HorizonSync

Synchronization covers the main inventory, armor, offhand, Ender Chest contents, experience, level, progress bar, health, food and saturation, and active potion effects, which return when a player joins. Accessory slots from the Accessories API and Trinkets slots are included, and backpack contents from the Sophisticated Backpacks unofficial Fabric port are serialized and kept in sync as well. Data is stored in MongoDB, a NoSQL database, and sent to players through synchronized network packets.

Saves run on an asynchronous timer every 3 minutes without slowing the server's ticks per second, and a synchronous final save runs at server shutdown so the last change is recorded. The database connection string is set in a configuration file that is generated on first launch, and the server's IP address must be allowed through the database firewall. HorizonSync creates the needed collections automatically, and the database name and per-player collection can be changed in the same file.

The mod runs on the server only, so players join without installing anything. It is published for Fabric and 1.21.1 only, and it depends on Fabric API. The license is Apache License 2.0, and the mod is not open source.