# ADR-003: Dash Web Framework for Visualization

## Status
Accepted

## Context
The orderflow backtest system requires a user interface for visualizing OHLC candlestick charts, volume data, orderbook depth, and derived metrics. Key requirements include:

- Real-time chart updates with minimal latency
- Professional financial data visualization capabilities
- Support for multiple chart types (candlesticks, bars, line charts)
- Interactive features (zooming, panning, hover details)
- Dark theme suitable for trading applications
- Python-native solution to avoid JavaScript development

## Decision
We will use Dash (by Plotly) as the web framework for building the visualization frontend, with Plotly.js for chart rendering.

## Consequences

### Positive
- **Python-native**: No JavaScript development required
- **Plotly integration**: Best-in-class financial charting capabilities
- **Reactive architecture**: Automatic UI updates via callback system
- **Professional appearance**: High-quality charts suitable for trading applications
- **Interactive features**: Built-in zooming, panning, hover tooltips
- **Responsive design**: Bootstrap integration for modern layouts
- **Development speed**: Rapid prototyping and iteration
- **WebGL acceleration**: Smooth performance for large datasets

### Negative
- **Performance overhead**: Heavier than custom JavaScript solutions
- **Limited customization**: Constrained by Dash component ecosystem
- **Single-page limitation**: Not suitable for complex multi-page applications
- **Memory usage**: Can be heavy for resource-constrained environments
- **Learning curve**: Callback patterns require understanding of reactive programming

## Implementation Details

### Application Structure
```python
# Main application with Bootstrap theme
app = dash.Dash(__name__, external_stylesheets=[dbc.themes.FLATLY])

# Responsive layout with 9:3 ratio for charts:depth
app.layout = dbc.Container([
    dbc.Row([
        dbc.Col([  # OHLC + Volume + Metrics
            dcc.Graph(id='ohlc-chart', style={'height': '100vh'})
        ], width=9),
        dbc.Col([  # Orderbook Depth
            dcc.Graph(id='depth-chart', style={'height': '100vh'})
        ], width=3)
    ]),
    dcc.Interval(id='interval-update', interval=500, n_intervals=0)
])
```

### Chart Architecture
```python
# Multi-subplot chart with shared x-axis
fig = make_subplots(
    rows=3, cols=1,
    row_heights=[0.6, 0.2, 0.2],  # OHLC, Volume, Metrics
    vertical_spacing=0.02,
    shared_xaxes=True,
    subplot_titles=['Price', 'Volume', 'OBI Metrics']
)

# Candlestick chart with dark theme
fig.add_trace(go.Candlestick(
    x=timestamps, open=opens, high=highs, low=lows, close=closes,
    increasing_line_color='#00ff00', decreasing_line_color='#ff0000'
), row=1, col=1)
```

### Real-time Updates
```python
@app.callback(
    [Output('ohlc-chart', 'figure'), Output('depth-chart', 'figure')],
    [Input('interval-update', 'n_intervals')]
)
def update_charts(n_intervals):
    # Read data from JSON files with error handling
    # Build and return updated figures
    return ohlc_fig, depth_fig
```

## Performance Characteristics

### Update Latency
- **Polling interval**: 500ms for near real-time updates
- **Chart render time**: 50-200ms depending on data size
- **Memory usage**: ~100MB for typical chart configurations
- **Browser requirements**: Modern browser with WebGL support

### Scalability Limits
- **Data points**: Up to 10,000 candlesticks without performance issues
- **Update frequency**: Optimal at 1-2 Hz, maximum ~10 Hz
- **Concurrent users**: Single user design (development server)
- **Memory growth**: Linear with data history size

## Alternatives Considered

### Streamlit
- **Rejected**: Less interactive, slower updates, limited charting
- **Pros**: Simpler programming model, good for prototypes
- **Cons**: Poor real-time performance, limited financial chart types

### Flask + Custom JavaScript
- **Rejected**: Requires JavaScript development, more complex
- **Pros**: Complete control, potentially better performance
- **Cons**: Significant development overhead, maintenance burden

### Jupyter Notebooks
- **Rejected**: Not suitable for production deployment
- **Pros**: Great for exploration and analysis
- **Cons**: No real-time updates, not web-deployable

### Bokeh
- **Rejected**: Less mature ecosystem, fewer financial chart types
- **Pros**: Good performance, Python-native
- **Cons**: Smaller community, limited examples for financial data

### Custom React Application
- **Rejected**: Requires separate frontend team, complex deployment
- **Pros**: Maximum flexibility, best performance potential
- **Cons**: High development cost, maintenance overhead

### Desktop GUI (Tkinter/PyQt)
- **Rejected**: Not web-accessible, limited styling options
- **Pros**: No browser dependency, good performance
- **Cons**: Deployment complexity, poor mobile support

## Configuration Options

### Theme and Styling
```python
# Dark theme configuration
dark_theme = {
    'plot_bgcolor': '#000000',
    'paper_bgcolor': '#000000',
    'font_color': '#ffffff',
    'grid_color': '#333333'
}
```

### Chart Types
- **Candlestick charts**: OHLC price data with volume
- **Bar charts**: Volume and metrics visualization
- **Line charts**: Cumulative depth and trend analysis
- **Scatter plots**: Trade-by-trade analysis (future)

### Interactive Features
- **Zoom and pan**: Time-based navigation
- **Hover tooltips**: Detailed data on mouse over
- **Crosshairs**: Precise value reading
- **Range selector**: Quick time period selection

## Future Enhancements

### Short-term (1-3 months)
- Add range selector for time navigation
- Implement chart annotation for significant events
- Add export functionality for charts and data

### Medium-term (3-6 months)
- Multi-instrument support with tabs
- Advanced indicators and overlays
- User preference persistence

### Long-term (6+ months)
- Real-time alerts and notifications
- Strategy backtesting visualization
- Portfolio-level analytics

## Monitoring and Metrics

### Performance Monitoring
- Chart render times and update frequencies
- Memory usage growth over time
- Browser compatibility and error rates
- User interaction patterns

### Quality Metrics
- Chart accuracy compared to source data
- Visual responsiveness during heavy updates
- Error recovery from data corruption

## Review Triggers
Reconsider this decision if:
- Update frequency requirements exceed 10 Hz consistently
- Memory usage becomes prohibitive (> 1GB)
- Custom visualization requirements cannot be met
- Multi-user deployment becomes necessary
- Mobile responsiveness becomes a priority
- Integration with external charting libraries is needed

## Migration Path
If replacement becomes necessary:
1. **Phase 1**: Abstract chart building logic from Dash specifics
2. **Phase 2**: Implement alternative frontend while maintaining data formats
3. **Phase 3**: A/B test performance and usability
4. **Phase 4**: Complete migration with feature parity