Deploying to Heroku

Heroku is a cloud application platform that makes it easy to deploy and scale applications. With the right project setup, deploying your application becomes as easy as git push.

See scalatra-heroku for a minimal and standalone project containing the example in this guide.

1. Sign up for a free a free account.

2. Install the Heroku Toolbelt.

3. Set up a project.

Create a Scalatra project from the usual Scalatra giter8 template. Check out the the installation and first project guides if this isn’t familiar.

$ g8 scalatra/scalatra-sbt -b develop

4. Tweak the app for Heroku

Make Jetty embeddable

Open project/build.scala in the root of your project. You will find two lines like these:

"org.eclipse.jetty" % "jetty-webapp" % "9.1.5.v20140505" % "container",
"org.eclipse.jetty" % "jetty-plus" % "9.1.5.v20140505" % "container",

Those are basically right, but we need to add compile scope because Heroku is not a servlet host. It can only run your app via an embedded Jetty server you provide. So replace the two lines above with these two:

"org.eclipse.jetty" % "jetty-webapp" % "9.1.5.v20140505" % "compile;container",
"org.eclipse.jetty" % "jetty-plus" % "9.1.5.v20140505" % "compile;container",

Add the sbt Native Packager plugin

You don’t want to use sbt to run your app in production. We’ll install an sbt plugin that will create a start script during compilation. Heroku will use that start script. Tell sbt where to find the plugin by adding this line to project/plugins.sbt (you may need to create the file first):

addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.0.0-RC1")

Then enable the plugin by calling the enablePlugins method on the Project object in your build.scala:

import com.typesafe.sbt.packager.archetypes.JavaAppPackaging
// ...
lazy val project = Project (
// ...
).enablePlugins(JavaAppPackaging)

Create a main method

Since Heroku launches your app without a container, you need to create a main method that launches the servlet.

Create src/main/scala/JettyLauncher.scala with this code:

import org.eclipse.jetty.server.Server
import org.eclipse.jetty.servlet.{ DefaultServlet, ServletContextHandler }
import org.eclipse.jetty.webapp.WebAppContext
import org.scalatra.servlet.ScalatraListener

object JettyLauncher {
  def main(args: Array[String]) {
    val port = if(System.getProperty("http.port") != null) System.getProperty("http.port").toInt else 8080

    val server = new Server(port)
    val context = new WebAppContext()
    context.setContextPath("/")
    context.setResourceBase("src/main/webapp")

    context.setEventListeners(Array(new ScalatraListener))

    server.setHandler(context)

    server.start
    server.join
  }
}

And don’t forget to set your servlet mapping (you probably already have something like this in ScalatraBootstrap):

context.mount(new MyScalatraServlet, "/*")

If you have a custom bootstrap class location:

context.setInitParameter(ScalatraListener.LifeCycleKey, "org.yourdomain.project.MyScalatraBootstrap")

Tell Heroku how to run your app (optional)

Heroku will detect the target/universal/stage/bin/<app-name> script generated by sbt-native-packager, so it will know how to run your app by default.

However, you may want to customize how your app starts. In that case, create a Procfile with contents such as:

web: target/universal/stage/bin/<app-name> -Dhttp.port=$PORT -Dother.prop=someValue

And replace <app-name> with the name of your app defined in build.scala.

5. Deploy

If you haven’t set up your project as a Git repo, do so.

$ cd [app root]
$ chmod u+x sbt
$ git init
$ git add .
$ git commit -m 'first commit'

Log into Heroku.

$ heroku login

Create your Heroku endpoint and deploy to it.

$ cd [app root]
$ heroku create
$ git push heroku master

After a couple minutes of streaming output, the last few lines will look like this:

-----> Discovering process types
Procfile declares types -> web
-----> Compiled slug size: 43.4MB
-----> Launching... done, v5
http://polar-atoll-9149.herokuapp.com deployed to Heroku

To git@heroku.com:polar-atoll-9149.git
* [new branch]      master -> master

Open your browser to to the URL provided right before deployed to Heroku in the output. Or just run:

$ heroku open

For more information on using Scala with Heroku see the Heroku DevCenter.