Displaying the percentage of hosts completed during a rolling Ansible Deploy using serial

Recently we were looking at enhancing our deployment pipeline so that we run validation checks between every batch that is upgraded. For our VM deployments in Ansible we normally have

serial: 10%

so that 10% of the fleet is operated on at once, this includes draining from a load balancer before we upgrade the node, and adding it back to the load balancer after.

With Ansible, it doesn’t seem like there is any built-in way to get the host percentage, so here is a hacky solution for others who want to do something similar:

Initialize a fact to an empty array:

If we put this in pre_tasks: you will need to keep in mind it is called on every batch, so you don’t want to override it:

    - name: Initialize completed_hosts
      set_fact:
        completed_hosts: "{{ hostvars['localhost']['completed_hosts'] | default('[]') }}"
      delegate_to: localhost
      delegate_facts: true
      run_once: true

Another option is initialize it in a play that runs before the play that is operating on hosts, in that case you can just initialize it to an empty array:

    - name: Initialize completed_hosts
      set_fact:
        completed_hosts: []

In post_tasks, append the current batch to completed_hosts:


    - name: Appends to completed hosts
      set_fact:
        completed_hosts: "{{ hostvars['localhost']['completed_hosts'] + ansible_play_batch }}"
      delegate_to: localhost
      delegate_facts: true

Calculate a percentage:


    - name: Percentage complete
      debug:
        msg: "Percentage complete: {{ ((hostvars['localhost']['completed_hosts'] | length) / (ansible_play_hosts | length) * 100) | int }}%"
      run_once: true
      delegate_to: localhost

And here is an example test play:

- name: test
  gather_facts: false
  hosts: some_host_group
  serial: '10%'
  pre_tasks:
    - debug:
        msg: drain
    - name: Initialize completed_hosts
      set_fact:
        completed_hosts: "{{ hostvars['localhost']['completed_hosts'] | default('[]') }}"
      delegate_to: localhost
      delegate_facts: true
      run_once: true
  tasks:
    - debug:
        msg: deploy
  post_tasks:
    - debug:
        msg: enable
    - name: Appends to completed hosts
      set_fact:
        completed_hosts: "{{ hostvars['localhost']['completed_hosts'] + ansible_play_batch }}"
      delegate_to: localhost
      delegate_facts: true
    - name: Display current batch
      debug:
        var: ansible_play_batch
      delegate_to: localhost
      run_once: true
    - name: Display completed hosts
      debug:
        msg: "Percentage complete: {{ ((hostvars['localhost']['completed_hosts'] | length) / (ansible_play_hosts | length) * 100) | int}}%"
      run_once: true
      delegate_to: localhost