How to make Ansible execute a playbook sequentially on one host at a time (for updating a Postgres cluster)?

To make Ansible execute a playbook sequentially on one host at a time, you need to use the serial parameter with a value of 1 in the playbook. This will force Ansible to process hosts one by one, which is useful for updating a cluster to avoid updating all nodes simultaneously.

Example:

- hosts: postgres_cluster
  serial: 1
  tasks:
    - name: Update Postgres
      yum:
        name: postgresql
        state: latest

You can also use --forks=1 when running ansible-playbook to limit the number of parallel processes to one.

ansible-playbook playbook.yml --forks=1