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Node.js Backend Development Bootcamp · Lección

Los eventos como fuente de verdad y el log de solo anexado

Sustituya el estado mutable por un flujo de eventos inmutable y reconstruya el estado reproduciendo los eventos.

Lección 1 de 413 pasos

Los eventos como fuente de verdad y el log de solo anexado es una lección gratuita de Node.js Backend Development Bootcamp en CoddyKit. Esta es la lección 1 de 4. Puedes leer la lección completa abajo gratuitamente — luego la practicas en el navegador con un editor de código integrado y un tutor de IA 24/7. Forma parte de la ruta de aprendizaje de Node.js Backend Development Bootcamp, y tu progreso se sincroniza en la web y la app de CoddyKit. El curso de Node.js Backend Development Bootcamp incluye 4 lecciones en total.

Partes de esta lección aún no han sido traducidas y se muestran en inglés.

The Problem with Mutable State

In a classic CRUD backend, you store the current state of an entity and overwrite it on every change. A user's balance is one row; when money moves, you UPDATE the number in place.

This is convenient but lossy. Once you overwrite the old value, the history is gone. You cannot answer:

  • How did the balance reach this value?
  • When and why did each change happen?
  • What did the state look like last Tuesday?

Event Sourcing flips this: instead of storing the latest state, you store the sequence of facts that produced it.

// Classic mutable CRUD: history is destroyed on every write
let account = { id: 'acc-1', balance: 100 };

function deposit(amount) {
  account.balance += amount; // old value is gone forever
}

function withdraw(amount) {
  account.balance -= amount; // no record of why or when
}

deposit(50);
withdraw(30);
console.log(account); // { id: 'acc-1', balance: 120 }
// We know the result, but not the journey.

Events as the Source of Truth

In Event Sourcing, an event is an immutable record of something that already happened. Events are named in the past tense: MoneyDeposited, MoneyWithdrawn, AccountOpened.

The event log becomes the source of truth. The current state is no longer stored directly — it is a derived value you compute by replaying events.

  • Events are facts: they cannot be changed or deleted.
  • State is an opinion: a projection of the facts at a point in time.

Each event captures intent and context, not just the resulting number.

// An event is an immutable, past-tense fact
const events = [
  { type: 'AccountOpened',  data: { accountId: 'acc-1', owner: 'Ada' }, at: '2026-01-01T09:00:00Z' },
  { type: 'MoneyDeposited', data: { accountId: 'acc-1', amount: 50 },  at: '2026-01-02T10:15:00Z' },
  { type: 'MoneyWithdrawn', data: { accountId: 'acc-1', amount: 30 },  at: '2026-01-03T11:30:00Z' }
];

// Each entry is a fact that already happened. Nothing is overwritten.
console.log(`Stored ${events.length} immutable facts.`);

The Append-Only Log

The store that holds events is an append-only log. There are only two real operations:

  • Append a new event to the end.
  • Read events in order, usually for a given stream (e.g. one account).

There is no UPDATE and no DELETE. This single constraint gives you a complete, ordered, tamper-evident audit trail for free.

Because the log is ordered, the position (or version) of each event matters: replaying in the same order always yields the same state.

// A minimal in-memory append-only log
class EventLog {
  constructor() { this.events = []; }

  append(event) {
    // Only ever push to the end — never mutate or remove
    const stored = { ...event, position: this.events.length + 1 };
    this.events.push(Object.freeze(stored));
    return stored;
  }

  read() {
    return [...this.events]; // ordered copy
  }
}

const log = new EventLog();
log.append({ type: 'AccountOpened', data: { accountId: 'acc-1' } });
log.append({ type: 'MoneyDeposited', data: { accountId: 'acc-1', amount: 50 } });
console.log(log.read());

Reconstructing State by Replaying

If state is derived, how do we get it back? We replay: start from an empty state and apply each event in order. This function is often called apply, evolve, or a reducer.

Notice the shape: (state, event) => newState. It is exactly a reducer — the same idea as Array.prototype.reduce.

  • The reducer is pure: same events in, same state out.
  • It handles each event type and returns a new state object.
function applyEvent(state, event) {
  switch (event.type) {
    case 'AccountOpened':
      return { id: event.data.accountId, owner: event.data.owner, balance: 0 };
    case 'MoneyDeposited':
      return { ...state, balance: state.balance + event.data.amount };
    case 'MoneyWithdrawn':
      return { ...state, balance: state.balance - event.data.amount };
    default:
      return state; // ignore unknown events
  }
}

const events = [
  { type: 'AccountOpened',  data: { accountId: 'acc-1', owner: 'Ada' } },
  { type: 'MoneyDeposited', data: { amount: 50 } },
  { type: 'MoneyWithdrawn', data: { amount: 30 } }
];

const state = events.reduce(applyEvent, null);
console.log(state); // { id: 'acc-1', owner: 'Ada', balance: 20 }

Streams and Aggregates

You rarely replay every event in the system. Events are grouped into streams, one per entity — for example account-acc-1. The entity reconstructed from a stream is called an aggregate.

