Question 1

How would you design Twitter's timeline at scale?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round. You know what a cache and a database are; the timeline-specific ideas are defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Twitter figures are from published talks and write-ups cited inline, checked in October 2026.

> **How to read this doc:** Plain language first, then `📌 Interview term:` callouts with the precise vocabulary. The cost table is computed by the

Question 2

How would you design Instagram?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round. You know what a server, a database and HTTP are; storage, CDN and sharding ideas are defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Instagram engineering facts are cited inline, checked in October 2026.

> **How to read this doc:** Plain language first, then `📌 Interview term:` callouts. Numbers come from the example program and are quote

Question 3

How would you design Twitter's timeline and search? (Facebook feed)

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** posts a tweet — 280 chars + media refs, persisted, then fanned out to followers' timelines.
- **User** reads home timeline — GET merged, ranked feed (paginated, <200ms p95).
- **Service** searches tweets — inverted index over tokens/hashtags with ranking and recency.
- **Service** fans out / backfills — asy

SYSTEM DESIGNDECODED.

Hard

How would you design Twitter's timeline at scale?

How would you design Instagram?

How would you design Twitter's timeline and search? (Facebook feed)