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How to get the current entity state from events?

Oskar Dudycz  |  15 July 2021



In Event Sourcing, the application state is stored in events. When we add an event, it is placed at the end of a structure called an append-only log (read more in this post What if I told you that Relational Databases are in fact Event Stores?). Events are the source of truth. This has many advantages, such as:

  • the history of changes in our system,
  • easier diagnostics,
  • closeness to business, as our code structures correspond to business facts.

However, it does not have to cause an automatic revolution in our code. We can still use aggregates/entities. In Event Sourcing, events are logically grouped linked into streams. Streams are ordered sequences of events. One stream includes all events for a given business object, e.g. InvoiceInitiated, InvoiceIssued, InvoiceSent.

Many people believe that Snapshots are a must-have in an Event-Sourced system. Instead of retrieving all stream events to rebuild the state, we could retrieve one record and use it for our business logic. It sounds promising and can be useful as a technical optimisation technique, but should not be consistently used as a starting point. Isn’t loading more than one event a performance issue? Frankly, it’s not. Downloading even a dozen, or several dozens of small events is not a significant overhead. Events are concise, containing only the information needed. Event Stores are optimised for such operations, and the reads scale well (read more in this article Snapshots in Event Sourcing).

Thus recommended approach is to build the current state from events. To we need to perform the following steps:

  1. Get all events for a given stream. We choose them based on the stream identifier (derived from the business object/record id).
  2. Sort them in the order of occurrence.
  3. Create a default, empty entity (e.g. using the default constructor).
  4. Apply each event sequentially to the entity.

The first three points are, I think it’s obvious, but what does "apply an event" mean? There are two ways:

  • Use the When function. We’re passing a generic event object as an input parameter. Inside the method, we can use “pattern matching” to determine what logic applies to the specific event type. It is a framework-independent solution. You have to write a bit more yourself, but there is less magic.
  • Some frameworks provide convention-based solutions that simplify handling and make it a bit more magical. You could build a convention based mechanism that would read events and apply them internally.

This process is also called stream aggregation. Let’s focus for now on the general approach to understand the flow properly. In C#, it might look like this:

public record Person(
    string Name,
    string Address

public record InvoiceInitiated(
    double Amount,
    string Number,
    Person IssuedTo,
    DateTime InitiatedAt

public record InvoiceIssued(
    string IssuedBy,
    DateTime IssuedAt

public enum InvoiceSendMethod

public record InvoiceSent(
    InvoiceSendMethod SentVia,
    DateTime SentAt

public enum InvoiceStatus
    Initiated = 1,
    Issued = 2,
    Sent = 3

public class Invoice
    public string Id { get;set; }
    public double Amount { get; private set; }
    public string Number { get; private set; }

    public InvoiceStatus Status { get; private set; }

    public Person IssuedTo { get; private set; }
    public DateTime InitiatedAt { get; private set; }

    public string IssuedBy { get; private set; }
    public DateTime IssuedAt { get; private set; }

    public InvoiceSendMethod SentVia { get; private set; }
    public DateTime SentAt { get; private set; }

    public void When(object @event)
        switch (@event)
            case InvoiceInitiated invoiceInitiated:
            case InvoiceIssued invoiceIssued:
            case InvoiceSent invoiceSent:

    private void Apply(InvoiceInitiated @event)
        Id = @event.Number;
        Amount = @event.Amount;
        Number = @event.Number;
        IssuedTo = @event.IssuedTo;
        InitiatedAt = @event.InitiatedAt;
        Status = InvoiceStatus.Initiated;

    private void Apply(InvoiceIssued @event)
        IssuedBy = @event.IssuedBy;
        IssuedAt = @event.IssuedAt;
        Status = InvoiceStatus.Issued;

    private void Apply(InvoiceSent @event)
        SentVia = @event.SentVia;
        SentAt = @event.SentAt;
        Status = InvoiceStatus.Sent;

The usage as follows:

var invoiceInitiated = new InvoiceInitiated(
    new Person("Oscar the Grouch", "123 Sesame Street"),
var invoiceIssued = new InvoiceIssued(
    "Cookie Monster",
var invoiceSent = new InvoiceSent(

// 1,2. Get all events and sort them in the order of appearance
var events = new object[] {invoiceInitiated, invoiceIssued, invoiceSent};

// 3. Construct empty Invoice object
var invoice = new Invoice();

// 4. Apply each event on the entity.
foreach (var @event in events)

If you prefer, you can add the base class with an abstract method to make sure that classes follow the convention and write the more generalised logic.

public abstract class Aggregate<TId>
    public TId Id { get; protected set; }
    public abstract void When(object @event);

Having that, we could write such a method for EventStoreDB to retrieve the aggregate state from events:

public async Task<TAggregate?> Find<TAggregate, TId>(Guid id, CancellationToken cancellationToken)
    where TAggregate: Aggregate<TId>, new ()
    var readResult = eventStore.ReadStreamAsync(
        cancellationToken: cancellationToken

    var aggregate = new TAggregate();

    await foreach (var @event in readResult)
        var eventData = Deserialize(@event);


    return aggregate;

Of course, this is a highly imperative approach. However, if we prefer a functional approach, we could use a pattern described in this article Why Partial is an extremely useful TypeScript feature?

