Table of Contents

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("outbox")]
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: "outbox")]
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;
}

Automatic Inbox Routing

When the event types handled by a reactor carry a [EventStore] attribute pointing to a different event store than the one the reactor is registered in, Chronicle automatically routes the reactor to the corresponding inbox sequence. No [EventSequence] annotation is needed in that case.

When the [EventStore] attribute points to the same event store as the current one, Chronicle routes to the event log instead.

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