Build Configuration

Railway will build and deploy your code with zero configuration, but when necessary, there are several ways to configure this behavior to suit your needs.

Railpack

Railpack is a new builder developed by Railway that produces smaller image sizes and supports all versions of packages. It is currently in beta and can be enabled from your service settings.

Find the full list of configuration options in the Railpack docs.

Nixpacks Options

Railway uses Nixpacks to build your code. It has a variety of options that can be configured with environment variables which can be defined in your services settings. 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.

You can find a complete list of languages we support out of the box here.

Customize the Build Command

Using the default Nixpacks builder, you can customize the build command that is run from within your service settings.

For those familiar with Nixpacks, this gets set as the --build-cmd argument during the Nixpacks build.

Set the Root Directory

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 the defined root directory.

Note: The Railway Config File does not follow the Root Directory path. You have to specify the absolute path for the railway.json or railway.toml file.

Configure 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:

# Match all TypeScript files under src/
/src/**/*.ts

# Match Go files in the root, but not in subdirectories
/*.go

# Ignore all Markdown files
**
!/*.md

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

Install a Specific Package Using Nixpacks

When using Nixpacks, you can install specific packages by defining them in a nixpacks configuration file. For example:

[phases.setup]
    aptPkgs = ['wget']

See the Nixpacks docs for more information.

Build Providers

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

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.

Procfiles

If using Nixpacks, you can override the start command with a Procfile at the root of your app. Only a single process type is supported at the moment.

HTTP servers should use the web process type. This process should listen on the PORT environment variable and will receive HTTP traffic.

Note: Some buildpacks specify a default start command

Specify a Custom Install Command

We do not expose a way to configure a custom install command in the UI, but you can control this using config as code (see Nixpacks Plan -> Install Command).

Disable Build Layer Caching

By default, Railway will cache build layers to provide faster build times. If you have a need to disable this behavior, set the following environment variable in your service:

NO_CACHE=1

Why Isn't My Build Using Cache?

Since Railway's build system scales up and down in response to demand, cache hit on builds is not guaranteed.

If you have a need for faster builds and rely on build cache to satisfy that requirement, you should consider creating a pipeline to build your own image and deploy your image directly.