This is going to be a short piece but I really want to share this because 1) I have to talk! It cost me several hours today to get a grip on this and 2) I couldn’t find any explanation of this Ansible behaviour on Stack Overflow or anywhere else (I actually posted this on SO to make sure it’s now there). By the way, I was reminded today that it can save you several hours of bug tracking, experimenting and general hair-tearing if you just know to ask. the right. question.

Here at Mesosphere (and especially in the Cluster Ops team I’m in) we use Ansible a lot for various stuff related to spinning up/down and maintaining clusters. We build tools around making all of the operations of DC/OS (and other) clusters insanely easy. Since I’ve joined the company recently coming more from an application developer background and mostly developing tools in Go here I’m not the most proficient Ansible user on this planet. So what I had to achieve today was to run some tasks on all of the cluster’s nodes and some tasks only on one special node. What I came up with looked a bit like this:

01 - hosts: all
02  name: Test Play
03  gather_facts: false
04
05  tasks:
06      - name: Create output directory
07        tempfile:
08            state: directory
09            suffix: diag
10        register: output_dir
11
12      - name: Create API resources directory
13        file:
14            path: "{{ output_dir.path }}/api-resources"
15            state: directory
16        delegate_to: "{{groups['control-plane'][0]}}"
17        run_once: yes
18        register: api_resources_dir

The intent of this playbook was to create temporary directories on every node (for storing some command output) and on one and only one host this temporary directory should contain a directory named api-resources. When I ran this playbook, though, that host ended up with two temporary directories, one of which had the same name as the temporary directory on another host (and the latter was surprisingly (or not) the one that conducted the delegation).

What happened here?

Turns out, the expression {{ output_dir.path }} in the second task is evaluated before the task is delegated to the other node. Therefore, the node creates the api-resources directory in another directory as the one that is created in the first task.

What’s the correct way to do this?

The correct way is to first figure out what you’re doing wrong and why. That took me 90% of the time today. It’s probably just a matter of Ansible experience and of not just applying the same pattern (using delegate_to) you’ve seen elsewhere. Interestingly enough, I figured out the correct question only after I found the answer to my problem: “How do I run a task on one specific node?”. But when you think that delegate_to is the right solution you don’t even arrive at asking that question (again).

There’s this nice thing called when in Ansible that comes in handy here. Here’s the corrected playbook:

 1  - hosts: all
 2    name: Test Play
 3    gather_facts: false
 4
 5    tasks:
 6        - name: Create output directory
 7          tempfile:
 8              state: directory
 9              suffix: diag
10          register: output_dir
11
12        - name: Create API resources directory
13          file:
14              path: "{{ output_dir.path }}/api-resources"
15              state: directory
16          when: inventory_hostname == groups['control-plane'][0]
17          register: api_resources_dir

Nice and slick. I hope this post will save someone a bad day.

Have a great one!