Last updated October 16, 2024

This article describes how to take an existing Scala app and deploy it to Heroku.

Prerequisites

Before you begin, make sure you have installed Scala, sbt, and the Heroku CLI. This article assumes that you are starting with an existing Scala app that is set up to build with sbt.

Overview

The way Heroku recognizes a Scala application is described in Heroku Scala Support.

Your application should already include a project/build.properties file that defines the sbt.version it is designed to use. It will look something like this:

sbt.version=0.13.9

For more information, see Build behavior.

Verify that the sbt-native-packager plugin is set up correctly

To package your app for deployment, you can use the sbt-native-packager sbt community plugin. If you are using the Play framework version 2.3 or higher, then this plugin will be automatically configured for you.

If you are not using Play framework, you must configure the plugin manually. The project/plugins.sbt file will specify which version of the sbt-native-packager to use. Add the following to your project/plugins.sbt file:

addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.0.3")

In your build.sbt enable the plugin you want. For example the JavaAppPackaging.

enablePlugins(JavaAppPackaging)

For more detailed information, see the sbt documentation, and the sbt-native-packager documentation.

Specify a JDK

Optionally, you can specify a JDK. For more information, see Specifying a Java version.

The Procfile

A Procfile is a text file in the root directory of your application, that defines process types and explicitly declares what command should be executed to start your app. If you are using the sbt-native-packager and the java_application package archetype, you will have a start script in target/universal/stage/bin. Your Procfile will look something like this, except instead of hello, you’d use the name of your app:

web: target/universal/stage/bin/hello

This declares a single process type, web, and the command needed to run it. The name, web, is important here. It declares that this process type will be attached to the HTTP routing stack of Heroku, and receive web traffic when deployed.

The command in a web process type must bind to the port number specified in the PORT environment variable. If it does not, the dyno will not start.

How to keep build artifacts out of git

Prevent build artifacts from going into revision control by creating a .gitignore file. Here’s a typical .gitignore file:

target/
project/boot

Build your app and run it locally

To build your app locally do this:

$ sbt compile stage
$ heroku local --port 5001

Your app should now be running on http://localhost:5001/.

Deploy your application to Heroku

After you commit your changes to git, you can deploy your app to Heroku.

$ git add .
$ git commit -m "Added a Procfile and packaging."
$ heroku login
...
$ heroku create
Creating warm-frost-1289... done, stack is heroku-18
http://warm-frost-1289.herokuapp.com/ | git@heroku.com:warm-frost-1289.git
Git remote heroku added
$ git push heroku master
...
-----> Scala app detected

To open the app in your browser, type heroku open.

Console

Heroku allows you to run one-off dynos - scripts and applications that only need to be executed when needed - using the heroku run command. The console has your application code available. Use this to launch a REPL process attached to your local terminal for experimenting in your app’s environment:

$ heroku run sbt console
Running sbt console attached to terminal... up, run.1
...
scala>

For more information, see Running a Remote sbt Console for a Scala or Play Application.

Troubleshooting

Major releases of sbt may not be compatible with each other. Most issues will be related to mismatched versions of sbt.