Event Sequence

By default, reducers observe the default event log. You can override this by specifying a different event sequence using either the [EventSequence] attribute or the eventSequence parameter on the [Reducer] attribute.

Using the [EventSequence] Attribute

Apply [EventSequence] directly to your reducer class to pin it to a specific event sequence:

using Cratis.Chronicle.EventSequences;
using Cratis.Chronicle.Reducers;

[EventSequence("fulfillment-events")]
public class ShipmentSummaryReducer : IReducerFor<ShipmentSummary>
{
    public ShipmentSummary OnShipmentDispatched(ShipmentDispatched @event, ShipmentSummary? current, EventContext context)
    {
        return new ShipmentSummary(
            @event.TrackingNumber,
            @event.Carrier,
            context.Occurred);
    }
}

Using the [Reducer] Attribute

When you are already using the [Reducer] attribute to set a custom identifier or other options, use its eventSequence parameter instead of adding a separate [EventSequence] attribute:

using Cratis.Chronicle.Reducers;

[Reducer(id: "shipment-summary", eventSequence: "fulfillment-events")]
public class ShipmentSummaryReducer : IReducerFor<ShipmentSummary>
{
    public ShipmentSummary OnShipmentDispatched(ShipmentDispatched @event, ShipmentSummary? current, EventContext context)
    {
        return new ShipmentSummary(
            @event.TrackingNumber,
            @event.Carrier,
            context.Occurred);
    }
}

Both approaches produce the same result. Prefer [Reducer(eventSequence: ...)] when you are already customizing the reducer with other parameters on that attribute.

Convenience: [EventLog]

Use [EventLog] when you want to be explicit that the reducer reads from the default event log, even when event types in the assembly carry a [EventStore] attribute pointing elsewhere:

using Cratis.Chronicle.EventSequences;
using Cratis.Chronicle.Reducers;

[EventLog]
public class LocalOrderSummaryReducer : IReducerFor<LocalOrderSummary>
{
    public LocalOrderSummary OnOrderPlaced(OrderPlaced @event, LocalOrderSummary? current, EventContext context)
    {
        var count = current?.OrderCount ?? 0;
        return new LocalOrderSummary(count + 1, context.Occurred);
    }
}

External Event Store Inbox Routing

For outbox-to-inbox subscriptions between event stores, including automatic inbox routing and observer-level [EventStore] usage, see Subscribe Reducers to External Event Stores.

When to Use [EventSequence] Explicitly

• When you need to read from a specific sequence other than the inferred one
• To suppress automatic inbox routing for a reducer that handles foreign events
• For reducers that target a specialized or partitioned event stream
• When an explicit sequence name improves readability and makes intent clear