Worlds

Every world is a directory under worlds/. The server loads the default world at startup and can load or unload the rest at any time, which is what lets a lobby, an arena and a survival map share a single process.

Adding a world

Copy the directory into worlds/ and restart, or load it at runtime from a plugin:

$manager = $this->getServer()->getWorldManager();

if(!$manager->isWorldLoaded("lobby")){
    $manager->loadWorld("lobby");
}

$world = $manager->getWorldByName("lobby");

unloadWorld() is the other half. Unloading saves the world and frees the memory its chunks were holding, which matters when you keep a dozen arena copies around.

Moving players between worlds

$world = $this->getServer()->getWorldManager()->getWorldByName("survival");

if($world !== null){
    $player->teleport($world->getSafeSpawn());
}

No transfer to another server node, no reconnect, no second IP to keep alive.

World formats

Keeping memory sane

Loaded chunks are the bulk of a running server's memory. If you keep many worlds loaded at once, lower view-distance, unload the worlds nobody is standing in, and watch the numbers with status before adding more. See Performance.