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mediumSystem Design

What is a message queue and why use one?

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01

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Explain message queues.

message-queue
02

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03

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What Is a Message Queue, and Why Use One?

Target Audience: Junior & Senior Software Engineers preparing for System Design Interviews — no prior system design knowledge assumed. Difficulty: Easy to Medium

How to read this doc: Every concept is explained in plain language first. Right after, you'll see

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04

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Message queue with visibility timeout (at-least-once redelivery)
Run Playground
class MessageQueue {
  constructor() {
    this.messages = [];
  }

  enqueue(id, body, now) {
    this.messages.push({ id, body, visibleAt: now });
  }

  dequeue(now, visibilityTimeout) {
    for (const msg of this.messages) {
      if (msg.visibleAt <= now) {
        msg.visibleAt = now + visibilityTimeout;
        return [msg.id, msg.body];
      }
    }
    return null;
  }

  ack(id) {
    this.messages = this.messages.filter((m) => m.id !== id);
  }
}

function printResult(result) {
  if (result === null) {
    console.log('None');
  } else {
    console.log(`('${result[0]}', '${result[1]}')`);
  }
}

const queue = new MessageQueue();
queue.enqueue('msg1', 'process order #1', 0);
queue.enqueue('msg2', 'process order #2', 0);

process.stdout.write('dequeue at t=1: ');
printResult(queue.dequeue(1, 10));

process.stdout.write('dequeue at t=2: ');
printResult(queue.dequeue(2, 10));
queue.ack('msg2');

process.stdout.write('dequeue at t=8 (msg1 still hidden): ');
printResult(queue.dequeue(8, 10));

process.stdout.write('dequeue at t=15 (msg1 visibility expired, redelivered): ');
printResult(queue.dequeue(15, 10));
queue.ack('msg1');

process.stdout.write('dequeue at t=20 (queue empty): ');
printResult(queue.dequeue(20, 10));
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