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 distributed systems detect node failures?

Accepted Answer

**Target Audience:** Beginners who know a cluster is several servers working together and want to understand how the cluster notices when one of them stops, and why it sometimes gets it wrong.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). The FLP result is from Fischer, Lynch and Paterson (JACM, 1985); Kubernetes lease behaviour from the Kubernetes node-status documentation; memberlist and Lifeguard from HashiCorp's "Making Gossip More Robust

Question 4

What is database connection pooling?

Accepted Answer

**Target Audience:** Backend beginners — you have written code that queries a database; no knowledge of 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 outpu

Question 5

What is the CAP theorem?

Accepted Answer

**Target Audience:** Beginners to distributed systems — you know a database stores data and that servers talk over a network; everything else is defined as we go.
**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. The numbers in the worked run come from the example program; th

Question 6

When do you use a read replica vs a cache?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a database and a query are; replication and caching are explained here.
**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 claims (stale reads, counts) come from running the example; the **Verified** sec

Question 7

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 8

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 9

What is two-phase commit and its drawbacks?

Accepted Answer

**Target Audience:** Developers who know what a database transaction is and now meet data spread over several databases or services. No distributed-systems background 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. All timings and counts come from running the examp

Question 10

Why prefer stateless services and how do you handle sessions?

Accepted Answer

**Target Audience:** Beginners to system design — you have built a web app with a login, nothing more.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the example prints the same output, byte for byte, including the token).

> **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 output shown was produced by actually running the example; the **Verified**

SYSTEM DESIGNDECODED.

Medium

When would you choose SQL vs NoSQL?

What is the difference between horizontal and vertical scaling?

How do distributed systems detect node failures?

What is database connection pooling?

What is the CAP theorem?

When do you use a read replica vs a cache?

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

What is database sharding?

Hard

How would you design a distributed job scheduler?

What is leader election in distributed systems?

How would you design a distributed cache?

Outbox pattern vs polling for CDC — how do you avoid dual-write failure?

How would you design Instagram?

Exactly-once processing — can Kafka or SQS guarantee it?

How would you design a unique ID generator at scale?

How would you design a real-time gaming leaderboard?