Q: How would you design a distributed key-value store?

**Target Audience:** Engineers preparing for the classic "design a key-value store" round, which tests whether you can combine partitioning, replication, consistency and storage into one coherent system. **Difficulty:** Hard **Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 for the cluster example (identical output); Node 22.22 and Python 3.12 for the storage-engine example. The design follows the Dynamo paper (DeCandia et al., SOSP 2007); the DynamoDB 400 KB item limit is from the

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

**Target Audience:** Readers who know the basic hash ring and virtual nodes; see [What is consistent hashing?](/interview-question/what-is-consistent-hashing) and [What is consistent hashing with virtual nodes and when do you need it?](/interview-question/what-is-consistent-hashing-with-virtual-nodes-and-when-do-you-need-it) if not. **Difficulty:** Hard **Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). History is cited from Thaler and Ravishankar

Question 1

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 2

What is consistent hashing?

Accepted Answer

**Target Audience:** Beginners to system design who know what a hash function and a cache are; no distributed-systems background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). History is cited from Karger et al., STOC 1997; system usage from the Dynamo paper (SOSP 2007) and the Apache Cassandra documentation.

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

Question 3

What is a load balancer and what strategies does it use?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a web server is; no networking background assumed.
**Difficulty:** Medium
**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 numbers come from actually running the example; the **Verified** section quotes the f

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

What is a Bloom filter and when is it useful?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a hash function and a set are; the probability is worked through step by step.
**Difficulty:** Medium
**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. Error rates and sizes come from running the example; the **Ve

Question 6

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 7

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 8

How would you design a distributed key-value store?

Accepted Answer

**Target Audience:** Engineers preparing for the classic "design a key-value store" round, which tests whether you can combine partitioning, replication, consistency and storage into one coherent system.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 for the cluster example (identical output); Node 22.22 and Python 3.12 for the storage-engine example. The design follows the Dynamo paper (DeCandia et al., SOSP 2007); the DynamoDB 400 KB item limit is from the

Question 9

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

Accepted Answer

**Target Audience:** Readers who know the basic hash ring and virtual nodes; see [What is consistent hashing?](/interview-question/what-is-consistent-hashing) and [What is consistent hashing with virtual nodes and when do you need it?](/interview-question/what-is-consistent-hashing-with-virtual-nodes-and-when-do-you-need-it) if not.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). History is cited from Thaler and Ravishankar

SYSTEM DESIGNDECODED.

Medium