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On ansible and the script module

2014/05/13

Ansible offers the convenience of running scripts on remote servers. But as the documentation notes:

It is usually preferable to write Ansible modules than pushing scripts. Convert your script to an Ansible module for bonus points!

There is a reason for this. Usually you have ansible run a script on your behalf when what you want to do is not achievable via a module or some combination of modules in a playbook. In extreme circumstances you will need to run a script via ansible when the receiving computer has no Python installed.

But there is a problem with running scripts this way: They are opaque.

A playbook that is applied to your machines is actually a model of that part of the machines that you want to manage. And ansible is your sensor that deals with the situation when things go sour.

It is very easy to write a script that does one thing well to one machine and does not check for failure. Now apply this to 100 or 500 machines that are similar, yet have some subtle differences between them. Can you imagine what a rewrite your script needs in order to account for all corner cases? And if you make it bullet-proof, congratulations! You’re half-way through to making your own incompatible version of ansible.

Having said that, I am guilty of running scripts instead of describing work to be done in a playbook. This mostly involves stuff that needs to be executed from a login shell (hello rvm!) which means the script begins with #!/bin/bash. However, in order to exercise better control in such situations I am not running more than one command plus checks for return codes in every script. This breaks the script down in many smaller ones, but allows me a better view when something goes wrong. Because my playbooks instead of having one script directive, they have 5 or six in a row.

You may have not described an accurate model of what you want to do using a playbook’s markup, but at least the name: directive for every single task is accurate enough to let you know what is executing, rather than having it issue a larger script where you wait whether it succeeded or not, and if not try to find out from which point exactly to roll back (if rolling back is possible).

So the new rule is:

When pushing a script through ansible, it should execute one command only plus any checks needed for return status.

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