contain ": " or #.
5. A file may start with --- .

---
server:
  name: web01
  ports:
    - 80
    - 443
  owner: "Asha Rao"

Common mistakes

• Using Ansible like a shell script: running shell or command modules for everything. Prefer a real module; it is idempotent and reports changes correctly.
• Tabs in YAML. Configure your editor to insert spaces.
• Forgetting that every managed node needs Python. RHEL 9 has python3 by default; minimal containers may not.