Event Sequence

By default, reactors 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 [Reactor] attribute.

Using the [EventSequence] Attribute

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

using Cratis.Chronicle.EventSequences;
using Cratis.Chronicle.Reactors;

[EventSequence("fulfillment-events")]
public class ShipmentReactor : IReactor
{
    public Task ShipmentDispatched(ShipmentDispatched @event, EventContext context)
    {
        return NotifyCarrierAsync(@event.TrackingNumber);
    }

    Task NotifyCarrierAsync(string trackingNumber) => Task.CompletedTask;
}

Using the [Reactor] Attribute

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

using Cratis.Chronicle.Reactors;

[Reactor(id: "shipment-reactor", eventSequence: "fulfillment-events")]
public class ShipmentReactor : IReactor
{
    public Task ShipmentDispatched(ShipmentDispatched @event, EventContext context)
    {
        return NotifyCarrierAsync(@event.TrackingNumber);
    }

    Task NotifyCarrierAsync(string trackingNumber) => Task.CompletedTask;
}

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

Convenience: [EventLog]

Use [EventLog] when you want to be explicit that the reactor 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.Reactors;

[EventLog]
public class LocalAuditReactor : IReactor
{
    public Task OrderPlaced(OrderPlaced @event, EventContext context)
    {
        return WriteAuditAsync(@event.OrderId, context.Occurred);
    }

    Task WriteAuditAsync(Guid orderId, DateTimeOffset occurred) => Task.CompletedTask;
}

External Event Store Inbox Routing

For outbox-to-inbox subscriptions between event stores, including automatic inbox routing and observer-level [EventStore] usage, see Subscribe Reactors 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 reactor that handles foreign events
• For reactors that target a specialized or partitioned event stream
• When an explicit sequence name improves readability and makes intent clear