Manage custom data inside chunks without hassle.

Client and server Library

Createda year ago
Updateda month ago

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Author: FalsePattern

License: GNU AGPLv3.0 with additional permissions (see below, or in the LICENSE file)


A mod for adding custom data to chunks without the hassle of writing custom packets, event handling, loading hooks, and more.


ChunkAPI is designed with players in mind, and as such, it uses slightly aggressive licensing to make sure that third party developers cannot break your worlds in an irrecoverable way by changing huge parts of the API.

For this reason, the mod itself is fully licensed under the GNU Affero General Public License, version 3.0 or later, with several additional permissions according to Section 7 of the AGPL.

You can find these permissions in the full LICENSE file. (TL;DR: Yes, you can use official releases of ChunkAPI in a modpack without getting trapped by AGPL's requirements).

Note that while these limitations might seem unnecessary, it is essential for protecting your saves from getting "trapped" in a random fork of ChunkAPI without any way to recover to vanilla or the official release of ChunkAPI without world corruption, in case no suitable backups are available.


It is a goal to provide a way to add custom data to chunks without having to modify any vanilla classes. The API will also provide a simple way to implement the custom serialization/deserialization of the data, both for the networking and the NBT format. An optional goal is to provide a way to register DFUs for the custom data, but this is not a priority.


It is not a goal to provide any kind of callback/hook system for anything except saving/loading and netcode, any kind of extra behavior should be implemented in the mod using ChunkAPI.

Storage is not provided by the API, the user is expected to implement their own storage system. A good example is using a mixin to add a field to the chunk class, and using that field to store the data.


Modifying chunk packet data involved incompatibly modifying network packets, the chunk class, and the anvil chunk loader, which is a lot of work for a simple feature. This mod provides a way to add custom data to chunks without having to modify any of the vanilla classes yourself.

API and spec

The API exposes a way to add custom data to chunks, and a way to register custom serializers for the data.


The ChunkDataManager itself is the primary class used for managing registrations, but does not implement functionality by itself. For that, you need to use the PacketDataManager, ChunkNBTDataManager, and SectionNBTDataManager interfaces included inside the ChunkDataManager class.


This interface is used for synchronizing data from the server to the client. If your data is only required on the server, you can freely skip implementing this interface.


This interface is used for saving/loading data from the chunk NBT. This is required if you want to keep persistent data across world reloads. Note, if you store data per block in the chunk, you should use SectionNBTData instead, as it is designed with the internal chunk format in mind.


This interface is mostly identical to ChunkNBTData, but is designed for storing data per block in the chunk. Instead of being called once per chunk, it is called once per chunk section (16x16x16 blocks, ExtendedBlockStage class).


This is where you actually register your manager. You need to do all registrations inside the init phase. You can also disable specific manager IDs by calling disableDataManager, but this is not recommended, and should only be used if you know what you are doing. You need to do all the disabling inside the postInit phase.

Packet specification

ChunkAPI modifies the S21PacketChunkData and S26PacketMapChunkBulk vanilla packets, and overwrites their default formats.

All sizes in the tables below are specified in bytes.

S21PacketChunkData new format:

Size (bytes) Datatype Name
4 int X position
4 int Z position
1 bool Force Update
2 short ExtendedBlockStorage mask
4 int Uncompressed data length
4 int (n) Compressed data length
n byte[n] Compressed data

S26PacketMapChunkBulk new format:

Size (bytes) Datatype Name
2 short (n) Chunk count in packet
n * 4 int[n] Uncompressed chunk data lengths
4 int (m) Compressed data length
1 1 Contains skylight data
m byte[m] Compressed data
n * 10 (int, int, short)[n] Chunk Headers (x, z, ebs mask)

In both cases, the compressed data is populated through the ChunkDataRegistryImpl.writeToBuffer method. The layout of this data is as follows:

Size (bytes) Datatype Name
4 int (n) Manager count
n * varying MGRData Manager data

Manager data:

Size (bytes) Datatype Name
4 int (n) Manager name length
n UTF-8 str Manager name
4 int (m) Manager data length
m byte[m] Manager data


This mod directly depends on FalsePatternLib, and some sort of Mixin injector that provides SpongePowered Mixin >= 0.8.5.

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Server side
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