Adam Johnson

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An Ansible MVP (Minimum Viable Playbook) for Testing Tasks

2015-04-18

MVPs are tiny.

I showed this briefly in my post on merging groups and hostvars, but I thought it was worth writing a whole post on. The Minimum Viable Playbook (MVP) is the shortest, most useful Ansible playbook I have. Whenever I need to write some Ansible code and I’m not entirely sure I’m doing it right (which is often), I implement it first in the MVP so I can test it quickly. Here’s my latest iteration:

- hosts: localhost
  connection: local
  gather_facts: no
  vars:
    my_name: a_b_c
  tasks:
    - debug: msg={{ my_name|replace('_', '-') }}

I keep the MVP saved as ~/tmp/test.yml, from which it can be run with:

$ ansible-playbook -i localhost, ~/tmp/test.yml

(The -i argument allows you to specify the inventory as a comma-separated string of hosts, but it looks a little weird with only one host. You can’t leave out the comma, otherwise it gets interpreted as a filename.)

The MVP runs in about 0.2 seconds giving me quick feedback:

PLAY [localhost] **************************************************************

TASK: [debug ] ****************************************************************
ok: [localhost] => {
    "msg": "a-b-c"
}

PLAY RECAP ********************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

Here I was testing a rather trivial use of the replace filter, but since I wasn’t sure I remembered how it worked, it was faster to use the MVP than to test the full task I was writing, or to go to the documentation. Running the actual playbook would take minutes rather than seconds (even with --start-at-task, since it relied on other tasks), and whilst going to the documentation is normally fast (I use Dash for quick local look-ups), but it can be tricky when the answer might be split between the Ansible and Jinja 2 docs.

Two things make the MVP particularly fast:

  1. When running tasks locally, Ansible always uses the local connection type, which means there is no SSH overhead.

  2. I’ve disabled fact-gathering which means my tasks are executed immediately. This saves about a second per run, which isn’t much but you do notice it.

Hope this helps you. If there’s something I’m missing, please let me know!


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Tags: ansible