led2/.planning/phases/02-audio-engine/02-02-SUMMARY.md

phase, plan, subsystem, tags, requires, provides, affects, tech-stack, key-files, key-decisions, patterns-established, requirements-completed, duration, completed

phase	plan	subsystem	tags	requires	provides	affects	tech-stack	key-files	key-decisions	patterns-established	requirements-completed	duration	completed
02-audio-engine	02	api	websocket fastapi mpv asyncio audio rest	phase provides 02-audio-engine/02-01 MPVEngine with get_state/play/pause/seek/load/stop, module-level engine in main.py	broadcast_loop sending position ticks at 10Hz over WebSocket WebSocket command dispatch (play/pause/seek/load) to MPVEngine REST POST /api/audio/load with file validation REST GET /api/audio/state returning current playback state	03-waveform 04-editor 05-timeline frontend-transport-ui	added patterns broadcast_loop uses get_engine callable to avoid circular import at module level asyncio.to_thread for blocking engine calls in async endpoints asyncio.create_task in lifespan for background loops with cancel on shutdown request.app.state.engine pattern for engine access in REST endpoints	created modified lightsync/api/audio.py lightsync/api/ws.py lightsync/main.py	broadcast_loop accepts get_engine callable instead of direct engine import — avoids circular import between ws.py and main.py asyncio.to_thread for engine.load in REST endpoint — engine.load is synchronous (python-mpv-jsonipc), must not block event loop AUD-02 (YouTube URL loading) noted as Phase 6 deferral — not in scope for this plan	Background loop pattern: asyncio.create_task in lifespan, cancel + await in shutdown Engine access in REST: request.app.state.engine (not module-level import) Engine access in WebSocket: deferred import from lightsync.main inside handler	AUD-02 AUD-04	8min	2026-04-06

Phase 02 Plan 02: Audio Engine WebSocket + REST Summary

WebSocket hub broadcasting MPV position at 10Hz and dispatching play/pause/seek/load commands; REST /api/audio/load with extension and existence validation

Performance

• Duration: 8 min
• Started: 2026-04-06T13:10:00Z
• Completed: 2026-04-06T13:18:00Z
• Tasks: 2
• Files modified: 3

Accomplishments

• Replaced WebSocket echo stub with full transport command dispatch (play/pause/seek/load) to MPVEngine
• Added broadcast_loop sending {type: "tick", position, paused, duration, loaded, file} at 10Hz
• Wired broadcast_loop as asyncio task in lifespan, properly cancelled on shutdown
• Created lightsync/api/audio.py with POST /load (validates path + extension) and GET /state
• Registered audio router at /api/audio prefix before static file catch-all in main.py

Task Commits

Each task was committed atomically:

1. Task 1: Wire WebSocket broadcast loop and command dispatch - 5e400a6 (feat)
2. Task 2: Add REST audio load endpoint with file validation - de45540 (feat)

Plan metadata: (to be committed)

Files Created/Modified

• lightsync/api/ws.py - broadcast_loop function; full command dispatch replacing echo stub
• lightsync/main.py - asyncio.create_task(broadcast_loop) in lifespan; audio router registration
• lightsync/api/audio.py - REST POST /load with file/extension validation; GET /state

Decisions Made

• broadcast_loop accepts get_engine: callable instead of importing engine directly — avoids circular import (ws.py would import main.py which imports ws.py)
• asyncio.to_thread wraps engine.load() in the REST endpoint since python-mpv-jsonipc's load is synchronous and would block the event loop
• AUD-02 (YouTube URL loading) deferred to Phase 6 — noted in docstring, not implemented

Deviations from Plan

None - plan executed exactly as written.

Issues Encountered

None. The get_engine callable pattern from the plan cleanly prevented the circular import issue that would otherwise occur if ws.py imported engine from main.py at module level.

User Setup Required

None - no external service configuration required.

Next Phase Readiness

• WebSocket + audio REST layer complete; browser can now receive real-time position ticks and send transport commands
• Phase 02-03 (waveform analysis) can proceed: GET /api/audio/state provides current file path; waveform endpoint can use that as input
• No blockers

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Phase: 02-audio-engine Completed: 2026-04-06