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Example apps

Game Servers: Minecraft

Game servers have expensive startup paths, loading assets and initializing world state, and long idle stretches between sessions, which usually means paying for a server that runs all day. Minecraft is a strong example of the pattern: JVM warm-up and world initialization happen once into a template snapshot, and an instance pauses when the last player disconnects, then resumes from that snapshot on the next connection.

This example runs a Minecraft Java server on Unikraft Cloud with:

• A reusable base image (minecraft:latest) built from itzg/minecraft-server (Java 25)
• Configuration overrides through auxiliary ROMs (optional)
• Template-based startup for faster instance creation
• TLS-exposed Minecraft and SSH endpoints

Prerequisites

1. Install the unikraft CLI. You need a BuildKit builder. The easiest way to get one is via Docker. Alternatively, you can also directly set up and use BuildKit, see the quick start.

2. Clone the examples repository and cd into the examples/minecraft/ directory:

   Code
   git clone https://github.com/unikraft-cloud/examples
   cd examples/minecraft/

3. Review and adjust the base server settings in base/.env. You can check out this documentation for available configuration options. Make sure to also set your PUBKEY for SSH access, and optionally set TEMPLATE_WITH_WORLD if you want the template to include the world (see below). Optionally, create per-config overrides in <config>/.env (for example bingo/.env). All the .env files are packaged as auxiliary ROMs and mounted at /rom/<config>.

Make sure to log into Unikraft Cloud and pick a metro close to you. This guide uses fra (Frankfurt, 🇩🇪):

Code
unikraft login

Deployment Workflow

Package the base image

First, package and push the base Minecraft server image:

Code
unikraft build . --output <my-org>/minecraft:latest

The image is built from the Docker image itzg/minecraft-server and includes a few tweaks:

• The entrypoint is wrapped around a custom wrapper.sh script that:
  • Starts an SSH server
  • Loads the environment configuration from the attached ROM(s)
  • Disables scale-to-zero before executing the original entrypoint
• The server configuration scripts from the original image are patched (see patches/) to trigger the template snapshot before the full warm-up of the server, which allows faster instance creation from the template. Scale-to-zero is enabled after server initialization.

Create an instance template from the base image

This step is required to enable fast startups from the base image, which is particularly important for a Minecraft server given the long initialization time. If you only customized the server using the base .env file, create the instance with one ROM:

Code
unikraft run --metro fra \
  --name minecraft-tpl \
  -m 4096M \
  --vcpus 4 \
  --rom dir=base,at=/rom/base \
  --image <my-org>/minecraft:latest

The output shows the instance details:

Code
metro:        fra
name:         minecraft-tpl
uuid:         d9d7be54-5495-45d0-b5af-48be7f30d1d8
state:        starting
image:        <my-org>/minecraft
resources:
  memory:     4GiB
  vcpus:      4
roms:
- name:       rom
  image:      c99125f6-6b8c-4f94-b94c-e2d9551b253b
  at:         /rom
networks:
- uuid:       123b0ed0-9f2f-417c-9307-34424d80e4cb
  private-ip: 10.0.0.29
  mac:        12:b0:0a:00:00:1d
timestamps:
  created:    just now

If you also have per-config overrides (for example in bingo/.env), create the instance with multiple ROMs:

Code
unikraft run --metro fra \
  --name minecraft-tpl \
  -m 4096M \
  --vcpus 4 \
  --rom dir=base,at=/rom/base \
  --rom dir=bingo,at=/rom/bingo \
  --image <my-org>/minecraft:latest

The instance will run until initialization is complete, then it will be snapshotted as a template and immediately deleted. The exact moment of snapshotting can be configured in patches/start-finalExec, but it can also be configured by the following environment variable:

• If TEMPLATE_WITH_WORLD=true, then the snapshot will be triggered after world generation
• Otherwise, the snapshot will be triggered after extracting the jar files and writing configuration files, before world generation

You can follow the logs of the instance to check the progress:

Code
unikraft instances logs minecraft-tpl -f

Once they stop, the template is ready and you can check it with:

Code
unikraft instances templates list

Code
METRO  NAME           STATE     IMAGE               ARGS  MEMORY  VCPUS  CREATED
fra    minecraft-tpl  template  <my-org>/minecraft        4GiB    4      5 seconds ago

