RegionCast

Pull region files from another server's world into this one, picked off a map.

RegionCast shows you which regions changed since the last time you copied them, you click the ones you want, and it brings terrain, points of interest and entities across during a controlled restart. The last few updates stay undoable.

Leer en español

What it is for

You run a second server that mirrors a build server: a creative copy, a testing world, a render world, a place to show people a base without letting them into the real one. Keeping it up to date means copying the world across, and the world is 8 GB, and 97% of it did not change.

On a real server the numbers look like this: 1812 overworld regions, 8.0 GB, and in a whole day of play only 59 of them change. That is 407 MB. RegionCast copies those 59.

The idea is the same one behind TISUnion's region_file_updater plugin and the old !!region commands, with one difference: you pick the regions on a map of the actual world instead of typing coordinates and hoping you got the right file.

How it works

• A map tile is 512 by 512 blocks. A region file is 512 by 512 blocks. So a tile is a region,

no conversion anywhere, and the map is drawn from the chunks your client already has.

• Colours come from a ledger, not from comparing the two files. Comparing does not work: the

copy gets a fresh timestamp, and the mirror rewrites its own region files the moment somebody walks around. The ledger records the timestamp the source file had when it was copied, and compares future source timestamps against that.

• Copying happens during shutdown. A mod lives inside the Minecraft process, so it cannot stop

the server, copy, and start it again: it would be killing itself half way through the job. What it can do is copy in SERVER_STOPPED, when the world is saved and every region file is closed but the JVM is still alive. Something outside then starts the server again (see MCDReforged plugin).

• Before copying, the source server is told save-off and save-all flush over RCON, and

save-on afterwards, with retries, because leaving somebody else's server with saving off is worse than anything this mod was trying to do.

Chunks

Zoom in far enough and the grid switches from regions to chunks, and clicking picks a single chunk. No mode button: at that zoom a region is wider than the screen, and zoomed out a chunk is a pixel, so the gesture can only mean one thing at a time.

Loose chunks are copied differently to whole regions. A region is a file copy. A chunk is opened out of the source's region file and written into ours, using Minecraft's own RegionFile, so the result is byte for byte what the game would have written itself. What that buys you is taking one build across without the 512 blocks of terrain it shares a file with.

What it costs:

• Lighting and anything crossing a chunk border arrives half done until the game recalculates

it. It does, but it is visible for a moment, mostly in caves.

• Slower per block: every chunk is decompressed and recompressed, and the source region file

is copied to a work folder first (RegionFile insists on opening its file for writing, and the source is supposed to be mounted read only). Still seconds, not minutes.

• The source only timestamps whole region files, so a chunk can only be judged against that.

A change anywhere in a region marks every chunk you took out of it as out of date, even ones nobody touched. Wrong in the direction that tells you to look.

Copied chunks are painted green, red once the source moves on, gold while queued.

Backups

Every update sets aside the region files it is about to replace. The last 3 updates stay undoable, and older ones are dropped automatically.

A backup is one update, not one region: you click a handful of regions, press Update, and later decide that whole batch was a mistake. Restoring puts the files back with their original timestamps and rewinds the ledger with them, so restored regions correctly read as out of date again rather than claiming to match a source they no longer match.

/rc backups          list what can be taken back
/rc restore          put the newest one back
/rc restore 2        put the second newest one back

Restoring takes the same route as an update: it is armed, the server restarts, and the files go back while the world is closed.

Two things worth knowing:

• Disk. A batch of 60 regions costs roughly the size of those 60 regions, around 400 MB on a

busy world. Three of those is over a gigabyte. Check you have the room, or lower backup_keep.

• Restoring does not back up what it overwrites. What a restore throws away is the copy

RegionCast pulled from the source, and that is the one thing here you can always fetch again by pressing Update.

Requirements

• Minecraft 1.21 / 1.21.1 or 26.3, Fabric and the matching Fabric API.
• Java 21 for 1.21; Java 25 and Fabric Loader 0.19.5 or newer for 26.3.
• Filesystem access to both worlds from the destination server. This is the real requirement:

the mod copies files, so the source world has to be visible to the destination process. A VPS or a dedicated machine, not a shared host like Aternos or the cheap end of Shockbyte where you only get your own folder.

• Optional but recommended: RCON on the source server, and something that restarts the destination

server after it stops.

The client mod is optional. Without it everything still works through /rc; you just do not get the map.

Setup

1. Install the jar on the destination server (the mirror). Install it on your client too if you want the map.