Limit ansible playbook task concurrency

Question

I'm updating several hosts with ansible at the same time, however I have a limitation...

I have to download artifacts from a common repository with no more than 3 simultaneous downloads!

The current solution I have is to limit the whole playbook to max three concurrent tasks

strategy: linear
serial: 3

Is it possible to limit concurrency only for particular task step rather than the whole playbook?

Answer 1

There's no direct way. Only workarounds like run_once loop with delegate_to or multiplying the task with loop and executing only one item per host.

See issue #12170, which is closed with "won't fix" status for details.

delegate_to loop:

- mytask: ..
  delegate_to: "{{item}}"
  run_once: true
  # many diff ways to make the loop
  with_inventory_hostnames: all

multiplied task:

- name: target task
  debug: msg="Performing task on {{ inventory_hostname }}, item is {{ item }}"
  with_items: "{{ play_hosts }}"
  when: "hostvars[item].inventory_hostname == inventory_hostname"

Answer 2

Yes, it's possible to only limit the concurrency of a certain task.

You just need to add the throttle keyword to your download task.

Example:

- name: Download payload.tar.gz
  get_url:
    url: https://example.com/path/payload.tar.gz
    dest: /mnt/scratch/payload.tar.gz
    mode: '0640'
  throttle: 3

Note that throttle was introduced in Ansible 2.9.