Question 1

When would you choose SQL vs NoSQL?

Accepted Answer

**Target Audience:** Beginners to system design — you have used a spreadsheet or a database table; no database internals assumed.
**Difficulty:** Medium
**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. Behaviour shown comes from running the example; the **Verified** section quotes the full output.

Question 2

What is the difference between horizontal and vertical scaling?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a server and a database are, nothing more.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the example prints the same table in both).

> **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. Every number below comes from actually running the example program; the **Verified** section quotes its f

Question 3

How do you choose between SQL and NoSQL and scale each (replication, sharding, federation)?

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** needs strong transactions — SQL (ACID, joins) for orders, payments, friendships.
- **Service** needs massive write scale — NoSQL (Cassandra/Dynamo wide-column/KV) for timelines, metrics, crawl frontier.
- **Service** scales reads — add read replicas (async), measure lag.
- **Service** scales writes — shard

Question 4

What is database sharding?

Accepted Answer

**Target Audience:** Beginners to system design who know what a database table is; no distributed-systems background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Notion figures are cited from the Notion engineering blog; DynamoDB limits from the AWS DynamoDB developer guide.

> **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 e

Question 5

Head vs tail sampling in OpenTelemetry — when does each break?

Accepted Answer

**Target Audience:** Engineers who have set up tracing and now have to choose a sampling strategy, and candidates who want to discuss sampling trade-offs with numbers rather than slogans.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Tail sampling processor settings (`decision_wait` default 30 s described as time since the first span, `num_traces` default 50000) from the Grafana Alloy reference for `otelcol.processor.tail_sampling` (a wrapper

Question 6

What is database replication?

Accepted Answer

**Target Audience:** Beginners to distributed data — you know what a database server is; no prior knowledge of replication assumed.
**Difficulty:** Medium
**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. Behaviour shown comes from running the example; the **Verified** section quotes the full outpu

Question 7

What are the trade-offs of microservices vs a monolith?

Accepted Answer

**Target Audience:** Beginners to architecture — you have built a web application; every term is defined the first time it is used.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Hop latencies, availabilities and memory sizes are stated assumptions. The case studies are cited to their published coverage (InfoQ, July 2018, on Segment; DevClass, May 2023, on Prime Video), checked October 2026.

> **How to read this doc:** Plain language first, t

Question 8

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 9

What is a message queue and why use one?

Accepted Answer

**Target Audience:** Beginners to system design who have built an API that does all its work inside the request.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Amazon SQS and Kafka facts are cited from AWS and Apache announcements and docs.

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

Question 10

How would you design a system that scales to millions of users on AWS?

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** signs up / logs in — auth on stateless API, session in Redis, email verify async.
- **User** reads feed/profile — cache-first, CDN for assets, read replicas for DB.
- **User** writes (post/like) — write to master, async fan-out, eventual read visibility.
- **Service** scales — milestone Users+ → Users+++++,

SYSTEM DESIGNDECODED.

Medium