Question 1

How do you handle asynchronism, message queues and backpressure at scale?

Accepted Answer

## Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

### Use cases
We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

- **User** triggers async work — POST returns 202, job enqueued (email, thumbnail, analytics), polled or webhook on done.
- **Service** consumes — workers pull with prefetch, ack on success, DLQ on poison, retry with backoff.
- **Service** handles backpressure — bounded queues, shed/slow producers, scale consumers on

Question 2

What is the difference between throttling and rate limiting?

Accepted Answer

**Target Audience:** Developers preparing for system design or backend interviews. No prior knowledge of rate limiting assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All behaviour shown comes from running the example; the **Verified** section quotes the full outpu

Question 3

What is backpressure and how do you handle it?

Accepted Answer

**Target Audience:** Beginners who have built a service or a data pipeline and want to understand why a fast producer and a slow consumer cause outages, and how systems push back.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the strategy simulation prints identical output in both); the Node.js stream example ran on Node 22.22 and reads its own default `highWaterMark`. The earlier 16 KiB default is cited from the Node.js v22.0.0 release announcement, as reported in commun

Question 4

How do you scale WebSockets to 1M connections with Redis Pub/Sub?

Accepted Answer

**Target Audience:** Engineers who have run a single WebSocket server and now need many. Read [When would you use WebSockets vs polling vs SSE?](/interview-question/when-would-you-use-websockets-vs-polling-vs-sse) and [WebSocket presence, sticky sessions, and resume — do you need sticky LB?](/interview-question/websocket-presence-sticky-sessions-and-resume-do-you-need-sticky-lb) first if those are new.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four

SYSTEM DESIGNDECODED.

Medium

How do you handle asynchronism, message queues and backpressure at scale?

What is the difference between throttling and rate limiting?

What is backpressure and how do you handle it?

Hard

How do you scale WebSockets to 1M connections with Redis Pub/Sub?