IV · The world

Verified against Minecraft 26.2 · Part IV · The machinery that turns a place you have walked to into a place that exists: chunks made, lit, sent, saved and forgotten, and the four side-systems that make the world they hold feel alive.

Part III was one thread going round. This part is what it goes round on. A world is too big to hold, so the server holds a moving window of it, and almost everything in this part exists to decide the window’s edge: which chunks are worth building, how far past the edge to build them, which of them tick, which of them are sent to you, and when one is finally written down and let go. A player recognises the part by its edge — the ring of half-made terrain past render distance, the mobs that stop moving when you fly away from them, the Saving world bar. Almost none of that edge is one number: render distance, simulation distance and the mob-spawning radius are three different radii, answered by three different mechanisms, and only two of them are settings.

The shape of the part

Part IV is a conveyor with a vocabulary page in front of it. Four pages are the conveyor and they hand a chunk to each other in order; the fifth page defines the thing being handed. The other five are not on the line at all — one is what the place and the hour decide, and four are about the world the conveyor delivers.

flowchart TD
    CA["Chunk anatomy: sections, palettes, heightmaps"]
    TL["Tickets and loading: which chunk, at what level"]
    GP["The generation pipeline: EMPTY to FULL"]
    LI["Lighting: two 4-bit fields, off the tick"]
    CS["Chunk storage: copy, encode, sectors"]
    LC["a live LevelChunk in a ticking world"]
    EA["Environment attributes: what the place and the hour decide"]
    ST["Scheduled ticks: the appointment book"]
    FL["Fluids: the book's biggest customer"]
    GV["Game events and vibrations: what just happened"]
    PI["Points of interest: what is worth going to"]
    CA -- "the vocabulary every page below spends" --> TL
    TL -- "a holder, a ceiling, three futures" --> GP
    GP -- "a chunk that still needs its light finished" --> LI
    LI -- "sections dirtied, one packet of them" --> LC
    LC -- "the level rises past 44, nobody needs it" --> CS
    CS -- "a ticket wants it back, and it is read in" --> TL
    LC --> ST
    ST -- "one customer, big enough for its own page" --> FL
    LC --> GV
    LC --> PI
    EA -- "read by fluids and by the villagers, and by Part III" --> LC

What the part hands forward, and what it does not: blocks and states in Part V assumes the section and palette model chunk anatomy defines, and Part XII’s terrain generation is the cargo on the conveyor the generation pipeline describes. Neither is a dependency of this part; both are parts that depend on it.

Before you start

The server tick and the level tick. Almost everything here happens on the Server thread inside that loop, or on a worker the loop is waiting for — the IO lane below is the exception the storage page is about — and the level tick is where the chunk source is asked to do its five things. Part II’s codecs and registries are assumed wherever a chunk is written to disk or a type is looked up by name, and tags wherever a behaviour is defined by a set the data pack owns — which is most of what the last two pages do.

Nothing in this part needs Part V or beyond.

Watch in this order

Lectures two to six are the chain — nothing later in it can be watched first. The first is off the chain on purpose, and the last four can be watched in any order once you have the vocabulary page.

1. Environment attributes and timelines — the one page here that depends on nothing else in the part, and the page the level tick already asked you to watch. Whether lava flows fast, what colour the sky is and when a villager goes to work are one mechanism. The night does not set the sky’s colour — it multiplies whatever the biome produced.
2. Chunk anatomy — what a chunk is made of, down to the bit storage. The vocabulary the rest of the part spends. A section holding two block states costs exactly what one holding sixteen costs.
3. Tickets and loading — a player takes one step east and a column twenty-one chunks wide is asked for. Nothing ever asks whether a chunk is loaded: it asks for a level, and two graphs reading one ticket store answer different questions about it.
4. The chunk generation pipeline — one chunk from EMPTY to FULL through twelve statuses and a pyramid of neighbour requirements. Asking for one chunk asks for 529.
5. Lighting — a torch is placed. Two 4-bit fields flooded on a worker and published as a copy. There is no light thread, and no light phase of the tick.
6. Chunk storage — a chunk nobody needs is copied, encoded and written, on three different threads, and the save path waits for none of it. Most of your world’s writes are ones nobody asked for.
7. Scheduled ticks — how anything happens later: an appointment book of two queues per chunk, and a dedup rule that quietly drops the second appointment even when it is sooner.
8. Fluids — a bucket of water on flat stone. Water finds a hole four blocks away because every side runs its own search, and a side the water cannot even enter still votes on where the rest of it goes.
9. Game events and vibrations — a footstep reaches a sculk sensor, through a cascade of tests that is most of the lecture. The sensor hears you one tick late by design.
10. Points of interest — a villager claims a bed from 48 blocks away, the moment a path to it exists. Going to sleep in it tells the index nothing, and the one behaviour that acts on the flag can only take a claim away.

Reference this part uses

Level data and rules — who owns the seed, the spawn, the rules, the border and the dimensions, and which file remembers each. Game rules — five of which this part reads. Math and primitives — ChunkPos and SectionPos, and the packings the conveyor pages assume. Block update flags — the flag word three pages here spend. Threads — the worker pool and the IO lane. Diagram lanes.

Rules: names, never code · how the system works, not how the code reads · newest version only · every backticked name passes tools/verify_names.py.