Heroku Buildpack for Scala

This is the official Heroku buildpack for Scala apps.

How it works

The buildpack will detect your app as Scala if it has a project/build.properties file and either a .sbt or .scala based build config (for example, a build.sbt file). It vendors a version of sbt into your slug (if you are not using sbt-native-packager, it also includes your popluated .ivy/cache in the slug). The .ivy2 directory will be cached between builds to allow for faster build times.

It is strongly recommended that you use sbt-native-packager with this buildpack instead of sbt-start-script. The latter is deprecated, and will result in exessively large slug sizes.

Documentation

For more information about using Scala and buildpacks on Heroku, see these articles:

• Heroku's Scala Support
• Play Documentation: Deploying to Heroku
• Customizing the JDK
• Running a Remote sbt Console for a Scala or Play Application
• Using Node.js to perform JavaScript optimization for Play and Scala applications
• Reducing the Slug Size of Play 2.x Applications

Examples

There are a number of example applications that demonstrate various ways of configuring a project for use on Heroku. Here are a few:

• Play Database seed
• Play Silhouette Angular seed
• Minimal sbt example
• Lift example

Customizing

This buildpack uses sbt-extras to run sbt. In this way, the execution of sbt can be customized either by setting the SBT_OPTS config variable, or by creating a .sbtopts file in the root directory of your project. When passing options to the underlying sbt JVM, you must prefix them with -J. Thus, setting stack size for the compile process would look like this:

$ heroku config:set SBT_OPTS="-J-Xss4m"

Running additional tasks before the build

Sometimes, it might be necessary to run additional sbt tasks before a build and deployment (for example, database migrations). Ideally, the tasks should be interdependent such that these tasks run automatically as pre-requisities to compile stage, but sometimes this might not be the case. To add any additional tasks, set the environment variable SBT_PRE_TASKS to a list of tasks that should be executed. If the following is set:

SBT_PRE_TASKS=flyway:migrate info

Then, the following command will be run for build:

sbt flyway:migrate info compile stage

Clean builds

In some cases, builds need to clean artifacts before compiling. If a clean build is necessary, configure builds to perform clean by setting SBT_CLEAN=true:

$ heroku config:set SBT_CLEAN=true
Setting config vars and restarting example-app... done, v17
SBT_CLEAN: true

All subsequent deploys will use the clean task. To remove the clean task, unset SBT_CLEAN:

$ heroku config:unset SBT_CLEAN
Unsetting SBT_CLEAN and restarting example-app... done, v18

Development

To make changes to this buildpack, fork it on Github. Push up changes to your fork, then create a new Heroku app to test it, or configure an existing app to use your buildpack:

# Create a new Heroku app that uses your buildpack
heroku create --buildpack <your-github-url>

# Configure an existing Heroku app to use your buildpack
heroku buildpacks:set <your-github-url>

# You can also use a git branch!
heroku buildpacks:set <your-github-url>#your-branch

License

Licensed under the MIT License. See LICENSE file.