Persistent state and trailing stop

sdk.state is the SDK mechanism for non-trivial strategies. It is a dictionary that the engine keeps alive between script calls.

Common patterns for managing state bar-by-bar, from flags to trailing stops.

def on_bar_strategy(sdk, params):
    # Pattern 1: Initialization
    if not isinstance(sdk.state, dict): sdk.state = {}
    
    # Pattern 2: Trail Stop Logic
    if sdk.position > 0:
        hw = sdk.state.get("high_water")
        sdk.state["high_water"] = max(hw, close) if hw else close
        new_stop = sdk.state["high_water"] * (1 - trail_pct)
        sdk.update_exits(stop_loss=new_stop)
    
    # Pattern 3: Cooldowns
    now = sdk.candles[-1]["time"]
    if now - sdk.state.get("last_entry", 0) < cooldown_ms:
        return

Visual flowchart of how data persists across engine calls and restarts.

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Principle

Always initialize keys before using them, at least for non-numeric objects. Missing numeric keys return 0.0 automatically, which simplifies counters but can hide bugs for lists or dicts.

Size limit

sdk.state lives in sandbox memory subject to the per-strategy memory ceiling. Avoid memory leaks by capping list sizes — see persistent state and sandbox limits.