Event-driven & Observer patterns

Let’s go through Event-Driven and Observer patterns in Node.js, which are fundamental for asynchronous, decoupled application design.

1️⃣ Event-Driven Pattern

Definition: Architecture where components react to events instead of calling each other directly.
Node.js is natively event-driven.

Key Points

Event producers emit events
Event consumers listen and react
Enables non-blocking, asynchronous behavior

Node.js Example: EventEmitter

const EventEmitter = require("events");
const emitter = new EventEmitter();
 
// Listener
emitter.on("orderCreated", (order) => {
  console.log("Processing order:", order.id);
});
 
// Emit event
emitter.emit("orderCreated", { id: 123, item: "Laptop" });

Multiple listeners can listen to the same event
.once() → listener triggers only once

2️⃣ Observer Pattern

Definition: Behavioral design pattern where observers subscribe to a subject to get notified on changes.
Event-driven programming is a real-world implementation of Observer pattern

Node.js Observer Analogy

class Subject extends EventEmitter {
  createOrder(order) {
    console.log("Order created:", order.id);
    this.emit("orderCreated", order); // notify observers
  }
}
 
const subject = new Subject();
 
// Observers
subject.on("orderCreated", (order) => console.log("Send email for order", order.id));
subject.on("orderCreated", (order) => console.log("Update analytics for order", order.id));
 
subject.createOrder({ id: 456, item: "Phone" });

Subject = emits events
Observers = listen and react
Decouples producer from consumers

3️⃣ Benefits of Event-Driven / Observer Patterns

Loose Coupling → producers don’t need to know consumers
Asynchronous Execution → avoids blocking
Scalability → add/remove listeners without changing core logic
Reusability → same event can trigger multiple independent actions

4️⃣ Real-World Use Cases in Node.js

HTTP Servers → req.on('data', ...)
Streams → readable.on('data')
WebSockets → socket.on('message')
Task queues → event-driven workers processing jobs
Microservices → publish-subscribe communication

5️⃣ Key Takeaways

Event-Driven = architecture where events trigger reactions
Observer Pattern = design pattern formalizing “publish-subscribe”
Node.js EventEmitter = core implementation
Widely used for asynchronous I/O, microservices, and scalable apps

We can next cover Node.js production best practices, combining performance optimization, monitoring, logging, clustering, and security for deployment-ready apps.

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