2Core

Run a second Fabric server as a separate JVM process — one mod, two roles.

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What is this?

2Core lets you run two Minecraft servers on the same machine from a single mod. The primary server launches and manages the secondary server as an external JVM process. Both servers use the same mod JAR — the role (primary or secondary) is determined automatically at startup.

No plugins, no proxies, no extra software required.

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Features

• One mod, two servers — install once, the mod handles both roles automatically
• Cross-server chat sync — chat messages, join/leave notifications, and death messages are relayed between servers in real time
• Player list — /listother shows online players on the other server (available to all players)
• Full process management — start, stop, restart the secondary server with simple commands
• Console relay — optionally mirror the secondary server's console output to the primary
• Auto-start — optionally start the secondary server automatically when the primary starts
• Configurable messages — customize join/leave/chat/death message formats independently for each server with color code support (&6, &a, etc.)
• Safe IPC — servers communicate over a localhost-only TCP socket, no external exposure

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Requirements

Minecraft	1.21.1
Fabric Loader	≥ 0.18.6
Fabric API	any
Fabric Language Kotlin	≥ 1.13.0

Server-side only. No client installation needed.

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Setup

1. Install the mod on your primary server (along with Fabric API and Fabric Language Kotlin)
2. Start the primary server — the mod creates a 2core/ folder and a 2core/2core.properties config file
3. Place your secondary server files inside 2core/ (or configure copy_mode to copy them automatically)
4. Run /2core start to launch the secondary server

The mod copies itself into 2core/mods/ automatically — no need to install it separately on the secondary.

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Commands

Available on both servers (all players)

Command	Description
/listother	Show online players on the other server

Admin commands — primary server only (/2core ...)

Command	Description
/2core start	Start the second server
/2core stop	Stop the second server
/2core restart	Restart the second server
/2core status	Show status, config, and IPC connection info
/2core chatlink on|off	Enable or disable cross-server sync
/2core console on|off	Toggle secondary console relay to primary
/2core cmd <command>	Send a command to the secondary server
/2core set <key> <value>	Change a config value without editing the file
/2core help	Show command list

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Configuration

Config file: 2core/2core.properties

Key	Default	Description
copy_mode	none	none / launcher / full — how to copy server files to 2core/
launcher_jar	fabric-server-launch.jar	Fabric launcher jar name
vanilla_jar	server.jar	Vanilla server jar name
port	25566	Port for the secondary server
max_ram_mb	1024	Max RAM for the secondary JVM (-Xmx)
auto_start	false	Start secondary automatically when primary starts
auto_console	false	Enable console relay automatically on startup
chat_link	false	Sync chat, join, leave, and death events between servers
ipc_port	0	IPC port for inter-server communication (0 = auto)
join_message_for_primary	&6%player% joined the second server	Shown on primary when a player joins the secondary
join_message_for_secondary	&6%player% joined the main server	Shown on secondary when a player joins the primary
leave_message_for_primary	&6%player% left the second server	Shown on primary when a player leaves the secondary
leave_message_for_secondary	&6%player% left the main server	Shown on secondary when a player leaves the primary
chat_format_for_primary	&7[&62nd&7] &f<%player%> %message%	Chat format on primary for messages from secondary
chat_format_for_secondary	&7[&61st&7] &f<%player%> %message%	Chat format on secondary for messages from primary
death_format_for_primary	&7[&62nd&7] &c%message%	Death message format on primary from secondary
death_format_for_secondary	&7[&61st&7] &c%message%	Death message format on secondary from primary

Placeholders: %player%, %message%. Color codes: &0–&9, &a–&f, &l, &o, &r, etc.

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How it works

When the primary server starts, it launches the secondary server as a child JVM process with -Dtwocore.role=secondary. Both JVMs run the same mod JAR — the secondary detects its role via the system property and skips all process management logic. Inter-server communication (chat sync, player list queries) happens over a localhost TCP socket managed by the mod.