Scala Native in NanoVM Unikernel

Exploring Scala Native + Nix Flakes + Nix Devshell + Direnv + NanoVM Unikernel.

• https://scala-native.org/en/latest
• https://github.com/numtide/devshell
• https://nixos-and-flakes.thiscute.world/nixos-with-flakes/introduction-to-flakes
• https://direnv.net
• https://github.com/typelevel/typelevel-nix
• https://nanovms.com

The Nix sets up the development environment with:

1. clang: C/C++ LLVM compiler https://clang.llvm.org
2. jdk: GraalVM Community Edition: https://www.graalvm.org
3. metals: Scala Metals LSP server: https://scalameta.org/metals
4. sbt: Scala Built Tool: https://www.scala-sbt.org/index.html
5. scala-cli: Scala command-line tool: https://scala-cli.virtuslab.org
6. scala-fix: Scala refactoring and linting tool for Scala: https://scalacenter.github.io/scalafix
7. ops: NanoVM Unikernel build tool: https://github.com/nanovms/ops
8. qemu: QEMU hypervisor: https://www.qemu.org

TODOs

• Static ELF.
• Non-Nix friendly.
• Build and package with Nix Flakes.
  • Running ops in stdenv is broken at the moment: nanovms/ops#1687
• Automatically build and publish the ELF and the unikernel image.
• Continuous deployment to DigitalOcean.

Note on Nix

You can use this repo without nix if qemu, ops, sbt and clang installed in your environment.

You'll also need to set LLVM_BIN and several SCALANATIVE_* environment variables, check the flake.nix.

As for nix users, cd into the repository directory and run nix develop to drop into the development environment.
Or, if you have direnv installed, simply cd into the repository directory and do direnv allow.
Now, whenever you cd into the repository directory the development environment will be activated automatically, and erased when you cd out of the repository directory.

Usage

Build the binary:
sbt nativeLink.

Now, let's use the ops command to run it as a QEMU virtual machine packaged as unikernel.
It binds to the port 80, so we'll need sudo:
sudo ops run --port 80 ./target/scala-3.6.3/unikernel-scala-out.

In another terminal window:
curl localhost.
Output:

Hello from Scala Native NanoVM Unikernel! Your request: Request(method=GET, uri=/, httpVersion=HTTP/1.1, headers=Headers(Host: localhost, User-Agent: curl/8.11.0, Accept: */*), entity=Entity.Empty)

Packaging:
ops build ./target/scala-3.6.3/unikernel-scala-out.

Verify the image created:
ops image list.
Output:

100% |████████████████████████████████████████|  [0s:0s]
100% |████████████████████████████████████████|  [0s:0s]
Bootable image file:/home/igor/.ops/images/unikernel-scala-out.img

The resulting image then can be deployed to any cloud hypervisor which uses QEMU, e.g. DigitalOcean.:

1. "Create Droplet".
2. "Choose Image" -> "Custom Images".
3. "Add Image".
4. Upload your image from ~/.ops/images/unikernel-scala-out.img.
5. Wait for uploading the image and verification.
6. On your image: "More" -> "Start Droplet".

The app is currently deployed as http://unikernel.igorramazanov.tech (you can simply open it in the browser):

curl -v http://unikernel.igorramazanov.tech

* Host unikernel.igorramazanov.tech:80 was resolved.
* IPv6: (none)
* IPv4: 138.68.108.40
*   Trying 138.68.108.40:80...
* Connected to unikernel.igorramazanov.tech (138.68.108.40) port 80
* using HTTP/1.x
> GET / HTTP/1.1
> Host: unikernel.igorramazanov.tech
> User-Agent: curl/8.11.0
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 200 OK
< Date: Wed, 12 Feb 2025 14:25:39 GMT
< Connection: keep-alive
< Content-Type: text/plain; charset=UTF-8
< Content-Length: 195
<
* Connection #0 to host unikernel.igorramazanov.tech left intact
Hello from Scala Native NanoVM Unikernel! Your request: Request(method=GET, uri=/, httpVersion=HTTP/1.1, headers=Headers(Host: unikernel.igorramazanov.tech, User-Agent: curl/8.11.0, Accept: */*))

CI

This repo uses nix based GitHub actions for caching the development environment dependencies, instead of the traditional approach with coursier/setup-action and coursier/cache-action:

1. https://github.com/DeterminateSystems/nix-installer-action
2. https://github.com/DeterminateSystems/flake-checker-action
3. https://github.com/DeterminateSystems/flakehub-cache-action

Debugging

Add --verbose and --show-debug flags to the ops run ./target/scala-3.6.3/unikernel-scala-out to see the qemu command:

qemu-system-x86_64 \
  -machine q35 \
  -device pcie-root-port,port=0x10,chassis=1,id=pci.1,bus=pcie.0,multifunction=on,addr=0x3 \
  -device pcie-root-port,port=0x11,chassis=2,id=pci.2,bus=pcie.0,addr=0x3.0x1 \
  -device pcie-root-port,port=0x12,chassis=3,id=pci.3,bus=pcie.0,addr=0x3.0x2 \
  -device virtio-scsi-pci,bus=pci.2,addr=0x0,id=scsi0 \
  -device scsi-hd,bus=scsi0.0,drive=hd0 \
  -vga none \
  -smp 1 \
  -device isa-debug-exit -m 2G \
  -device virtio-rng-pci \
  -machine accel=kvm:tcg \
  -cpu host \
  -no-reboot \
  -cpu max \
  -drive file=/root/.ops/images/unikernel-scala-out.img,format=raw,if=none,id=hd0 \
  -device virtio-net,bus=pci.3,addr=0x0,netdev=n0,mac=3e:bd:d3:d8:e0:3f \
  -netdev user,id=n0,hostfwd=tcp::8080-:80 \
  -display none \
  -serial stdio