An Ansible MVP (Minimum Viable Playbook) for Testing Tasks

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
    my_name: a_b_c
    - 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 while 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!

If your Django project’s long test runs bore you, I wrote a book that can help.

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