Use ansible for manual staged rollout using `serial` and unknown inventory size

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Consider an Ansible inventory with an unknown number of servers in a nodes key.

The script I'm writing should be usable with different inventories that should be as simple as possible and are out of my control, so I don't know the number of nodes ahead of time.

My command to run the playbook is pretty vanilla and I can freely change it. There could be two separate commands for both rollout stages.

ansible-playbook -i $INVENTORY_PATH playbooks/example.yml

And the playbook is pretty standard as well and can be adjusted:

- hosts: nodes
  vars:
    ...
  remote_user: '{{ sudo_user }}'
  gather_facts: no
  tasks:
    ...

How would I go about implementing a staged execution without changing the inventory?

I'd like to run one command to execute the playbook for 50% of the inventory first. Here the result needs to be checked manually by a human. Then I'd like to use another command to execute the playbook for the other half. The author of the inventory should not have to worry about this. All machines below the nodes key are the same.

I've looked into the serial keyword, but it doesn't seem like I could automatically end execution after one batch and then later come back to continue with the second half.

Maybe something creative could be done with variables passed to ansible-playbook? I'm just wondering, shouldn't this be a common use-case? Are all staged rollouts supposed to be fully automated?

ansible
1 Answers

Without even using serial here is a possible very simple scenario.

First get a calculation of $half of the inventory by inspecting the inventory itself. The following is enabling the json callback plugin for the ad hoc command and making sure it is the only plugin enabled. It is also using jq to parse the result. You can adapt to any other json parser (or even use the yaml callback with a yaml parser if your prefer). Anyway, adapt to your own needs.

half=$( \
ANSIBLE_LOAD_CALLBACK_PLUGINS=1 \
ANSIBLE_STDOUT_CALLBACK=json \
ANSIBLE_CALLBACK_WHITELIST=json \
ansible localhost -i yourinventory.yml -m debug -a "msg={{ (groups['nodes'] | length / 2) | round(0, 'ceil') | int }}" \
| jq -r ".plays[0].tasks[0].hosts.localhost.msg" \
)

Then launch your playbook limiting to the first $half nodes with whatever vars are needed for human check, and launch it again for the remainder nodes without check.

ansible-playbook -i yourinventory.yml example_playbook.yml -l nodes[0:$(($half-1))] -e human_check=true
ansible-playbook -i yourinventory.yml example_playbook.yml -l nodes[$half:] -e human_check=false
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