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:

C#

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

[EventType]
public record ReducersEventSequenceShipmentDispatched(string TrackingNumber, string Carrier);

public record ReducersEventSequenceShipmentSummary(string TrackingNumber, string Carrier, DateTimeOffset DispatchedAt);

[EventSequence("fulfillment-events")]
public class ReducersEventSequenceShipmentSummaryReducer : IReducerFor<ReducersEventSequenceShipmentSummary>
{
    public ReducersEventSequenceShipmentSummary Dispatched(ReducersEventSequenceShipmentDispatched @event, ReducersEventSequenceShipmentSummary? current, EventContext context) =>
        new(@event.TrackingNumber, @event.Carrier, context.Occurred);
}

TypeScript reaches the same result through the reducer() decorator’s event-sequence parameter shown below rather than a separate attribute. Kotlin, Java, and Elixir don’t currently support targeting a non-default event sequence for a reducer at all.

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:

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reducers;

[EventType]
public record ReducersEventSequenceReducerAttributeShipmentDispatched(string TrackingNumber, string Carrier);

public record ReducersEventSequenceReducerAttributeShipmentSummary(string TrackingNumber, string Carrier, DateTimeOffset DispatchedAt);

[Reducer(id: "shipment-summary", eventSequence: "fulfillment-events")]
public class ReducersEventSequenceReducerAttributeShipmentSummaryReducer : IReducerFor<ReducersEventSequenceReducerAttributeShipmentSummary>
{
    public ReducersEventSequenceReducerAttributeShipmentSummary Dispatched(ReducersEventSequenceReducerAttributeShipmentDispatched @event, ReducersEventSequenceReducerAttributeShipmentSummary? current, EventContext context) =>
        new(@event.TrackingNumber, @event.Carrier, context.Occurred);
}

TypeScript

import { eventType, reducer } from '@cratis/chronicle';

@eventType()
class ReducersEventSequenceReducerAttributeShipmentDispatched {
    trackingNumber = '';
    carrier = '';
}

class ReducersEventSequenceReducerAttributeShipmentSummary {
    trackingNumber = '';
    carrier = '';
    dispatchedAt: Date = new Date(0);
}

@reducer('shipment-summary', 'fulfillment-events', ReducersEventSequenceReducerAttributeShipmentSummary)
class ReducersEventSequenceReducerAttributeShipmentSummaryReducer {
    // Method name must be the exact camelCase of the event's class name
    reducersEventSequenceReducerAttributeShipmentDispatched(
        event: ReducersEventSequenceReducerAttributeShipmentDispatched,
        _current: ReducersEventSequenceReducerAttributeShipmentSummary | undefined
    ): ReducersEventSequenceReducerAttributeShipmentSummary {
        return { trackingNumber: event.trackingNumber, carrier: event.carrier, dispatchedAt: new Date() };
    }
}

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:

C#

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

[EventType]
public record ReducersEventSequenceLogOrderPlaced(Guid OrderId);

public record ReducersEventSequenceLocalOrderSummary(int OrderCount, DateTimeOffset LastOrderAt);

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

This convenience is C#-only — it exists to disambiguate against C#‘s [EventStore] attribute on event types, which the other clients don’t have.

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