To load an aggregate:

  • Read only that stream's events (filtered by streamId).
  • Replay them through the reducer.
  • Return the resulting in-memory state.

Keeping streams small keeps replay fast and keeps consistency boundaries clear.

function loadAggregate(allEvents, streamId, reducer) {
  return allEvents
    .filter(e => e.streamId === streamId)
    .sort((a, b) => a.position - b.position)
    .reduce(reducer, null);
}

const allEvents = [
  { streamId: 'account-acc-1', position: 1, type: 'AccountOpened',  data: { accountId: 'acc-1', owner: 'Ada' } },
  { streamId: 'account-acc-2', position: 1, type: 'AccountOpened',  data: { accountId: 'acc-2', owner: 'Lin' } },
  { streamId: 'account-acc-1', position: 2, type: 'MoneyDeposited', data: { amount: 75 } }
];

const reducer = (s, e) => {
  if (e.type === 'AccountOpened') return { id: e.data.accountId, balance: 0 };
  if (e.type === 'MoneyDeposited') return { ...s, balance: s.balance + e.data.amount };
  return s;
};

console.log(loadAggregate(allEvents, 'account-acc-1', reducer));

Commands vs Events

A crucial distinction: a command is a request to do something (it may be rejected); an event is the record that it happened (it cannot be undone).

  • Withdraw is a command — imperative, present tense, may fail.
  • MoneyWithdrawn is an event — past tense, a settled fact.

The aggregate's job is to take the current state plus a command, run business rules, and decide which event(s) to append — or reject the command entirely.

function decide(state, command) {
  switch (command.type) {
    case 'Withdraw':
      if (command.amount > state.balance) {
        throw new Error('Insufficient funds'); // command rejected
      }
      return [{ type: 'MoneyWithdrawn', data: { amount: command.amount } }];
    case 'Deposit':
      return [{ type: 'MoneyDeposited', data: { amount: command.amount } }];
    default:
      throw new Error('Unknown command: ' + command.type);
  }
}

const state = { balance: 40 };
console.log(decide(state, { type: 'Withdraw', amount: 30 }));
try { decide(state, { type: 'Withdraw', amount: 100 }); }
catch (e) { console.log('Rejected:', e.message); }

The Decide / Evolve Cycle

Put the pieces together and you get the core write flow of an event-sourced aggregate:

  • Load: replay the stream to rebuild current state.
  • Decide: run the command against that state to produce new events.
  • Append: write the new events to the log.
  • Evolve: the same reducer that loaded state also keeps it current.

The decide function never writes; the evolve reducer never validates. This separation keeps the domain logic clean and testable.

function evolve(state, event) {
  if (event.type === 'MoneyDeposited') return { ...state, balance: state.balance + event.data.amount };
  if (event.type === 'MoneyWithdrawn') return { ...state, balance: state.balance - event.data.amount };
  return state;
}
function decide(state, cmd) {
  if (cmd.type === 'Deposit') return [{ type: 'MoneyDeposited', data: { amount: cmd.amount } }];
  if (cmd.type === 'Withdraw' && cmd.amount <= state.balance)
    return [{ type: 'MoneyWithdrawn', data: { amount: cmd.amount } }];
  throw new Error('Invalid command');
}

let history = [{ type: 'MoneyDeposited', data: { amount: 100 } }];
let state = history.reduce(evolve, { balance: 0 });   // load
const newEvents = decide(state, { type: 'Withdraw', amount: 60 }); // decide
history = [...history, ...newEvents];                 // append
state = newEvents.reduce(evolve, state);              // evolve
console.log(state); // { balance: 40 }

Optimistic Concurrency with Expected Version

Two requests may try to change the same aggregate at once. Because the log is append-only, we guard writes with the expected version: the position the writer believes the stream is at.

When appending, you say "I expect this stream to be at version N." If another writer already advanced it, the append fails and the caller retries by reloading.

  • No locks held across the request.
  • Conflicts are detected, never silently lost.
class VersionedStore {
  constructor() { this.streams = new Map(); }

  append(streamId, expectedVersion, newEvents) {
    const current = this.streams.get(streamId) || [];
    if (current.length !== expectedVersion) {
      throw new Error(
        `Concurrency conflict: expected v${expectedVersion}, got v${current.length}`
      );
    }
    this.streams.set(streamId, [...current, ...newEvents]);
    return current.length + newEvents.length;
  }
}

const store = new VersionedStore();
store.append('acc-1', 0, [{ type: 'AccountOpened' }]);   // ok -> v1
try {
  store.append('acc-1', 0, [{ type: 'MoneyDeposited' }]); // stale version
} catch (e) { console.log(e.message); }
store.append('acc-1', 1, [{ type: 'MoneyDeposited' }]);  // correct -> v2
console.log('Final version:', store.streams.get('acc-1').length);

Snapshots: Replay Without Re-reading Everything

Replaying thousands of events on every load gets slow. A snapshot is a cached copy of the aggregate state at a known version. To load, you start from the snapshot and only replay events after it.

