Add Deploy A Review App To A Different Stack as a Heroku til

README.md

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 ### Heroku
 
+- [Deploy A Review App To A Different Stack](heroku/deploy-a-review-app-to-a-different-stack.md)
 - [Set And Show Heroku Env Variables](heroku/set-and-show-heroku-env-variables.md)
 
 ### HTML

heroku/deploy-a-review-app-to-a-different-stack.md

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+# Deploy A Review App To A Different Stack
+
+Heroku has different stacks for deploying apps. As newer versions of Linux
+distros (such as Ubuntu) come out and software packages need patching and
+updating, Heroku releases new stacks. And deprecates older ones.
+
+Before upgrading a production app to a new stack, you should test it out first.
+The recommended way to test this out is with a Review App.
+
+If you need to bump the Ruby version or make any other changes, do that first.
+
+Then add or update your `app.json` file. This is a Heroku-specific file that
+tells Heroku what stack to use when creating a new app or review app.
+
+```json
+{
+  "stack": "heroku-18"
+}
+```
+
+Let's say my app is currently on the Heroku-16 stack. I can set the `stack` to
+be `heroku-18` in `app.json`. Then I can push up a branch with all these
+changes and turn it into a PR. From the Heroku dashboard, I can click the
+Heroku Button that builds a Review App from the PR.
+
+Once it it finishes building and deploying, Heroku will give me a custom URL
+for visiting the app so that I can manually evaluate it.
+
+[source](https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack#testing-an-app-on-a-new-stack)