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

WebSocket presence, sticky sessions, and resume — do you need sticky LB?

Accepted Answer

**Target Audience:** Engineers who know what a WebSocket is (see [When would you use WebSockets vs polling vs SSE?](/interview-question/when-would-you-use-websockets-vs-polling-vs-sse)) and want to run them on more than one server.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Socket.IO and AWS facts are cited from their documentation.

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

Question 3

How would you design a distributed job scheduler?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round. You know what a database, a queue and a server are; leases, fencing and misfire policies are explained from zero.
**Difficulty:** Hard
**Verified with:** Node 22 and Python 3.12 (identical output). Kubernetes CronJob and PostgreSQL facts are cited inline, checked in October 2026.

> **How to read this doc:** Plain language first, then `📌 Interview term:` callouts with the vocabulary an interviewer expects. Every number in the d

Question 4

What is leader election in distributed systems?

Accepted Answer

**Target Audience:** Engineers who know what a server, a network and a replica are; consensus is introduced from scratch, and every term 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. Election times, vote counts and failure beh

Question 5

How does CDC with Debezium keep search and cache in sync with Postgres?

Accepted Answer

**Target Audience:** Back-end and platform engineers who keep derived stores (search indexes, caches, analytics) in sync with a primary database. You know what Postgres, Kafka and a cache are; CDC is explained from zero.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Debezium and Postgres behaviour is from their documentation, cited inline; it was not run here.

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

SYSTEM DESIGNDECODED.

Medium

How do distributed systems detect node failures?

WebSocket presence, sticky sessions, and resume — do you need sticky LB?

Hard

How would you design a distributed job scheduler?

What is leader election in distributed systems?

How does CDC with Debezium keep search and cache in sync with Postgres?