Question 1

When would you use WebSockets vs polling vs SSE?

Accepted Answer

**Target Audience:** Beginners to system design who have built a web page that calls an API; no networking background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Protocol facts are cited from RFC 6455, RFC 8441, MDN and the AWS load balancer docs.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the words an interviewer expects. Numbers come from r

Question 2

How would you design a notification or chat system (fan-out, presence)?

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** sends message/notification — POST to API, persisted, fanned out to online recipients via WebSocket, offline via push (APNs/FCM).
- **User** receives — subscribed via WebSocket/SSE, presence shows online/offline/typing.
- **Service** fans out — group chat fan-out O(members), notification fan-out via queue +

Question 3

WebSocket presence, sticky sessions, and resume — do you need sticky LB?

Accepted Answer

**Target Audience:** Engineers who know what a WebSocket is (see [When would you use WebSockets vs polling vs SSE?](/interview-question/when-would-you-use-websockets-vs-polling-vs-sse)) and want to run them on more than one server.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Socket.IO and AWS facts are cited from their documentation.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview

Question 4

How would you design a real-time chat system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round. You know HTTP and what a database is; persistent connections, sequencing and sync are explained from scratch.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product facts are cited inline, checked in October 2026.

> **How to read this doc:** Plain language first, then `📌 Interview term:` callouts with the vocabulary an interviewer expects. The capacity numbers and the delivery

Question 5

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

Question 6

How would you design AI inference infra with GPU queue and KV-cache?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round who may never have served a model. GPUs, tokens, prefill, decode and the KV-cache are explained from zero.
**Difficulty:** Hard
**Verified with:** Node 22 and Python 3.12 (identical output). Paper and product facts (Orca, vLLM, model shapes, GPU memory) are cited inline, checked in October 2026; throughput per node is an **assumption**, stated as such.

> **How to read this doc:** Plain language first, then `📌 Interview term:` c

SYSTEM DESIGNDECODED.

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