# Trigger Rules — Apache Airflow: Orchestrate Data Pipelines

Source: https://www.geekswithgeeks.com/en/airflow/flow-trigger-rules

> Control when a task runs depending on the states of its upstream tasks.

## Not only "all succeeded"

By default a task runs when **all** upstream tasks succeeded (`all_success`). Other rules include `all_done` (run once everything finished, whatever the outcome: good for cleanup and notifications), `one_failed` (run an alert when something failed), `none_failed` and `none_failed_min_one_success` (continue after skips, fail if any upstream failed). Choosing the right rule is how you build cleanup steps that always run and alerts that only run on failure.

## Evaluating rules, run

I ran a plain-Python model of four rules for upstream states `success` and `failed`. Only `all_done` and `one_failed` would let the task run; `all_success` and `none_failed` would not.

```python
def ok(rule, states):
    s = list(states)
    return {"all_success": all(x == "success" for x in s),
            "all_done": all(x in ("success", "failed", "skipped") for x in s),
            "one_failed": any(x == "failed" for x in s),
            "none_failed": all(x != "failed" for x in s)}[rule]

ups = ["success", "failed"]
print({r: ok(r, ups) for r in ("all_success", "all_done", "one_failed", "none_failed")})
```

Output:

```
{'all_success': False, 'all_done': True, 'one_failed': True, 'none_failed': False}
```

## Use all_done for cleanup

Delete temporary tables and files in a task with `trigger_rule="all_done"` so cleanup runs even after failures.

**Quiz:** Which rule runs a failure alert only when an upstream task failed?

- [ ] all_success
- [x] one_failed
- [ ] always_skip
- [ ] none_failed

*Answer:* one_failed. one_failed triggers as soon as at least one upstream task has failed.