Split monolith

split-monolith

A file split done wrong breaks every caller. Follow this procedure exactly — it is reversible at every step.

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Step 0 — Confirm the file needs splitting

wc -l app/services/<file>.py

Lines	Action
< 400	Do not split — you're solving a non-problem
400–600	Plan the split now, execute when convenient
> 600	Split immediately (Principle A7 hard cap)

Also ask: does this file mix multiple concerns? A file that is long but cohesive is better than a premature split.

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Step 1 — Identify the domain splits

Do NOT split by size. Split by type of responsibility.

Good splits (by domain):

wallet_service.py (1200 LOC) →
  wallet/user.py      ← user-facing operations (charge, refund)
  wallet/admin.py     ← admin operations (top-up, override)
  wallet/history.py   ← read-only queries

Good splits (by layer):

generation_service.py (1000 LOC) →
  generation/orchestrator.py   ← coordinates the flow
  generation/cost.py           ← cost calculation logic
  generation/storage.py        ← result persistence

Bad splits (by size only — don't do this):

big_service.py →
  big_service_part1.py   ← meaningless
  big_service_part2.py   ← meaningless

Write the target structure before touching any file.

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Step 2 — Create the package directory

mkdir app/services/<domain>/

Do NOT move any code yet.

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Step 3 — Create sub-files one at a time

For each sub-file, copy (not move) the relevant functions:

# Create the new file with the relevant subset
touch app/services/<domain>/user.py
# Copy relevant classes/functions from the original

Each sub-file must:

• Have its own imports (do not rely on * imports)
• Be under 200 LOC (you're splitting — keep it lean)
• Contain one cohesive responsibility

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Step 4 — Create __init__.py with ALL old names re-exported

This is the most important step. Every name that existed in the original file must still be importable from the same path.

# app/services/<domain>/__init__.py

# Principle A8: re-export everything so callers don't change.
from app.services.<domain>.user import CreditsUserService
from app.services.<domain>.admin import CreditsAdminService
from app.services.<domain>.user import get_credits_user_service

# Backward-compat alias if the old class had a different name
CreditsService = CreditsUserService  # old name → new class

__all__ = [
    "CreditsUserService",
    "CreditsAdminService",
    "CreditsService",           # backward compat
    "get_credits_user_service",
]

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Step 5 — Verify no import breaks

# Check every file that imported from the old module still works
python3 -c "from app.services.<domain> import <OldClassName>"
python3 -c "from app.services.<domain> import <AnotherClass>"

# Run the full test suite
pytest tests/ -x -q

All tests must be GREEN before deleting the original file.

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Step 6 — Delete the original file

Only after Step 5 passes:

rm app/services/<original_file>.py

Run tests again:

pytest tests/ -x -q
bash scripts/lint-architecture.sh

Both must pass.

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Common mistakes

Mistake	Consequence	Prevention
Split before writing __init__.py	Import errors everywhere	Always create __init__.py first
Split by size, not responsibility	Sub-files still coupled	Ask: "what is the single job of this file?"
Forget to re-export old names	Callers break silently	List every public name before splitting
Move code instead of copy+verify	Can't roll back	Copy first, delete only after tests pass
Split and refactor at same time	Impossible to debug	One PR = one split. No logic changes.

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Verification

The split was done correctly when:

• All sub-files are under 200 LOC
• __init__.py re-exports every name that existed before
• python3 -c "from app.services.<domain> import <OldName>" works
• pytest tests/ -x -q is green
• bash scripts/lint-architecture.sh exits 0
• Original file is deleted