Question 1

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 2

How would you design a news feed system?

Accepted Answer

**Target Audience:** Engineers preparing for a senior system design round. You know what a server, a database and a cache are; feed-specific ideas are defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product facts are cited inline and were checked in October 2026.

> **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.

Question 3

How would you design the data structures for a social network (Social Graph)?

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** adds/removes friend — mutual edge write, idempotent, replicated with quorum.
- **User** reads friends / friends-of-friends — adjacency lookup, paginated, filtered by activity.
- **User** reads feed derived from graph — 2-hop traversal (friends' posts) with cache.
- **Service** suggests friends — FoF (friend

Question 4

How would you design Nearby Friends?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a cache and a WebSocket are; the geo indexing side is covered on the sibling page about nearby drivers.
**Difficulty:** Hard
**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

SYSTEM DESIGNDECODED.

Medium

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

Hard

How would you design a news feed system?

How would you design the data structures for a social network (Social Graph)?

How would you design Nearby Friends?