Ansible

Ansible is a extra-simple tool/API for doing 'parallel remote things' over SSH -- whether executing commands, running declarative 'modules', or executing larger 'playbooks' that can serve as a configuration management or deployment system.

While Func, which I co-wrote, aspired to avoid using SSH and have it's own daemon infrastructure, Ansible aspires to be quite different and more minimal, but still able to grow more modularly over time. This is based on talking to a lot of users of various tools and wishing to eliminate problems with connectivity and long running daemons, or not picking tool X because they preferred to code in Y.

Why use Ansible versus something else? (Fabric, Capistrano, mCollective, Func, SaltStack, etc?) It will have far less code, it will be more correct, and it will be the easiest thing to hack on and use you'll ever see -- regardless of your favorite language of choice. Want to only code plugins in bash or clojure? Ansible doesn't care. The docs will fit on one page and the source will be blindingly obvious.

Design Principles

* Dead simple setup
* Super fast & parallel by default
* No server or client daemons, uses existing SSHd
* No additional software required on client boxes
* Everything is self updating on the clients  
* Encourages use of ssh-agent
* Plugins can be written in ANY language
* API usage is an equal citizen to CLI usage
* Can be controlled/installed/used as non-root

Requirements

For the server the tool is running from, only:

* python 2.6 -- or the 2.4/2.5 backport of the multiprocessing module
* PyYAML (if using playbooks)
* paramiko

Inventory file

The inventory file is a required list of hostnames that can be potentially managed by ansible. Eventually this file may be editable via the CLI, but for now, is edited with your favorite text editor.

The default inventory file (-H) is /etc/ansible/hosts and is a list of all hostnames to target with ansible, one per line. These can be hostnames or IPs

Example:

abc.example.com
def.example.com
192.168.10.50
192.168.10.51

This list is further filtered by the pattern wildcard (-P) to target specific hosts. This is covered below.

You can organize groups of systems by having multiple inventory files (i.e. keeping webservers different from dbservers, etc)

Command line usage example

Run a module by name with arguments

• ssh-agent bash
• ssh-add ~/.ssh/id_rsa.pub
• ansible -p "*.example.com" -n modName -a "arg1 arg2"

API Example

The API is simple and returns basic datastructures. Ansible will keep track of which hosts were successfully contacted seperately from hosts that had communication problems. The format of the return, if successful, is entirely up to the module.

import ansible
runner = ansible.runner.Runner(
    pattern='*',
    module_name='inventory',
    module_args='...' 
)
data = runner.run()

{ 
    'contacted' : {
        'xyz.example.com' : [ 'any kind of datastructure is returnable' ],
        'foo.example.com' : [ '...' ]
    },
    'dark' : {
        'bar.example.com' : [ 'failure message' ]
    }
}

Additional options to Runner include the number of forks, hostname exclusion pattern, library path, arguments, and so on.

Read the source, it's not complicated.

Patterns

To target only hosts starting with "rtp", for example:

• ansible -p "rtp*" -n command -a "yum update apache"

Parallelism

Specify the number of forks to use, to run things in greater parallelism.

* ansible -f 10 "*.example.com" -n command -a "yum update apache"

10 forks. The default is 3. 5 is right out.

File Transfer

Ansible can SCP lots of files to lots of places in parallel.

• ansible -p "web-*.acme.net" -f 10 -n copy -a "/etc/hosts /tmp/hosts"

Ansible Library (Bundled Modules)

See the example library for modules, they can be written in any language and simply return JSON to stdout. The path to your ansible library is specified with the "-L" flag should you wish to use a different location than "/usr/share/ansible". This means anyone can use Ansible, even without root permissions.

There is potential for a sizeable community to build up around the library scripts.

Modules include:

• command -- runs commands, giving output, return codes, and run time info
• ping - just returns if the system is up or not
• facter - retrieves facts about the host OS
• ohai - similar to facter, but returns structured data
• copy - add files to remote systems

Playbooks

Playbooks are loosely equivalent to recipes or manifests in most configuration management or deployment tools and describe a set of operations to run on a set of hosts. Some tasks can choose to only fire when certain conditions are true, and if a task in a chain fails the dependent tasks will not proceed. Playbooks are described in (YAML)[http://yaml.org] format.

Future plans

• see TODO.md

License

• MIT

Author

Michael DeHaan -- michael.dehaan@gmail.com

http://michaeldehaan.net