Event Sourcing as Middleware

In this lesson, you will learn how to shift your middleware logic from storing final state to recording an immutable sequence of events for every write request.

The Shift from State to History

In standard middleware, a POST or PUT request usually results in an "Update" operation—you overwrite the current state of a database record. With Event Sourcing as Middleware, your middleware intercepts the request and transforms the intent into an Event object. Instead of saving the result, you append this event to an "Event Store."

Think of it like a bank statement. You don't just see your current balance; you see the chronological list of deposits and withdrawals that led to that balance. By using middleware to capture these intent-based events, you create a perfect audit trail that can reconstruct any entity's state at any point in time.

Implementing Event Capture

You can inject this behavior into your existing pipeline after your Command Query Responsibility Segregation (CQRS) layer handles the initial request validation. Here is how you might structure a basic middleware function:

const eventSourcingMiddleware = (req, res, next) => {
  const event = {
    type: 'USER_UPDATED',
    payload: req.body,
    timestamp: new Date(),
    metadata: { userId: req.headers['x-user-id'] }
  };

  // Append event to the store instead of executing a direct 'save'
  eventStore.append(event)
    .then(() => next())
    .catch(next);
};

In this setup, your application no longer cares about the "final state" during the request phase. It only cares that the command was successfully recorded as an event. Because this acts as middleware, it remains decoupled from your business logic, ensuring that every write request is treated as an immutable fact rather than a destructive update.

This approach pairs perfectly with your Audit Logging Middleware; however, while audit logs are reactive (recording what happened), Event Sourcing is proactive (the event is the data source). By storing these events, you can "replay" them later to generate your Aggregated Data Responses or verify current state, providing a robust, high-integrity architecture.

Key Takeaway: Event Sourcing as middleware replaces destructive state updates with an append-only log of intent, ensuring your system history remains as accessible as your current state.