delegate_to & run_once for Orchestration

By default each task runs on its target host. For orchestration you sometimes need a task to run somewhere else entirely. 🎯

The delegate_to keyword runs a task on a different host than the current one, while still using that host's variables.

While looping over web nodes, delegate the load balancer change to the LB host. The play stays per-node but the action lands on the LB.

Run a task on your control node by delegating to localhost. Great for calling an API or sending a notification from one place.

Delegation moves where a task runs, not whose facts it sees. inventory_hostname still names the original target, not the delegate.

The run_once keyword runs a task a single time for the whole batch, not once per host. Ansible picks the first host to execute it.

Some actions must happen only once, like a migration or a leader election. run_once stops every host from repeating the same shared step.

Combine run_once with delegate_to to do one action on one chosen host, like running a migration from a single app node.

With run_once, a registered result is shared across all hosts in the batch. They all see the output from that single execution.

Use delegate_facts if you want gathered facts to attach to the delegate host instead of the current one. Off by default.

Together these keywords turn per-host config into true orchestration: coordinating one action across a fleet from a single point.

Let us check these orchestration keywords.

Use delegate_to to run a task on another host and run_once to run it just once per batch, the core of orchestration. ✅