Question 1

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 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 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 4

Explain database indexing and B-trees

Accepted Answer

**Target Audience:** Beginners to databases — you know what a table and a row are; no data-structures background needed beyond "sorted lists are easy to search".
**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. Behaviour shown comes from running the exa

Question 5

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 6

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 7

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 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

SYSTEM DESIGNDECODED.

Medium