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Use Cases

Game Servers

Running multiplayer game servers often means keeping long-lived VMs online, pre-warming for peak hours, and paying for idle time between sessions. On Unikraft Cloud, you can run game servers in microVMs with stateful scale-to-zero, so instances pause when idle and resume when players reconnect.

Minecraft is a strong example of this pattern. JVM startup and world initialization are expensive, but template snapshots and stateful resume keep player experience smooth without running infrastructure every day.

Why use Unikraft Cloud for game servers

Fast restarts from template snapshots

Game servers often have expensive startup paths: loading assets, initializing runtime state, and binding network services. Unikraft Cloud's instance templates snapshot the server after initialization, so later starts resume from a warm state instead of repeating full initialization. For Minecraft specifically, this avoids repeated JVM warm-up and world initialization work.

Stateful scale-to-zero

When all players disconnect, the instance enters standby and consumes no active resources. On the next connection it wakes from the same in-memory snapshot, preserving server state without a full restart. For Minecraft, this means your world and server process resume from snapshot instead of cold booting.

Isolation per server

Each server runs in its own microVM: no shared kernel with other tenants, no container escape surface, and no noisy-neighbour risk.

Quick iteration with ROM

Server settings can ship as auxiliary ROMs at instance creation time. You can run distinct game-server variants by swapping ROM configuration rather than rebuilding the base image.

Getting started

This guide uses the minecraft example. The base image is built from itzg/minecraft-server (Java 25) and includes:

• A wrapper.sh entrypoint that starts SSH, loads ROM configuration, and disables scale-to-zero during initialization.
• Patched startup scripts that snapshot the instance into a template before full server warm-up.

Prerequisites

1. Install the CLI. Use the unikraft CLI or the legacy kraft CLI. You need a BuildKit builder. The easiest way to get one is via Docker. You can also set up and use BuildKit directly; see the quick start.

   The unikraft CLI is the current standard, while kraft is the legacy version. Choose one of the CLIs below and only run the commands associated with it for the rest of this guide.

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. All .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, 🇩🇪):

Step 1: Package and push the base image

First, package and push the base Minecraft server image:

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

• The entrypoint uses a custom wrapper.sh script that:
  • Starts an SSH server
  • Loads the environment configuration from the attached ROMs
  • Disables scale-to-zero before executing the original entrypoint
• The server configuration scripts from the original image have 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 then re-enabled after server initialization.

Step 2: Create a template from the base image

Boot a short-lived instance from the base image. The startup script snapshots it into a template before world generation, then the instance exits.

Pass your server settings as a ROM:

The output shows the instance details:

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

The instance runs until initialization completes, then the platform snapshots it as a template and immediately deletes it. You can configure the exact moment of snapshotting in patches/start-finalExec or with the following environment variable:

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

Follow the template instance logs until they stop:

Confirm the template is ready:

Step 3: Launch a server from the template

The output includes the service address:

The instance address is https://hidden-water-ewr8l9sp.fra.unikraft.app. Follow the logs until the server is ready:

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 the server

Minecraft

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

Connect your Minecraft client to localhost:25565.

SSH

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

Connect to localhost:2222 with username root and the SSH key set in .env.

Administration

Remote console

SSH into the instance, then:

Code
rcon-cli --host 127.0.0.1

Example commands:

say Hello from Unikraft Cloud!
/op <username>

Whitelist in offline mode

Offline mode generates UUIDs that differ from online UUIDs. Compute the correct UUID per username and add it to /data/whitelist.json:

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

Cleanup

Learn more

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

Or visit the CLI Reference or the legacy CLI reference.

For more information on the features used in this use case and how to use them, check out the following documentation:

• Instance templates
• Scale-to-zero
• ROMs
• minecraft example
• itzg/minecraft-server documentation