Scalingo Review Apps: On-Demand Preview Environments for Git Branches

Problem: Testing feature branches without polluting production

When a team pushes a new branch they need a sandbox to verify the changes. Deploying to the same environment risks breaking production or leaking secrets.

Solution: Scalingo Review Apps

Review Apps automatically spin up isolated, on-demand preview URLs for each branch, using the same buildpacks and configuration as the production app.

How Review Apps Work

• Each push to a feature branch creates a new dyno set with a unique sub-domain, e.g. feature-login-myapp.scalingo.io.
• Environment variables from the parent app are inherited, but you can override or add review-specific variables with the SCALINGO_REVIEW_APP flag or variable groups.
• Dyno types and sizes are identical to the parent, so resource consumption scales with the number of active previews.
• When a branch is deleted or merged, the corresponding review app is torn down automatically.

Step-by-Step Example

1. Create or select an app in the Scalingo dashboard.
2. Enable Review Apps in the Deploy tab.
3. Push a feature branch:

   git push scalingo feature-login

4. Verify the preview URL in the dashboard or by running:

   scalingo apps:review-apps:list --app myapp

5. Make a change in the branch, push again, and confirm the preview updates without affecting production.
6. To set a review-specific secret:

   scalingo --app myapp variables:create REVIEW_SECRET s3cr3t --scope review

7. When the branch is merged or deleted, the preview app disappears automatically.

Cost & Resource Considerations

• Review Apps consume dyno hours from your account quota. A large number of long-lived branches can inflate costs.
• Use the Scalingo dashboard to set a maximum number of concurrent review apps or to monitor dyno usage.
• Some add-ons (e.g. dedicated databases) are not clonable for review apps. In those cases, keep the preview on a shared service plan or use lightweight substitutes.

Trade-offs & Limitations

While Review Apps provide instant feedback, they can quickly exhaust your dyno budget if not monitored. Additionally, certain add-ons may not be available in preview environments, requiring architectural adjustments.

Actionable Take-aways

• Enable Review Apps in your Scalingo project and let the platform spin up previews automatically.
• Use variable groups or the SCALINGO_REVIEW_APP flag to keep secrets scoped to each preview.
• Set dyno limits or monitor usage through the dashboard to avoid unexpected charges.
• For databases or other heavy add-ons, consider using a shared plan for previews or refactor the architecture to use a lightweight clone.
• After merging, verify that the preview app is removed and that no orphaned dynos remain.

Scalingo Review Apps turn every branch into a safe, isolated testing ground, improving confidence before promotion to production.