# Concurrency Groups — GitHub Actions

Source: https://www.skillbyai.com/en/github-actions/s-concurrency

> Cancel superseded runs.

## One run per group

A `concurrency` group allows only one active run per group key; with `cancel-in-progress: true`, a new run cancels the older one in the same group. Grouping by `github.ref` cancels outdated runs when someone pushes again to a branch, saving minutes and giving faster feedback. For deployments, use a group per environment **without** cancelling, so deploys queue instead of being interrupted halfway.

## Runs cancelling older runs on the same ref, run

I ran this with Python 3. It is a simplified model of GitHub Actions behaviour for learning, not GitHub's implementation. Each push to feature-x cancels the previous run on that branch, while the run on main is in its own group and continues: runs 103 and 104 remain.

```python
pushes = [("refs/heads/feature-x", "run 101"), ("refs/heads/feature-x", "run 102"),
          ("refs/heads/main", "run 103"), ("refs/heads/feature-x", "run 104")]
running = {}
for ref, run in pushes:                       # concurrency: group ci-<ref>, cancel-in-progress: true
    group = f"ci-{ref}"
    if group in running:
        print(f"{run}: cancels {running[group]} in group {group}")
    else:
        print(f"{run}: starts in group {group}")
    running[group] = run
print("still running:", sorted(running.values()))
```

Output:

```
run 101: starts in group ci-refs/heads/feature-x
run 102: cancels run 101 in group ci-refs/heads/feature-x
run 103: starts in group ci-refs/heads/main
run 104: cancels run 102 in group ci-refs/heads/feature-x
still running: ['run 103', 'run 104']
```

## Do not cancel deployments

Use concurrency: { group: deploy-production, cancel-in-progress: false } so a deployment always finishes.

**Quiz:** What does cancel-in-progress: true do for a new run in the same group?

- [ ] Deletes the group
- [ ] Cancels the new run
- [ ] Runs both in parallel
- [x] Cancels the older in-progress run

*Answer:* Cancels the older in-progress run. Newest run wins.