Subscribe Reducers to External Event Stores

Automatic Inbox Routing

When all event types reduced by a reducer come from the same source event store, Chronicle routes the reducer to the matching inbox sequence automatically:

• Source event store: fulfillment-service
• Inbox sequence observed by the reducer: inbox-fulfillment-service

This works when the source is declared on event types (or assembly) using [EventStore("...")], and no explicit event sequence is configured on the reducer.

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

[EventType]
[EventStore("fulfillment-service")]
public record ShipmentDispatched(Guid OrderId, string TrackingNumber);

public record FulfillmentStatus(string Status, string TrackingNumber);

public class FulfillmentStatusReducer : IReducerFor<FulfillmentStatus>
{
    public FulfillmentStatus Reduce(ShipmentDispatched @event, FulfillmentStatus? current, EventContext context) =>
        new("Dispatched", @event.TrackingNumber);
}

Observer-Level EventStore for Missing Event Metadata

When event contracts cannot carry [EventStore] metadata, apply [EventStore] directly on the reducer.

Chronicle resolves the reducer to the inbox for that source event store and implicitly creates or reuses the outbox-to-inbox subscription.

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

[EventType]
public record ExternalUserInvited(Guid UserId);

public record ExternalUserTotals(int Count);

[EventStore("identity-service")]
[Reducer]
public class ExternalUserTotalsReducer : IReducerFor<ExternalUserTotals>
{
    public ExternalUserTotals Reduce(ExternalUserInvited @event, ExternalUserTotals? current, EventContext context) =>
        new((current?.Count ?? 0) + 1);
}

Implicit Combined Event Type Filtering

For each source event store, Chronicle combines all reducer and reactor event types into one implicit subscription filter.

This means:

• You get one subscription per source event store.
• The filter is the union of handled event types from all participating observers.
• Only those event types are forwarded from source outbox to target inbox.

Explicit Event Sequence Is Not Allowed with Observer EventStore

When a reducer has [EventStore("...")], do not combine it with:

• [EventSequence("...")]
• [EventLog]
• [Reducer(eventSequence: "...")]

This is invalid and fails:

• At runtime with EventStoreCannotBeCombinedWithExplicitEventSequence.
• At compile time with analyzer rule CHR0014.

See Also

• Implicit Event Store Subscriptions
• Explicit Event Store Subscriptions
• Event Sequence