40-02-module-boundaries

Development

When to extract a module, what belongs together, what stays coupled. Prevents premature extraction and god modules alike.

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40.02 Module Boundaries

A module is a file (or small group of files) that owns one concern. The question is never "can I extract this?" — it's "does this concern have its own reason to change?"

When to Extract

Extract when two or more of these are true:

> 500 lines — too much to hold in your head
Multiple concerns — entity management AND connection lifecycle AND queries
Independent change reasons — query functions change when UI needs change, not when connection protocol changes
Can't test in isolation — the thing you want to test is trapped inside something you don't want to set up

Do NOT extract just because a file is large. A 800-line file with one concern (subscription wiring that genuinely shares lifecycle) is fine.

What Belongs Together

Things that change for the same reason stay in the same module:

Together	Why
`connect()` + `cleanupConnection()` + `scheduleReconnect()`	Connection lifecycle — all change when protocol changes
`upsertShip()` + `removeShip()` + `shipEntities` Map	Ship ECS management — all change when entity schema changes
`questAccept()` + `questComplete()` + `questAbandon()`	Quest domain actions — all change when quest system changes

Things that change for different reasons go in different modules:

Separate	Why
SQL query definitions vs. subscription orchestration	Queries change when schema changes; orchestration changes when connection protocol changes
Table → store callbacks vs. ECS entity management	Store wiring changes when store shape changes; ECS changes when rendering changes
Reducer wrappers vs. read-only queries	Writes change when server API changes; reads change when UI needs change

The Extraction Test

Before extracting, answer:

What shared state does it need? If it only reads from shared refs/Maps → easy extraction to standalone functions.
What does it write? If it writes closure-local let variables → move the state first (see 40.01), then extract.
Does it need constructor-time values? (world, renderer) → put them in a ref object set once at init. Not a factory.
Does it reset on disconnect? → It's connection lifecycle. Stays in the closure.

The God Module Test

Your module might be a god module if:

It has functions that don't call each other
It imports from > 15 different modules
You can draw a line through the file where "above" and "below" don't interact
New features keep getting added to it because "it has access to everything"

File Size Guidelines

Lines	Assessment
< 200	Fine. Don't merge small modules to "reduce files."
200-500	Normal. Single concern, well-structured.
500-1000	Review. Probably has 2-3 concerns. Consider extraction.
> 1000	Almost certainly a god module. Decompose.