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Data flow

Both modes share the same conceptual pipeline; only the last stage differs (simulate vs. execute).

Backtest​

SymbolScanner.scan(candidates)            # marketdata -> scan signals
        │  universe
        ▼
MarketDataClient.get_bars(universe, tf, start, end)   # {symbol: OHLCV}
        │
        ▼  per symbol
Strategy.process_data(bars)               # + indicator columns
Strategy.calculate_scores(processed)      # {timestamp: signed score}  ← the one decision
Strategy.generate_signals(processed)      # base class derives {timestamp: BUY/SELL/HOLD/...}
        │
        ▼
BacktestEngine._replay(...)               # one merged timeline, one capital pool
                                          # mark -> exit -> rank -> admit -> record
        │  trades
        ▼
analytics.performance.compute_backtest_metrics(...)   # -> metrics dict
analytics.reporting.log_backtest_report(...)          # -> text

Every symbol is simulated on a single merged timeline against one shared capital pool, so positions compete for the same dollars as they would live: exits settle before entries within a bar, and simultaneous candidates are admitted in conviction order while position limits and free cash allow. Any position still open at the final bar is force-closed. See portfolio accounting.

Live​

LiveEngine.start(universe)
   ├── _warm_up: get_bars(lookback) -> process_data -> strategy.warm_up(buffer)
   └── MarketDataClient.stream(universe, on_bar)   # auto-reconnecting bar stream
            │ BarEvent (full OHLCV) per bar
            ▼
        Strategy.process_bar(symbol, {o,h,l,c,v}, ts)   # -> signal
            │ actionable signal
            ▼
        LiveTrader.handle_signal(symbol, signal, price)
            ├── entry: PositionSizer.size(...) -> submit_bracket_order
            └── exit:  Broker.close_position
            │
            ▼
        AlpacaBroker -> Alpaca SDK

The stream feeds the strategy the full streamed OHLCV bar (not just the close), and the PositionSizer is pluggable — RiskBasedSizer by default, or PortfolioWeightSizer when running live --portfolio. See Portfolio manager.

Why the pipeline is shared​

process_data and generate_signals are identical in both modes — the strategy doesn't know whether it's being backtested or traded live. The difference is only what consumes the signal: a simulator (engine) or a broker (execution). This is the payoff of separating concerns.