Publisher & Subscriber Pattern

 The publisher-subscriber pattern with a broker, also known

as the pub-sub pattern, is a messaging pattern used in

distributed systems, where publishers publish messages to a topic,

and subscribers receive messages from the topic through a broker.

class Broker {

constructor() {

this.topics = new Map();

this.subscribers = new Map();

}

subscribe(topic, subscriber) {

if (!this.subscribers.has(subscriber)) {

this.subscribers.set(subscriber, new Set());

}

this.subscribers.get(subscriber).add(topic);

if (!this.topics.has(topic)) {

this.topics.set(topic, new Set());

}

this.topics.get(topic).add(subscriber);

}

debugMap() {

console.log('---------------Topics-------------')

for (var [key, value] of this.topics) {

var arr = new Array(...value)

console.log(key + ' = ' + JSON.stringify(arr));

}

console.log('---------------Subcriber-------------')

for (var [key, value] of this.subscribers) {

var arr = new Array(...value)

console.log(JSON.stringify(key) + ' = ' + JSON.stringify(arr));

}

}

unsubscribe(topic, subscriber) {

if (this.subscribers.has(subscriber)) {

this.subscribers.get(subscriber).delete(topic)

}

if (this.topics.has(topic)) {

this.topics.get(topic).delete(subscriber)

}

}

publish(topic, data) {

//this.debugMap()

if (this.topics.has(topic)) {

this.topics.get(topic).forEach(subscriber => {

subscriber.receive(topic, data)

});

} else {

console.log("Broker -->Topic is not available", topic, data)

}

}

}

class Subscriber {

constructor(name,broker) {

this.name = name;

this.broker = broker;

}

receive(topic, data) {

console.log(`${this.name} received event on topic "${topic}": ${data}`);

}

subscribe(topic) {

this.broker.subscribe(topic, this);

}

unsubscribe(topic) {

this.broker.unsubscribe(topic, this);

}

}

class Publisher {

constructor(broker) {

this.broker = broker;

}

publish(topic, data) {

this.broker.publish(topic, data);

}

}

// Example usage:

const broker = new Broker();

const subscriber1 = new Subscriber("Alice",broker);

const subscriber2 = new Subscriber("Bob",broker);

const subscriber3 = new Subscriber("Charlie",broker);

subscriber1.subscribe("topic1");

subscriber2.subscribe("topic1");

subscriber3.subscribe("topic2");

//broker.debugMap()

const publisher = new Publisher(broker);

publisher.publish("topic1", "Event 1");

// Alice received event on topic "topic1": Event 1, Bob

received event on topic "topic1": Event 1

console.log("Alice Unsubscribed to topic1")

subscriber1.unsubscribe("topic1", broker);

publisher.publish("topic1", "Event 2");

// Bob received event on topic "topic1": Event 2

publisher.publish("topic2", "Event 3");

// Charlie received event on topic "topic2": Event 3