Event sourcing is a design pattern that allows you to capture and store all changes made to an application’s state as a sequence of events. These events can then be used to reconstruct the state of the application at any point in time. In this blog post, we will explore how to implement a proxy-based event sourcing system in JavaScript.

What is event sourcing?

Event sourcing is a technique that treats the state of a system as a series of events rather than a single, mutable state. Instead of updating the state directly, every change to the state is captured as an event and appended to a log. The log of events can be used to recreate the state of the system by replaying the events in order.

Implementing a proxy-based event sourcing system

In JavaScript, we can leverage the power of JavaScript Proxy objects to implement an event sourcing system. Proxies allow us to intercept and customize the behavior of an object, which is perfect for capturing events.

Let’s start by defining an EventSourcedObject class that will act as the base class for our event sourcing system:

class EventSourcedObject {
  constructor() {
    this.events = [];
    this.eventHandlers = {};
    this.proxy = new Proxy(this, {
      set: (target, property, value) => {
        // Capture the event
        const event = { property, value };
        this.events.push(event);

        // Update the target object
        target[property] = value;

        // Dispatch the event to the registered event handlers
        if (this.eventHandlers[property]) {
          this.eventHandlers[property].forEach(handler => handler(event));
        }

        return true;
      }
    });
  }

  subscribe(property, handler) {
    if (!this.eventHandlers[property]) {
      this.eventHandlers[property] = [];
    }

    this.eventHandlers[property].push(handler);
  }

  get proxy() {
    return this.proxy;
  }
}

The EventSourcedObject class has an events array to store all the captured events, an eventHandlers map to keep track of the registered event handlers, and a proxy property that returns the proxy object.

To capture events, we define a setter trap inside the proxy’s set handler. Whenever a property is set on the object, we create an event object with the property name and value, store it in the events array, update the target object, and dispatch the event to the registered handlers.

To subscribe to events, we provide a subscribe method that takes the property name and a handler function. The handler function will be called with the event object whenever the property is updated.

Now, let’s create a subclass of EventSourcedObject to demonstrate how our event sourcing system works:

class User extends EventSourcedObject {
  constructor(name, age) {
    super();
    this.name = name;
    this.age = age;
  }
}

const user = new User("John", 30);

user.subscribe("name", event => {
  console.log(`Name changed to ${event.value}`);
});

user.name = "Jane";

In this example, we create a User class that extends EventSourcedObject. When a new user is created, the constructor sets the initial values for name and age. We then subscribe to the name property and log a message whenever it changes. Finally, we update the name property, triggering the event and the corresponding handler.

Running this code will output "Name changed to Jane".

Conclusion

Implementing a proxy-based event sourcing system in JavaScript can provide a powerful way to capture and replay events to recreate the state of an application. By leveraging the capabilities of JavaScript Proxy objects, we can easily intercept property changes and build a robust event sourcing mechanism.

#eventSourcing #JavaScript