Question 1

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 2

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 3

How would you design a file storage service like Dropbox?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a hash, a database and object storage are; nothing about sync engines is assumed.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 for the chunking example; Node 22.22 and Python 3.12 for the journal example (identical output in every language). Dropbox facts are from Dropbox engineering posts, cited inline.

> **How to read this doc:** Every idea is explained in plain langua

Question 4

How would you design a collaborative editor like Google Docs?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a WebSocket and a database log are; OT and CRDTs are explained from scratch.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four 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

Question 5

How would you design Amazon's category sales ranking?

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.

- **Service** ingests orders — order events (item, category, qty) into durable log (Kinesis/Kafka).
- **Service** aggregates rank — sliding window (hourly/daily) count per category, sorted set, rank = position in sorted order.
- **User** reads rank — GET `/category/{id}/rank?item_id=...`, <50ms p95, stale <1m acceptab

Question 6

How would you design a real-time gaming leaderboard?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a database index and a cache are; sorted sets are explained from scratch.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Redis behaviour and memory were checked on Redis 7.0.15.

> **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 interviewe

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