Why not use something that's already available, such as Bourne shell script or PowerShell? For simple deployments, shell scripts may be a better approach. But as our deployment process becomes more complex, it is much harder to handle every possible initial state using the shell's conditional statements.
Dealing with differences between initial states is actually something Ansible is especially good at. Unlike traditional shell scripts, which use the imperative form (move this file, edit that file, run a particular command), Ansible playbooks, as they are called, use the declarative form (make sure the file is available in this path, make sure the file contains specified lines, make sure the program is running, make sure the program completes successfully).
This declarative approach also helps to achieve idempotence. Idempotence is a feature of a function that means applying the function several times over will have exactly the same results as a single application. If the first run of an Ansible playbook introduces some changes to the configuration, each subsequent run will already start in the desired state. This prevents Ansible from performing any additional changes.
In other words, when you invoke Ansible, it will first assess the current state of all the machines you wish to configure:
- If any of them requires any changes, Ansible will only run the tasks required to achieve the desired state.
- If there's no need to modify a particular thing, Ansible won't touch it. Only when the desired and actual states differ will you see Ansible taking action to converge the actual state toward the desired one described by the contents of the playbook.
