App reference
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To define your app, you can either use one of Upsun’s single-runtime image or its composable image (BETA).
Single-runtime image 
Upsun provides and maintains a list of single-runtime images you can use for each of your application containers. See all of the options you can use to define your app using a single-runtime image.
Composable image (BETA)
The Upsun composable image provides more flexibility than single-runtime images. When using a composable image, you can define a stack (or group of packages) for your application container to use.
There are over 80,000 packages available from the Nix Packages collection that you can add to your stack. You can add as many packages to your application container as you need.
Note
Upsun guarantees optimal user experience with the specific set of packages it supports. You can use any other package available from the Nix Packages collection, including unstable ones, but NixOs is responsible for their support.
See all of the options you can use to define your app using the composable image.