If we already have instances running, the previous tasks are skipped and the single task in roles/ec2/existing_deployment.yml is run. This task simply takes one of the running hosts and adds it to the host group named ec2_instance:

- name: add one of our running instances to a host group for use in the next step
  add_host:
    name: "{{ groups['already_running'][0] }}"
    ansible_ssh_host: "{{ groups['already_running'][0] }}"
    groups: "ec2_instance"

This leaves us in the same position as we were in at the end of the new deployment tasks, with a host called ec2_instance with a single instance that is accessible over SSH.