---
phase: 03-communication-protocol
plan: "03"
subsystem: protocol
tags: [udp, simulator, e2e-testing, fastapi, tdd]
dependency_graph:
  requires: ["03-01", "03-02"]
  provides: ["UDP simulator with packet logging", "UDPSender in FastAPI lifespan", "end-to-end protocol tests"]
  affects: ["lightsync/main.py", "lightsync/protocol/simulator.py", "tests/test_e2e_protocol.py"]
tech_stack:
  added: ["asyncio.DatagramProtocol for UDP simulation"]
  patterns: ["TDD RED-GREEN", "asyncio.run() in tests (no pytest-asyncio)", "dataclass for structured packet logs"]
key_files:
  created:
    - lightsync/protocol/simulator.py
    - tests/test_e2e_protocol.py
  modified:
    - lightsync/main.py
decisions:
  - "asyncio.run() test pattern chosen over pytest-asyncio — not installed, plan explicitly provided this fallback"
  - "SimulatorProtocol uses port=0 in tests — OS assigns free port, read back via get_extra_info('sockname')[1]"
  - "UDPSender stopped before registry.save() in shutdown — orderly teardown, no sends after socket close"
metrics:
  duration: "2 min"
  completed_date: "2026-04-06"
  tasks_completed: 2
  files_created: 2
  files_modified: 1
---

# Phase 03 Plan 03: UDP Simulator and FastAPI Integration Summary

**One-liner:** Async UDP simulator parses all 4 WLED packet types (DRGB/DRGBW/DNRGB/ANIM_CMD) with structured packet logging; UDPSender integrated into FastAPI lifespan; 7 end-to-end tests prove full animation-to-wire pipeline.

## What Was Built

### Task 1: UDP Simulator with Packet Logging (TDD)

`lightsync/protocol/simulator.py` — a `SimulatorProtocol(asyncio.DatagramProtocol)` that:
- Parses DRGB (0x02): extracts `led_count = (len(data)-2) // 3`
- Parses DRGBW (0x03): extracts `led_count = (len(data)-2) // 4`
- Parses DNRGB (0x04): extracts big-endian uint16 start index at bytes 2-3, `led_count = (len(data)-4) // 3`
- Parses ANIM_CMD (0xAC): calls `decode_animation_cmd`, stores result in `PacketLog.decoded`
- Stores all received packets as `PacketLog` dataclasses for test assertions

`tests/test_e2e_protocol.py` — 7 end-to-end tests using `asyncio.run()`:
- `test_e2e_drgb_10_pixels` — DRGB 10 pixels, led_count verified
- `test_e2e_drgbw_10_pixels` — DRGBW 10 pixels, led_count verified
- `test_e2e_dnrgb_600_pixels` — 600-LED frame splits into 2 packets, start_indices 0 and 489
- `test_e2e_animation_cmd_chase` — animation command round-trip, decoded animation=="chase"
- `test_e2e_full_pipeline` — solid_color render -> encode_drgb -> send -> 20-LED DRGB frame verified
- `test_e2e_sk6812_rgbw_pipeline` — solid_color -> RGBW encode -> DRGBW 15-LED frame verified
- `test_simulator_standalone_import` — run_simulator importable without error

TDD flow: RED commit (failing import) -> GREEN commit (simulator implemented, all 7 pass).

### Task 2: UDPSender into FastAPI Lifespan

`lightsync/main.py` updated:
- Added `from lightsync.protocol.udp_sender import UDPSender` import
- Lifespan startup: `UDPSender()` created, `await udp_sender.start()`, assigned to `app.state.udp_sender`
- Lifespan shutdown: `await app.state.udp_sender.stop()` before `await registry.save()`
- All existing lifespan code unchanged (beats cache, registry, show_store)

## Deviations from Plan

None — plan executed exactly as written.

## Known Stubs

None — all implemented functionality is fully wired.

## Self-Check: PASSED

See below.