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

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

What is a read/write quorum?

Accepted Answer

**Target Audience:** Beginners to distributed systems who know that databases can keep copies of data on several machines.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Cassandra consistency levels are cited from the DataStax Cassandra documentation; Dynamo techniques from the Dynamo paper (SOSP 2007) as summarised in course material.

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

Question 4

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 5

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

SYSTEM DESIGNDECODED.

Medium

When would you choose SQL vs NoSQL?

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

What is a read/write quorum?

Hard

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

How would you design a distributed key-value store?