In the functional approach, we don’t need base classes. We don’t need aggregates. Instead, we’re splitting the behaviour (functions) from the state (entity).

In TypeScript, having event and entity defined as:

type Event<
  EventType extends string = string,
  EventData extends Record<string, unknown> = Record<string, unknown>,
  EventMetadata extends Record<string, unknown> = Record<string, unknown>
> = Readonly<{
  type: Readonly<EventType>;
  data: Readonly<EventData>;
  metadata?: Readonly<EventMetadata>;

type Person = Readonly<{
  name: string;
  address: string;

type InvoiceInitiated = Event<
    number: string;
    amount: number;
    issuedTo: Person;
    initiatedAt: Date;

type InvoiceIssued = Event<
    number: string;
    issuedBy: string;
    issuedAt: Date;

type InvoiceSent = Event<
    number: string;
    sentVia: InvoiceSendMethod;
    sentAt: Date;

type InvoiceEvent =
  | InvoiceInitiated 
  | InvoiceIssued 
  | InvoiceSent;

type Invoice = Readonly<{
  number: string;
  amount: number;
  status: InvoiceStatus;

  issuedTo: Person;
  initiatedAt: Date;

  issued?: Readonly<{
    by?: string;
    at?: Date;

  sent?: Readonly<{
    via?: InvoiceSendMethod;
    at?: Date;

We can define the When method as:

function when(
  currentState: Partial,
  event: CashRegisterEvent
): Partial {
  switch (event.type) {
    case 'invoice-initiated':
      return {
        number: event.data.number,
        amount: event.data.amount,
        status: InvoiceStatus.INITIATED,
        issuedTo: event.data.issuedTo,
        initiatedAt: event.data.initiatedAt,
    case 'invoice-issued': {
      return {
        status: InvoiceStatus.ISSUED,
        issued: {
          by: event.data.issuedBy,
          at: event.data.issuedAt,
    case 'invoice-sent': {
      return {
        status: InvoiceStatus.SENT,
        sent: {
          via: event.data.sentVia,
          at: event.data.sentAt,
      return {

Using the reduce method and partial type described in the previously mentioned article, we can define the generic stream aggregation method as:

function aggregateStream<Aggregate, StreamEvents extends Event>(
  events: StreamEvents[],
  when: (
    currentState: Partial<Aggregate>,
    event: StreamEvents
  ) => Partial<Aggregate>,
  check?: (state: Partial<Aggregate>) => state is Aggregate
): Aggregate {
  const state = events.reduce<Partial<Aggregate>>(when, {});

  if (!check) {
    console.warn('No type check method was provided in the aggregate method');
    return <Aggregate>state;

  if (!check(state)) throw 'Aggregate state is not valid';

  return state;

Then we could use it as follows to rebuild the current state:

const events: InvoiceEvent[] = [];

for await (const resolvedEvent of eventStore.readStream(
)) {
  if (!resolvedEvent.event) continue;

    type: resolvedEvent.event!.type,
    data: resolvedEvent.event!.data,
    metadata: resolvedEvent.event?.metadata,

const invoice = aggregateStream<Invoice, InvoiceEvent>(

Object-oriented way brings more ceremony. However, it has an advantage against the functional approach, keeping object state and behaviour grouped together.

Stream aggregation is a simple but powerful pattern. It allows for easy debugging, writing unit tests and better control over what is happening. It's also the basis for doing the essence of Event Sourcing, so treating events as the source of truth.

Check out the detailed samples in these repositories:

This post was written and published by Oskar Dudycz on his blog, it has been edited and republished here with his permission.

Photo of Oskar Dudycz

Oskar Dudycz Oskar was a Developer Advocate at Event Store. His focus is on helping to create applications closer to business needs. He believes that Event Sourcing, CQRS, and Event-Driven Design are enablers to achieving that. Oskar is an Open Source contributor, co-maintainer of Marten library and always focuses on practical, hands-on development experience. You can check Oskar's blog https://event-driven.io/ and follow him on Twitter at @oskar_at_net. He has now left Event Store.