Rolling Release with Ansible#

Problem#

• How can we use Ansible to follow a Rolling Release / Rolling Deployment strategy?
• How can we ensure that our service remains available at deployment time?

Solution#

Use serial in a Play:

Run the tasks on one host before going to the next host.

- hosts: nameservers
  serial: 1
  tasks:
  ...

For larger clusters, we can define serial as a list and define how much of hosts are allowed to fail at max using max_fail_percentage:

- hosts: apps
  max_fail_percentage: 30
  serial:
    - 1
    - 3
    - 10%
    - 30%
  tasks:
  ...

Explanation#

Especially when using a logical cluster, e.g. several authoritative DNS servers, CDN hosts or a worker queue, a rolling release strategy helps to improve the user experience to keep the some hosts untouched if anything goes wrong.

With serial we define the batch size, i.e. the maximum number of hosts (rounded up to 1) that Ansible is allowed to change on the next run. The Play now runs in a loop until all hosts have been taken into account.

If serial is defined as a list, the Batch size is recalculated for each run based on the value in the list, the last value is then applied as long as it still has hosts.