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Scala on Miren

Scala isn't auto-detected, so you deploy it with a Dockerfile.miren that packages a runnable jar. This guide uses scala-cli with the Cask framework — the least-ceremony path. The same jar-and-java -jar pattern works for sbt builds with http4s or Play.

Let your agent do this

Ask your AI coding agent to "set up this Scala app on Miren" after installing the Miren agent skills. It adds the Dockerfile.miren, binds the server to 0.0.0.0:$PORT, and deploys — using this page as its reference.

Do you need a Dockerfile?​

Yes. Miren doesn't auto-detect Scala, so add a Dockerfile.miren to your project root. Miren builds from it instead of guessing the stack — see Using Dockerfile.miren.

Want native support?

Miren auto-detects and builds common stacks (Python, Node, Bun, Go, Ruby, Rust) without a Dockerfile. This language isn't one of them yet — if you'd like first-class support, request it.

Bind to the injected port​

Miren injects PORT and routes traffic to it, so read PORT and bind 0.0.0.0. A Cask app declares its port and host:

//> using dep com.lihaoyi::cask:0.9.4

object App extends cask.MainRoutes {
  override def port = sys.env.getOrElse("PORT", "8080").toInt
  override def host = "0.0.0.0"

  @cask.get("/")
  def hello() = "Hello from Scala on Miren!\n"

  initialize()
}

The //> using dep line lets scala-cli resolve dependencies without a separate build file.

The Dockerfile​

Create Dockerfile.miren in your project root. scala-cli packages an assembly jar, which runs on a plain JRE:

# ----- Build stage -----
FROM virtuslab/scala-cli:latest AS builder
WORKDIR /app
COPY . .
RUN scala-cli --power package app.scala -o app.jar --assembly

# ----- Runtime stage -----
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=builder /app/app.jar /app/app.jar
EXPOSE 8080
CMD ["java", "-jar", "app.jar"]

.dockerignore​

.git

Set up the app​

Even with a Dockerfile.miren, Miren needs at least one service defined — it doesn't use the image's CMD as the start command. Add a Procfile:

web: java -jar /app/app.jar

Then create .miren/app.toml naming your app and deploy from your project root:

name = "scala-bench"
miren deploy
Deploying without a service fails

If no service is defined, the build succeeds but the deploy stops with no services defined: please define at least one service in a Procfile or .miren/app.toml.

Environment variables​

Set variables with miren env set — -e for plain values, -s for secrets (masked in output and logs). Read them with sys.env.get("KEY"):

miren env set -e LOG_LEVEL=info
miren env set -s DATABASE_URL

You can also declare variables in .miren/app.toml:

[[env]]
key = "DATABASE_URL"
value = ""
required = true
sensitive = true

See App Configuration — Environment Variables.

Agent quick reference​

  • Detection: none — requires Dockerfile.miren
  • Build: scala-cli --power package app.scala -o app.jar --assembly; run on a JRE image
  • Service is required: define a Procfile (web: java -jar /app/app.jar) — the image CMD is not used
  • Port: sys.env.getOrElse("PORT", "8080").toInt; Cask override def host = "0.0.0.0"
  • Env vars: miren env set -e/-s; read with sys.env.get

Next steps​