DevOps - Ansible: Using 'delegate_to' and 'run_once' for Multi-Tier Orchestration

Beyond Simple Playbooks

Most Ansible tutorials show you how to install a package on a group of servers. In a highly skilled infrastructure role, you often need to orchestrate actions across different tiers. For example, you need to tell your Load Balancer to stop sending traffic to web01 before you begin the update on web01.

The Tool: delegate_to

delegate_to allows a task to execute on a machine other than the current inventory_hostname.

Scenario: Zero-Downtime Deployment

We want to update 10 web servers, one by one, ensuring each is removed from the HAProxy load balancer first.

- hosts: webservers
  serial: 1 # Crucial: Process one server at a time
  tasks:
    - name: Disable server in HAProxy
      community.network.haproxy:
        state: disabled
        host: ""
        backend: app_pool
      delegate_to: lb01 # This runs on the load balancer, NOT the web server

    - name: Update Web Application
      apt:
        name: my-app
        state: latest

    - name: Enable server in HAProxy
      community.network.haproxy:
        state: enabled
        host: ""
        backend: app_pool
      delegate_to: lb01

The Tool: run_once

Sometimes you have a task that only needs to happen once for the entire play, regardless of how many hosts are in the inventory (e.g., running a database migration).

- name: Run DB Migrations
  command: /usr/bin/php artisan migrate
  run_once: true

Troubleshooting Key Considerations

• Variable Scope: When you use delegate_to, the variables available are still those of the original host, not the delegated host. If you need the delegated host’s IP, you must access it via hostvars['lb01']['ansible_default_ipv4']['address'].
• Connection Overhead: If you delegate 100 tasks to a single master server, Ansible will open 100 separate SSH connections. Use pipelining = True in your ansible.cfg to mitigate this.
• Local Delegation: delegate_to: localhost is frequently used to interact with cloud APIs (AWS/Azure) or to send a Slack notification after a deployment is complete.

Summary

Advanced orchestration is about managing dependencies between hosts. By mastering delegate_to and run_once, you move from being a “server configurer” to an “infrastructure orchestrator.”