Builds

Railway uses Nixpacks to build and deploy your code with zero configuration.

Currently, we support the following languages out of the box.

Build Configuration

Nixpacks has a variety of options that can be configured with environment variables. These include things like

Install/build/start commands
Nix/Apt packages to install
Directories to cache

For a full list of these options, please view the Nixpacks docs.

If you have a language or feature that you want us to support, please don't hesitate to reach out on Discord or on the Nixpacks GitHub repo.

Build Command

If using the Nixpacks builder, you can customize the build command that is run. For those familiar with Nixpacks, this gets set as the --build-cmd argument during the Nixpacks build.

The root directory defaults to / but can be changed for various use-cases like monorepo projects. When specified, all build and deploy commands will operate within that root directory. Additionally, files changed outside the root directory will not trigger a new build.

Watch Paths

Watch paths are gitignore-style patterns that can be used to trigger a new deployment based on what file paths have changed. For example, a monorepo might want to only trigger builds if files are changed in the /packages/backend directory. When specified, any changes that don't match the patterns will skip creating a new deployment. Multiple patterns can be combined, one per line.

Note, if a Root Directory is provided, patterns still operate from /. For a root directory of /app, /app/**.js would be used as a pattern to match files in the new root.

Here are a few examples of common use-cases.

Note, negations will only work if you include files in a preceding rule.

Build Providers

With Nixpacks, we analyze the app source directory and generate a build plan. This determines which language provider to install packages and runtimes for.

If there is a detected nixpacks.toml file, it is possible to define a build provider ahead of time like so:

Within your Service's settings, under the Builds section, you can define within the multi-select box which Nixpacks language providers you would like to use for your builds. This is useful if you have code that calls libraries that need to be built from another language within your repo.

When multiple providers are defined, Railway will build your service with the language providers (in the order you defined) and ensure your binaries are ready to be called. The runtime(s) will then initialize as soon as you start your application.