Create an instance from the template

You can now create new instances from the template, which will boot much faster than the original base image:

Code
unikraft run --metro fra \
  --name minecraft \
  -p 2222:2222/tls \
  -p 25565:25565/tls \
  --scale-to-zero policy=on,cooldown-time=5000,stateful=true \
  --template minecraft-tpl

The output shows the instance address and other details:

Code
metro:         fra
name:          minecraft
uuid:          56787b5d-14fb-4908-88d0-e9609d1ac1e0
state:         running
image:         <my-org>/minecraft
resources:
  memory:      4GiB
  vcpus:       4
service:       hidden-water-ewr8l9sp
roms:
- name:        rom
  image:       c99125f6-6b8c-4f94-b94c-e2d9551b253b
  at:          /rom
networks:
- uuid:        b6c7d615-7e74-49d6-a8b6-14c39323233b
  private-ip:  10.0.0.29
  mac:         12:b0:0a:00:00:1d
timestamps:
  created:     just now
scale-to-zero: policy=on,stateful=true,cooldown-time=5s

In this case, the instance name is minecraft and the address is https://hidden-water-ewr8l9sp.fra.unikraft.app.

A sample log output of the server getting ready:

Code
unikraft instances logs minecraft -f

Code
[12:59:45] [Server thread/INFO]: Preparing spawn area: 100%
[12:59:45] [Server thread/INFO]: Time elapsed: 3641 ms
[12:59:45] [Server thread/INFO]: Done (3.850s)! For help, type "help"
[12:59:45] [Server thread/INFO]: Starting remote control listener
[12:59:45] [Server thread/INFO]: Thread RCON Listener started
[12:59:45] [Server thread/INFO]: RCON running on 0.0.0.0:25575
[12:59:45] [Server thread/INFO]: Saving chunks for level 'ServerLevel[world]'/minecraft:overworld
[12:59:45] [Server thread/INFO]: Saving chunks for level 'ServerLevel[world]'/minecraft:the_end
[12:59:45] [Server thread/INFO]: Saving chunks for level 'ServerLevel[world]'/minecraft:the_nether
[12:59:45] [Server thread/INFO]: ThreadedAnvilChunkStorage (world): All chunks are saved
[12:59:45] [Server thread/INFO]: ThreadedAnvilChunkStorage (DIM1): All chunks are saved
[12:59:45] [Server thread/INFO]: ThreadedAnvilChunkStorage (DIM-1): All chunks are saved
[12:59:45] [Server thread/INFO]: ThreadedAnvilChunkStorage: All dimensions are saved

Connect to Minecraft and SSH

At the moment, Unikraft Cloud can only expose these services over TLS. Since Minecraft clients and SSH clients do not support TLS, you can use socat to terminate TLS locally and expose plain TCP endpoints for your clients to connect to.

Minecraft proxy

Code
socat TCP-LISTEN:25565,reuseaddr,fork OPENSSL:hidden-water-ewr8l9sp.fra.unikraft.app:25565,verify=0

Then connect your Minecraft client to localhost:25565.

SSH proxy

Code
socat TCP-LISTEN:2222,reuseaddr,fork OPENSSL:hidden-water-ewr8l9sp.fra.unikraft.app:2222,verify=0

Then connect your SSH client to localhost:2222 with the username root and the SSH key you configured in the .env file.

Administration

RCON

SSH into the instance (via the local TLS proxy), then run:

Code
rcon-cli --host 127.0.0.1

You can then execute any Minecraft server command through the RCON interface, for example:

Code
say Hello from Unikraft Cloud!

You can also give yourself operator permissions to execute these commands in-game:

Code
/op <username>

Whitelist in offline mode

Offline mode does not produce online UUIDs. If you whitelist players manually, compute UUIDs per username and update /data/whitelist.json.

Code
import hashlib, uuid; h = hashlib.md5(b'OfflinePlayer:<username>').digest(); u = uuid.UUID(bytes=h); print(u)

Format:

Code
[
    {
        "uuid": "<uuid>",
        "name": "<username>"
    }
]

Learn more

Use the --help option for detailed information on using Unikraft Cloud:

Code
unikraft --help

Or visit the CLI Reference.