community.general/docsite/rst/dev_guide/overview_architecture.rst

Ansible Architecture
====================

Ansible is a radically simple IT automation engine that automates cloud provisioning, configuration management, application deployment, intra-service orchestration, and many other IT needs.

Being designed for multi-tier deployments since day one, Ansible models your IT infrastructure by describing how all of your systems inter-relate, rather than just managing one system at a time.

It uses no agents and no additional custom security infrastructure, so it's easy to deploy - and most importantly, it uses a very simple language (YAML, in the form of Ansible Playbooks) that allow you to describe your automation jobs in a way that approaches plain English.

In this section, we'll give you a really quick overview of how Ansible works so you can see how the pieces fit together.

Modules
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Ansible works by connecting to your nodes and pushing out small programs, called "Ansible Modules" to them. These programs are written to be resource models of the desired state of the system. Ansible then executes these modules (over SSH by default), and removes them when finished.

Your library of modules can reside on any machine, and there are no servers, daemons, or databases required. Typically you'll work with your favorite terminal program, a text editor, and probably a version control system to keep track of changes to your content.

Inventory
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By default, Ansible represents what machines it manages using a very simple INI file that puts all of your managed machines in groups of your own choosing.  

To add new machines, there is no additional SSL signing server involved, so there's never any hassle deciding why a particular machine didn’t get linked up due to obscure NTP or DNS issues.

If there's another source of truth in your infrastructure, Ansible can also plugin to that, such as drawing inventory, group, and variable information from sources like EC2, Rackspace, OpenStack, and more.

Here's what a plain text inventory file looks like::

    ---
    webservers]
    www1.example.com
    www2.example.com

    [dbservers]
    db0.example.com
    db1.example.com

Once inventory hosts are listed, variables can be assigned to them in simple text files (in a subdirectory called 'group_vars/' or 'host_vars/' or directly in the inventory file.

Or, as already mentioned, use a dynamic inventory to pull your inventory from data sources like EC2, Rackspace, or OpenStack.

Playbooks
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Playbooks can finely orchestrate multiple slices of your infrastructure topology, with very detailed control over how many machines to tackle at a time.  This is where Ansible starts to get most interesting.

Ansible's approach to orchestration is one of finely-tuned simplicity, as we believe your automation code should make perfect sense to you years down the road and there should be very little to remember about special syntax or features.

Here's what a simple playbook looks like:: 

    ---
    - hosts: webservers
    serial: 5 # update 5 machines at a time
    roles:
    - common
    - webapp

    - hosts: content_servers
    roles:
    - common
    - content


Extending Ansible with Plug-ins and the API
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Should you want to write your own, Ansible modules can be written in any language that can return JSON (Ruby, Python, bash, etc). Inventory can also plug in to any datasource by writing a program that speaks to that datasource and returns JSON. There's also various Python APIs for extending Ansible’s connection types (SSH is not the only transport possible), callbacks (how Ansible logs, etc), and even for adding new server side behaviors.