REPLAY_ENGINE_ENHANCEMENTS.md

Replay Engine Enhancements: True Session Replay

Overview

Enhanced the Replay Engine to provide true session replay with complete state reconstruction, not just event log playback. The replay now behaves as if the user is watching a recording of the live simulator.

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Key Enhancements

1. State Reconstruction System

Problem: Original replay only streamed raw events without maintaining market state.

Solution: Implemented comprehensive state reconstruction:

• Initial State Loading: Loads SESSION_START or first CHECKPOINT to establish baseline state
• Incremental State Updates: Applies each event (TICK, TRADE, STRATEGY_TOGGLE) to current state
• Checkpoint-Based Seeking: Finds nearest checkpoint before target position, then replays events forward
• Full State Reconstruction: At any point in timeline, complete market state can be rebuilt

Implementation:

• _build_initial_state() - Extracts initial state from session
• _apply_event_to_state() - Updates state based on event type
• _reconstruct_state_at_index() - Rebuilds state at specific event index

2. Snapshot Broadcasting

Problem: Frontend components expected SNAPSHOT messages like in live mode, but replay sent raw events.

Solution: Replay server now emits reconstructed SNAPSHOT messages:

{
  "type": "SNAPSHOT",
  "symbol": "AAPL",
  "bids": [...],  # Full order book
  "asks": [...],
  "history": [...],  # Price history
  "recent_trades": [...],
  "portfolio": {...},
  "strategy_states": {...},
  "replay_meta": {
    "index": 42,
    "total": 1000,
    "progress": 0.042,
    "mode": "playing",
    "speed": 1.0,
    "is_replay": true
  }
}

Benefits:

• Frontend components work identically in live and replay modes
• Order book updates progressively
• Price chart redraws with historical data
• Portfolio metrics update frame-by-frame
• No component modifications needed

3. Enhanced Seek Functionality

Problem: Seeking only moved event index without reconstructing state.

Solution:

• Finds nearest checkpoint before target
• Replays all events from checkpoint to target
• Broadcasts reconstructed snapshot at target position
• Handles seeks by index, timestamp, or progress ratio

User Experience:

• Dragging timeline slider instantly shows market state at that point
• All components (order book, chart, portfolio, strategies) update to match

4. Dual WebSocket System

Problem: Single WebSocket couldn't differentiate between live and replay modes.

Solution:

• Live Mode: ws://127.0.0.1:8000/ws/stream
• Replay Mode: ws://127.0.0.1:8000/ws/replay
• Frontend automatically switches based on replayMode state
• WebSocket reconnects when mode changes

5. Lifecycle Event Preservation

Problem: Order Journey component requires LIFECYCLE events to animate order flow.

Solution:

• LIFECYCLE events broadcast separately from snapshots
• Order Journey animates during replay exactly as in live mode
• User can see orders progress through: Submit → Risk → Book → Match → Fill

6. Replay Mode Management

Frontend State:

const [replayMode, setReplayMode] = useState<boolean>(false);

Mode Transitions:

• Enter Replay: Loading a session sets replayMode = true
• Exit Replay: Stopping replay or clicking "Exit Replay" sets replayMode = false
• WebSocket automatically reconnects to appropriate endpoint

Visual Indicators:

• Header shows green "LIVE" badge or orange "REPLAY MODE" badge
• ReplayPanel shows "Replay Mode Active" banner when in replay
• Mode tabs disabled during active replay

7. Progressive Data Updates

Component Behavior During Replay:

Component	Update Behavior
Order Book	Bids/asks appear/disappear, quantities change, spread updates
Price Chart	Candles redraw progressively, price line moves, volume bars appear
Execution Feed	Trades appear one-by-one at recorded timestamps
Portfolio	Cash, positions, P&L update with each trade
Order Journey	Animates through pipeline stages
System Pipeline	Shows order flow through matching engine
Strategy Panel	Enable/disable toggles reflect recorded state

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Recording Format Validation

Events Already Recorded ✅

1. SESSION_START - Initial configuration
2. TICK - Market price updates (symbol, price, timestamp)
3. LIFECYCLE - Order pipeline stages (submit, risk, book, match, fill)
4. TRADE - Executions (buyer, seller, quantity, price)
5. STRATEGY_TOGGLE - Strategy enable/disable
6. USER_ORDER - User order submissions
7. USER_CANCEL - User order cancellations
8. CHECKPOINT (every 30s) - Full snapshot:
   • Order book (bids/asks)
   • Best bid/ask/spread
   • Price history (last 60 candles)
   • Recent trades (last 20)
   • Portfolio (cash, positions, P&L)
   • User open orders
   • Strategy states
9. SESSION_END - Recording stopped

No Additional Recording Needed

The existing recording format is comprehensive. Checkpoints capture full market state every 30 seconds, and incremental events (TICK, TRADE, LIFECYCLE) fill the gaps.