Important: snapshots are an optimization, not a source of truth. You can delete every snapshot and still rebuild perfect state from the events. The log remains authoritative.

  • Store snapshot + the version it represents.
  • On load: hydrate from snapshot, then replay the tail.
function loadWithSnapshot(snapshot, events, evolve) {
  // snapshot = { state, version } or null
  let state = snapshot ? snapshot.state : { balance: 0 };
  const fromVersion = snapshot ? snapshot.version : 0;
  return events
    .filter(e => e.position > fromVersion)
    .reduce(evolve, state);
}

const evolve = (s, e) =>
  e.type === 'MoneyDeposited' ? { balance: s.balance + e.data.amount } : s;

const events = [
  { position: 1, type: 'MoneyDeposited', data: { amount: 100 } },
  { position: 2, type: 'MoneyDeposited', data: { amount: 50 } },
  { position: 3, type: 'MoneyDeposited', data: { amount: 25 } }
];
const snapshot = { state: { balance: 150 }, version: 2 };
console.log(loadWithSnapshot(snapshot, events, evolve)); // { balance: 175 }

Schema Evolution and Upcasting

Events live forever, so their shape will outlive the code that wrote them. You can never edit an old event in place, but the new code must still understand it.

The standard tool is upcasting: a function that transforms an old event version into the current shape at read time, before it reaches the reducer.

  • Add a version field to each event type.
  • Provide new fields with sensible defaults; rename via the upcaster.
  • Never mutate stored events — transform a copy on the way in.
// v1 had `amount` (cents implied); v2 adds explicit `currency`
function upcast(event) {
  if (event.type === 'MoneyDeposited' && (event.version || 1) === 1) {
    return {
      ...event,
      version: 2,
      data: { ...event.data, currency: 'USD' } // default for legacy events
    };
  }
  return event;
}

const legacy = { type: 'MoneyDeposited', data: { amount: 50 } };
console.log(upcast(legacy));
// { type: 'MoneyDeposited', data: { amount: 50, currency: 'USD' }, version: 2 }

Persisting the Log in Node.js

In production the append-only log is backed by durable storage: a dedicated store like EventStoreDB, or a relational table used as a log. A typical Postgres design:

  • One events table with (stream_id, version, type, data jsonb, recorded_at).
  • A unique constraint on (stream_id, version) — this is what enforces optimistic concurrency at the database level.
  • Inserts only; the application code never issues UPDATE or DELETE on it.

The same load/decide/append cycle runs on top, just with SQL behind the store interface.

// Sketch of an append against a Postgres-backed log (pg client `db`).
// The UNIQUE (stream_id, version) constraint rejects concurrent duplicates.
async function appendEvents(db, streamId, expectedVersion, events) {
  const client = await db.connect();
  try {
    await client.query('BEGIN');
    let version = expectedVersion;
    for (const e of events) {
      version += 1;
      await client.query(
        `INSERT INTO events (stream_id, version, type, data)
         VALUES ($1, $2, $3, $4)`,
        [streamId, version, e.type, JSON.stringify(e.data)]
      );
    }
    await client.query('COMMIT');
    return version;
  } catch (err) {
    await client.query('ROLLBACK'); // unique violation => concurrency conflict
    throw err;
  } finally {
    client.release();
  }
}

Quick Check

Consider an event-sourced account aggregate.

Responde en la lección interactiva

Recap

You replaced mutable state with an immutable event stream:

  • Events are past-tense, immutable facts; the append-only log that holds them is the source of truth.
  • State is derived, not stored — you rebuild it by replaying events through a pure reducer: (state, event) => newState.
  • Events are grouped into streams, one per aggregate.
  • The write cycle is load → decide → append → evolve; commands may be rejected, events never are.
  • Expected version gives optimistic concurrency; a UNIQUE(stream_id, version) constraint enforces it in the database.
  • Snapshots speed up replay but are optional; upcasting keeps old events readable as schemas evolve.

Next, you'll build read models (projections) and complete the CQRS picture.

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Sustituya el estado mutable por un flujo de eventos inmutable y reconstruya el estado reproduciendo los eventos. Practicas Node.js Backend Development Bootcamp con código real que ejecutas directamente en el navegador, y un tutor de IA 24/7 responde tus preguntas mientras trabajas en la lección.

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Todas las lecciones de este curso

  1. Los eventos como fuente de verdad y el log de solo anexado
  2. Agregados, comandos y modelado de eventos de dominio
  3. Construcción de modelos de lectura y proyecciones
  4. Instantáneas, versionado y evolución del esquema de eventos
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