---
phase: 02-app-core-audio
plan: "02"
subsystem: transport
tags: [asyncio, udp, esp32, datagram-protocol, json, led-control]

# Dependency graph
requires:
  - phase: 01-esp32-firmware
    provides: "UDP JSON protocol spec (docs/protocol.md) — port 4210, v:1 commands, STATUS response format"
provides:
  - "ESP32Transport: asyncio.DatagramProtocol for fire-and-forget UDP JSON commands to ESP32"
  - "create_esp32_transport(): async factory coroutine returning ready ESP32Transport"
  - "Connection state tracking (connected/disconnected) via STATUS ok responses"
  - "led_sync.transport package with clean re-exports"
affects: [02-04-scheduler, 02-05-repl, 03-tui]

# Tech tracking
tech-stack:
  added: [pytest-asyncio>=0.23]
  patterns:
    - "asyncio.DatagramProtocol for non-blocking UDP fire-and-forget"
    - "v:1 protocol version injected by transport layer, not callers"
    - "TDD red-green cycle for protocol-critical transport behavior"

key-files:
  created:
    - src/led_sync/transport/__init__.py
    - src/led_sync/transport/udp_client.py
    - tests/test_transport.py
  modified:
    - pyproject.toml

key-decisions:
  - "D-10: asyncio DatagramProtocol, fire-and-forget sendto() — no blocking anywhere in send path"
  - "D-11: send_status_ping() for optional heartbeat, non-blocking"
  - "D-12: connected=True set only upon STATUS ok response, not upon socket bind"
  - "D-14: Minimal error handling — silent drop on oversized payload, malformed JSON, pre-connection calls"
  - "Protocol version v:1 injected by send_command(), callers never need to include it"

patterns-established:
  - "Pattern 1: asyncio.DatagramProtocol subclass for ESP32 UDP transport (fire-and-forget, no awaits in send path)"
  - "Pattern 2: Silent drop pattern — no exceptions from transport layer, only debug/warning logs"
  - "Pattern 3: on_status callback for reactive STATUS response handling without polling"

requirements-completed: [CHR-05]

# Metrics
duration: 4min
completed: 2026-04-03
---

# Phase 02 Plan 02: ESP32 UDP Transport Client Summary

**asyncio DatagramProtocol UDP client for ESP32 on port 4210 — fire-and-forget JSON commands with STATUS-based connection tracking**

## Performance

- **Duration:** 4 min
- **Started:** 2026-04-03T12:15:55Z
- **Completed:** 2026-04-03T12:19:45Z
- **Tasks:** 1 (TDD: test commit + feat commit)
- **Files modified:** 4

## Accomplishments

- ESP32Transport implements asyncio.DatagramProtocol with all protocol behaviors matching docs/protocol.md exactly
- send_command() injects v:1, enforces 512-byte limit, silently drops if pre-connection — all non-blocking
- STATUS ok responses parsed in datagram_received() to set connected=True and store last_status
- 15 unit tests covering all public behaviors including async factory and oversized payload drop
- create_esp32_transport() async factory sends initial STATUS ping for D-12 connection bootstrap

## Task Commits

1. **RED — Failing tests for ESP32Transport** - `f10fcf0` (test)
2. **GREEN — ESP32Transport implementation** - `378620e` (feat)

## Files Created/Modified

- `src/led_sync/transport/udp_client.py` — ESP32Transport class + create_esp32_transport() factory
- `src/led_sync/transport/__init__.py` — Package marker, re-exports ESP32Transport and create_esp32_transport
- `tests/test_transport.py` — 15 unit tests covering all protocol behaviors
- `pyproject.toml` — Added pytest-asyncio to dev deps, asyncio_mode=auto

## Decisions Made

- Protocol version field v:1 is injected by send_command() — callers never include it (cleaner downstream API)
- on_status callback parameter in ESP32Transport constructor and create_esp32_transport() factory for reactive status handling without polling
- Silent drop on pre-connection and oversized payload (no exceptions) matches D-14 fire-and-forget philosophy

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 3 - Blocking] Added pytest-asyncio dependency for async test support**
- **Found during:** Task 1 (TDD RED phase)
- **Issue:** Tests use `@pytest.mark.asyncio` but pytest-asyncio was not in pyproject.toml dev deps; test would fail with UnknownMarkWarning and no async test support
- **Fix:** `uv add --dev pytest-asyncio` and added `asyncio_mode = "auto"` in `[tool.pytest.ini_options]`
- **Files modified:** pyproject.toml, uv.lock
- **Verification:** All 15 tests pass including async factory test
- **Committed in:** 378620e (Task 1 GREEN commit)

**2. [Rule 3 - Blocking] Discovered pyproject.toml and led_sync package already existed**
- **Found during:** Task 1 setup
- **Issue:** Plan 02-01 (scaffold) has not been committed to git yet, but the pyproject.toml and src/led_sync/ structure already existed on disk from a previous partial execution
- **Fix:** Worked with existing files; only added transport package and pytest-asyncio
- **Files modified:** None (existing structure was sufficient)
- **Verification:** `uv run python -c "from led_sync.transport import create_esp32_transport"` exits 0

---

**Total deviations:** 2 auto-fixed (2x Rule 3 blocking)
**Impact on plan:** Both fixes necessary for test infrastructure and working with existing project state. No scope creep.

## Issues Encountered

None beyond the deviations documented above.

## User Setup Required

None — no external service configuration required.

## Next Phase Readiness

- `from led_sync.transport import create_esp32_transport` is importable and ready for use
- Scheduler (02-04) and REPL (02-05) can depend on `transport.send_command(dict)` and `transport.connected` interfaces
- Transport is fully tested with 15 unit tests covering all edge cases

## Self-Check: PASSED

All created files verified present. Both commits (f10fcf0, 378620e) confirmed in git log.

---
*Phase: 02-app-core-audio*
*Completed: 2026-04-03*