Table of Contents

Getting state

Event sequence state provides information about how far a sequence has progressed. The most common state value is the tail sequence number, which represents the latest event appended to the sequence. Use the IEventSequence APIs, such as GetTailSequenceNumber and related state calls, to capture the current position.

Use sequence state for scenarios such as:

  • Tracking progress for consumers and observers
  • Capturing a point in time for read model time travel
  • Avoiding duplicate processing when resuming work

For point-in-time reads of read models, capture the sequence position from the event sequence state and use it alongside the read model APIs described in the read models guides.

Related reading:

Examples

Capture the tail for a checkpoint

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

public class CheckpointStore(IEventLog eventLog)
{
    public async Task<EventSequenceNumber> CaptureTail()
    {
        // Persists the current tail so processing can resume later.
        return await eventLog.GetTailSequenceNumber();
    }
}

Capture the tail for a specific event source and event types

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

[EventType]
public record InventoryAdjusted(string Sku, int Delta);

[EventType]
public record InventoryReserved(string Sku, int Quantity);

public class InventoryCheckpoint(IEventLog eventLog)
{
    public async Task<EventSequenceNumber> CaptureFor(EventSourceId inventoryId)
    {
        // Scopes the tail to a specific stream of inventory events.
        var eventTypes = new[]
        {
            typeof(InventoryAdjusted).GetEventType(),
            typeof(InventoryReserved).GetEventType()
        };

        return await eventLog.GetTailSequenceNumber(
            eventSourceId: inventoryId,
            filterEventTypes: eventTypes
        );
    }
}

Check whether an observer is caught up

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

public class ObserverProgress(IEventSequence eventSequence)
{
    public async Task<EventSequenceNumber> GetRelevantTail(Type observerType)
    {
        // Uses the observer's event type filters to compute the relevant tail.
        return await eventSequence.GetTailSequenceNumberForObserver(observerType);
    }
}