# Conditional backend builds (Gen 1 apps only)
<a name="conditional-backends"></a>

**Note**  
The information in this section is for Gen 1 apps only. Amplify Gen 2 introduces a TypeScript-based, code-first developer experience. Therefore, this feature isn't necessary for Gen 2 backends.

Amplify supports conditional backend builds on all branches in a Gen 1 app. To configure conditional backend builds, set the `AMPLIFY_DIFF_BACKEND` environment variable to `true`. Enabling conditional backend builds will help speed up builds where changes are made only to the frontend.

When you enable diff based backend builds, at the start of each build, Amplify attempts to run a diff on the `amplify` folder in your repository. If Amplify doesn't find any differences, it skips the backend build step, and doesn't update your backend resources. If your project doesn't have an `amplify` folder in your repository, Amplify ignores the value of the `AMPLIFY_DIFF_BACKEND` environment variable. For instructions on setting the `AMPLIFY_DIFF_BACKEND` environment variable, see [Configuring diff based backend builds for a Gen 1 app](edit-build-settings.md#enable-diff-backend).

If you currently have custom commands specified in the build settings of your backend phase, conditional backend builds won't work. If you want those custom commands to run, you must move them to the frontend phase of your build settings in your app's `amplify.yml` file. For more information about updating the `amplify.yml` file, see [Build specification reference](yml-specification-syntax.md).