Events

NautilusTrader models execution, position, account, and time changes as events. The MessageBus routes these events to interested components and, where supported, to strategy handlers. This guide covers the event types, their dispatch, and how order fills and corrections produce position events.

Event categories

Handler dispatch

When an event reaches a strategy, the system calls handlers in a fixed order. The specific handler runs before the aggregate handler, so you can handle events at either granularity or use both. A strategy passes events to its handlers only while it is running; events that arrive before start or after stop are not dispatched.

Python data actors do not expose order event callbacks or the raw message bus. Use signals to send derived values from a strategy to a data actor. See Actors: order event handling.

Order events

1. Specific handler (for example, on_order_filled).
2. on_order_event (receives all order events).

Position events

For the position lifecycle events dispatched to strategies:

1. Specific handler (for example, on_position_opened).
2. on_position_event (receives all dispatched position lifecycle events).

Time events

Timers and alerts produce TimeEvent objects. Pass a callback when calling set_timer or set_time_alert to direct events to your own method. If you omit the callback, a callback previously registered under the same name is used when present; otherwise the event is delivered to on_time_event.

Order events

Order events initialize an order, change its state, or correct its fill history. The execution pipeline applies them to the order and cache, then publishes them on the MessageBus. The table below shows the primary transitions; partially filled, external, and triggered orders support additional transitions documented in the full order state flow.

Common Python order event fields

Every concrete Python order event exposes these fields:

Each order event page lists its type-specific fields. These include venue_order_id, account_id, and reconciliation only on the Python event classes that expose them. For example, OrderFilled adds last_qty, last_px, trade_id, and commission. OrderFillVoided identifies the corrected trade and carries its cumulative voided quantity.

Position events

Position lifecycle events describe cached position changes caused by fills and fill corrections. The ExecutionEngine processes each OrderFilled, updates or creates a position, and emits the corresponding lifecycle event.

When an OrderFillVoided corrects a locally applied fill, it rebuilds each affected cached position from its effective fill history. It does not emit an opposite fill. After publishing the correction, the engine emits PositionChanged for a corrected position that remains open or PositionClosed for one that is closed. An order-only correction does not produce a position event.

From fill to position: the causal chain

The following diagram shows how a single OrderFilled event produces a position event, the link between order management and position tracking.

Step by step:

1. Fill arrives. The ExecutionEngine receives an OrderFilled event through the execution pipeline.
2. Order state updates. The engine applies the fill to the order object and writes the updated order to the Cache.
3. Position ID resolved. The engine determines which position this fill belongs to, based on OMS type and strategy configuration.
4. Position created or updated. Three outcomes:
   • No position exists for this ID: the engine creates a Position from the fill, adds it to the Cache, and emits PositionOpened.
   • Position exists and remains open after the fill: the engine applies the fill to the position, updates the Cache, and emits PositionChanged.
   • Position exists and closes (quantity reaches zero): the engine applies the fill, updates the Cache, and emits PositionClosed.
5. Flip case. When a fill reverses the position, for example a sell of 15 against a long 10, the engine splits the fill into two parts: one that closes the original position (PositionClosed) and one that opens the new position (PositionOpened).

Position event fields

The three position lifecycle event classes share a core field set and expose additional fields as the position develops. A check mark means the Python class exposes the field; a dash means the field is absent from that class.

Tracing orders to positions

The Cache provides methods to navigate between orders and positions:

# From a position, find all orders that contributed fills
orders = self.cache.orders_for_position(position.id)

# From an order, find the position it belongs to
position = self.cache.position_for_order(order.client_order_id)

# The opening order is stored directly on the position
opening_order_id = position.opening_order_id

Account events

AccountState events represent balance and margin snapshots. They fire when:

• The venue reports an account update (via the execution client).
• The Portfolio recalculates account state after a position update (for margin accounts with calculate_account_state enabled).

Account state contains balances, margins, account type, and base currency. The Portfolio subscribes to these events internally to maintain exposure and balance tracking. See AccountState for the full field list.

Related guides

• Orders - Order types and state machine.
• Positions - Position lifecycle and PnL.
• Execution - Execution flow and risk checks.
• Strategies - Handler implementations in strategies.
• Architecture - Data and execution flow patterns.