Events

Events are immutable facts that describe what happened in your system. Chronicle identifies events by their event type rather than the .NET CLR type, which makes the CLR type a convenient vessel for expressing intent and structure.

C#
TypeScript

using Cratis.Chronicle.Events;

[EventType]
public record EventsIndexEmployeeRegistered(string FirstName, string LastName);

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

@eventType()
class EventsIndexEmployeeRegistered {
    constructor(
        readonly firstName: string,
        readonly lastName: string
    ) {}
}

Event Sequences and the Event Log

Events live in event sequences, which are ordered, append-only streams. Each event receives a monotonically increasing sequence number that makes it possible to replay events, resume processing, and reason about how far consumers have progressed.

Chronicle includes a specialized event sequence called the event log. It is the default sequence used throughout the system and is exposed through the IEventLog API.

C#
TypeScript

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

public class EventsIndexEmployeesService(IEventLog eventLog)
{
    public Task RegisterEmployee(EventSourceId employeeId, string firstName, string lastName) =>
        eventLog.Append(employeeId, new EventsIndexEmployeeRegistered(firstName, lastName));
}

import { IEventStore } from '@cratis/chronicle';

class EventsIndexEmployeesService {
    constructor(private readonly store: IEventStore) {}

    async registerEmployee(employeeId: string, firstName: string, lastName: string): Promise<void> {
        await this.store.eventLog.append(employeeId, new EventsIndexEmployeeRegistered(firstName, lastName));
    }
}

Working with Events

Use these topics to append, read, and manage events: