Q: How would you design a distributed cache?

**Target Audience:** Intermediate — you know what a cache is and what a hash function does; distributed-systems vocabulary is defined as it appears. **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. Slot numbers, key movements and node loads come from runn

Q: How would you design a typeahead/autocomplete system?

**Target Audience:** Engineers preparing for a system design round; you know what a server and a database are, and data structures like tries 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). The capacity figures are worked arithmetic from stated assumptions, not measurements of a real service. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Intervi

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 consistent hashing with virtual nodes and when do you need it?

Accepted Answer

**Target Audience:** Readers who know the basic hash ring; if you do not, read [What is consistent hashing?](/interview-question/what-is-consistent-hashing) first, as this page builds on it.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Cassandra facts are cited from The Last Pickle, Instaclustr and the Apache Cassandra 4.0 production docs.

> **How to read this doc:** Every idea is explained in plain language first, the

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

How would you design a URL shortener like TinyURL? (Pastebin / Bit.ly)

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** creates a paste / short URL — POST long URL or text, get back `https://sho.rt/Ab3x9K` (302 redirect for URL, render for paste).
- **User** reads a paste/short URL — GET `/Ab3x9K`, cache-hit serves in <10ms, miss hits DB then populates cache.
- **Service** generates short code — deterministic Base62 of auto-

Question 6

How would you design a distributed cache?

Accepted Answer

**Target Audience:** Intermediate — you know what a cache is and what a hash function does; distributed-systems vocabulary is defined as it appears.
**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. Slot numbers, key movements and node loads come from runn

Question 7

How would you design a 'nearby drivers' / proximity search?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a database index and a cache are; no maps or geometry background needed.
**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 interviewer

Question 8

How would you design the data structures for a social network (Social Graph)?

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** adds/removes friend — mutual edge write, idempotent, replicated with quorum.
- **User** reads friends / friends-of-friends — adjacency lookup, paginated, filtered by activity.
- **User** reads feed derived from graph — 2-hop traversal (friends' posts) with cache.
- **Service** suggests friends — FoF (friend

Question 9

How would you design a distributed key-value store / query cache for search?

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** searches — GET `/search?q=...`, cache lookup by normalized query hash, hit returns in <20ms, miss fetches search tier.
- **Service** caches query results — write-through on miss with TTL + jitter, LRU eviction, per-tenant isolation.
- **Service** invalidates — TTL expiry plus explicit invalidation on index

Question 10

How would you design a typeahead/autocomplete system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; you know what a server and a database are, and data structures like tries 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). The capacity figures are worked arithmetic from stated assumptions, not measurements of a real service.

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

SYSTEM DESIGNDECODED.

Medium

When would you choose SQL vs NoSQL?

What is consistent hashing with virtual nodes and when do you need it?

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

What is database sharding?

Hard

How would you design a URL shortener like TinyURL? (Pastebin / Bit.ly)

How would you design a distributed cache?

How would you design a 'nearby drivers' / proximity search?

How would you design the data structures for a social network (Social Graph)?

How would you design a distributed key-value store / query cache for search?

How would you design a typeahead/autocomplete system?

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

Consistent hashing vs rendezvous (HRW) hashing — which for what scale?