Question 1

What is the publish-subscribe pattern?

Accepted Answer

**Target Audience:** Beginners to system design who have written services that call each other; no messaging background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Redis and Kafka behaviour is cited from their documentation.

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

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

Kafka vs SQS — how do you choose for an event-driven system in 2025?

Accepted Answer

**Target Audience:** Beginners to messaging — you know what a server and a database are; queues, logs, offsets and consumer groups are defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product limits are from the AWS SQS Developer Guide and the Apache Kafka release announcements, checked in October 2026.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you als

Question 4

How would you design a notification system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; queues, retries and push notifications are explained from the start.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Provider facts (FCM HTTP v1 migration, FCM payload size, SMS segment lengths, Gmail and Yahoo bulk sender rules) checked in October 2026 and cited inline.

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

Question 5

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 6

How would you design Google Drive?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. This page builds on the [Dropbox design](/interview-question/how-would-you-design-a-file-storage-service-like-dropbox) (block sync) and the [S3-like object storage design](/interview-question/how-would-you-design-s3-like-object-storage-572x) (blob storage) and focuses on what is different: an ID-based shared tree, permissions, the change feed and uploads.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (bo

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