Chunking

Chunking is the process that decides which modules are placed into which bundles, and the relationship between these bundles.

For this process a few intermediate concepts are used:

• ChunkingContext: A context trait which controls the process.
• ChunkItem: A derived object from a Module which combines the module with the ChunkingContext.
• ChunkableModule: A trait which defines how a specific Module can be converted into a ChunkItem.
• ChunkableModuleReference: A trait which defines how a specific ModuleReference will interact with chunking.
• ChunkType: A trait which defines how to create a Chunk from ChunkItems.
• Chunk: A trait which represents a bundle of ChunkItems.

graph TB
convert{{convert to chunk item}}
ty{{get chunk type}}
create{{create chunk}}

EcmascriptModule -. has trait .-> Module
CssModule -. has trait .-> Module
Module -. has trait .-> ChunkableModule
ChunkableModule === convert
ChunkingContext --- convert
convert ==> ChunkItem
ChunkItem ==== ty
EcmascriptChunkType -. has trait ..-> ChunkType
CssChunkType -. has trait ..-> ChunkType
ty ==> ChunkType
ChunkType ==== create
create ==> Chunk
EcmascriptChunk -. has trait ..-> Chunk
CssChunk -. has trait ..-> Chunk

BrowserChunkingContext -. has trait .-> ChunkingContext
NodeJsChunkingContext -. has trait .-> ChunkingContext

A Module need to implement the ChunkableModule trait to be considered for chunking. All references of these module that should be considered for chunking need to implement the ChunkableModuleReference trait.

The chunking algorithm walks the module graph following these chunkable references to find all modules that should be bundled. These modules are converted into ChunkItems and a special algorithm decides which ChunkItems are placed into which Chunks.

One factor is for example the ChunkType of each ChunkItem since only ChunkItems with the same ChunkType can be placed into the same Chunk. Other factors are directory structure and chunk size based.

Once the ChunkItems that should be placed together are found, a Chunk is created for each group of ChunkItems by using the method on ChunkType.

An OutputAsset is then created for each Chunk which is eventually emitted to disk. The output format decides how a Chunk is transformed into an OutputAsset. These OutputAssets that are loaded together are called a chunk group.

The chunking also collects all modules into a AvailableModules struct which is e. g. used for nested chunk groups to avoid duplicating modules that are already in the parent chunk group.