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Technical Implementation

Backend Changes

trading_simulator/replay/server.py:

• Added _build_initial_state() - Extract initial state from session
• Added _apply_event_to_state() - Update state based on event type
• Added _reconstruct_state_at_index() - Rebuild state at any point
• Added _broadcast_reconstructed_snapshot() - Emit SNAPSHOT-like messages
• Enhanced _playback_loop() - Maintain and broadcast state during playback
• Enhanced seek() - Reconstruct state when seeking
• Enhanced step() - Reconstruct state when stepping
• Enhanced load_session() - Broadcast initial state on load (now async)

web_server.py:

• Made /api/replay/load endpoint async to support initial state broadcast

Frontend Changes

frontend/src/App.tsx:

• Added replayMode state
• Modified WebSocket connection to switch between /ws/stream and /ws/replay
• Added replayMode dependency to WebSocket useEffect (reconnects on mode change)
• Passed replayMode prop to Header component
• Passed replayMode and onReplayModeChange to ReplayPanel

frontend/src/components/Header.tsx:

• Added replayMode prop
• Displays "LIVE" (green, pulsing) or "REPLAY MODE" (orange) badge
• Badge always visible in header

frontend/src/components/ReplayPanel.tsx:

• Added replayMode and onReplayModeChange props
• Enters replay mode when session loaded
• Exits replay mode when replay stopped
• Shows "Replay Mode Active" banner during replay
• Disables mode tabs during active replay
• "Exit Replay" button to quickly return to live mode

frontend/src/index.css:

• Added --replay-orange color variable
• Added pulse animation for live mode indicator

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User Experience Flow

Recording a Session

1. User clicks "Record" tab
2. Optionally enters session name
3. Clicks "Start Recording"
4. Red pulsing indicator shows recording active
5. Duration, events, file size display in real-time
6. Clicks "Stop Recording"
7. Session saved to replay_sessions/ folder

Replaying a Session

1. User clicks "Replay" tab
2. Sees list of available sessions
3. Clicks a session to load
4. UI enters Replay Mode:
   • Header shows orange "REPLAY MODE" badge
   • ReplayPanel shows "Replay Mode Active" banner
   • WebSocket switches to /ws/replay
   • Initial state displays immediately
5. User controls playback:
   • Play/Pause - Start/stop replay
   • Timeline Slider - Seek to any point (state reconstructs instantly)
   • Step - Move forward/backward one event at a time
   • Speed - 0.25x, 0.5x, 1x, 2x, 5x
6. All dashboard components update progressively:
   • Order book depth changes
   • Price chart redraws
   • Executions appear in feed
   • Portfolio metrics update
   • Strategies enable/disable
7. At replay end, session summary modal displays
8. User clicks "Exit Replay" to return to live mode

Seeking Behavior

When user drags timeline slider:

1. Replay pauses
2. Server finds nearest checkpoint before target
3. Server replays events from checkpoint to target
4. Server broadcasts reconstructed SNAPSHOT
5. Frontend updates all components instantly
6. User sees complete market state at that timestamp
7. If replay was playing, resumes from new position

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Performance Optimizations

1. Checkpoint Strategy: Full snapshots every 30s (or every 100 events) enable fast seeking
2. Incremental Updates: Between checkpoints, only changed data transmitted
3. Lazy State Reconstruction: State only rebuilt on seek/step, not every event during playback
4. Efficient Event Format: JSON Lines allows streaming without loading entire file
5. WebSocket Streaming: Real-time data delivery with low latency

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Testing Checklist

Recording

• Start recording → real-time status updates
• Submit orders → captured in recording
• Enable/disable strategies → captured in recording
• Stop recording → file created with correct size
• Checkpoints appear every ~30 seconds

Playback - State Reconstruction

• Load session → initial state displays
• Play → order book updates progressively
• Play → price chart redraws with historical data
• Play → executions appear one-by-one
• Play → portfolio updates with trades
• Play → strategies reflect recorded state
• Pause → state freezes
• Resume → continues from paused point

Playback - Seeking

• Seek to middle → complete state reconstructed
• Seek to beginning → returns to initial state
• Seek to end → shows final state
• Step forward → next event, state updates
• Step backward → previous event, state reconstructs
• Seek during playback → pauses, reconstructs, resumes

Playback - Lifecycle

• Order Journey animates during replay
• System Pipeline shows order flow
• LIFECYCLE events appear in terminal journal

Mode Switching

• Load session → enters replay mode
• Header shows "REPLAY MODE" badge
• WebSocket switches to /ws/replay
• Stop replay → exits replay mode
• Header shows "LIVE" badge
• WebSocket switches back to /ws/stream

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Known Limitations

1. Order Book Detail: Between checkpoints, order-level book changes may be aggregated
2. Large Sessions: Very long recordings (>1 hour) may take time to load
3. Memory Usage: Full event array loaded into memory on session load
4. Playback Speed Limits: Maximum 10x speed (can be increased if needed)

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Future Enhancements

Short-term

• Lazy loading for large sessions (load events in chunks)
• Compression for recording files (gzip .jsonl)
• Session trimming (remove unwanted segments)
• Bookmarks (mark important timestamps)

Long-term

• Multi-session comparison (side-by-side replay)
• Export to video (screen recording of replay)
• Replay filtering (show only specific strategies/symbols)
• Collaborative replay (share sessions with team)
• Cloud storage integration

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Files Modified

Backend:

• trading_simulator/replay/server.py (major enhancements)
• web_server.py (async load endpoint)

Frontend:

• frontend/src/App.tsx (replay mode state + WebSocket switching)
• frontend/src/components/Header.tsx (mode indicator)
• frontend/src/components/ReplayPanel.tsx (mode management)
• frontend/src/index.css (replay color variable)

Documentation:

• REPLAY_ENGINE_ENHANCEMENTS.md (this file)
• REPLAY_ENGINE_IMPLEMENTATION_STATUS.md (updated)

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Summary

The enhanced Replay Engine now provides true session replay rather than event log playback:

✅ Complete state reconstruction at any point in timeline
✅ Progressive component updates (order book, chart, portfolio, strategies)
✅ Instant seeking with full state restoration
✅ Lifecycle event preservation for Order Journey animation
✅ Dual WebSocket system for live/replay mode switching
✅ Visual mode indicators (header badge, replay banner)
✅ No component modifications - works with existing dashboard
✅ Comprehensive recording format - all state changes captured

The replay behaves as if the user is watching a recording of the live simulator. All dashboard components update naturally, exactly as they did during the original trading session.