Question 1

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 2

How would you design a notification system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; queues, retries and push notifications are explained from the start.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Provider facts (FCM HTTP v1 migration, FCM payload size, SMS segment lengths, Gmail and Yahoo bulk sender rules) checked in October 2026 and cited inline.

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

Question 3

Exactly-once processing — can Kafka or SQS guarantee it?

Accepted Answer

**Target Audience:** Engineers who have used a queue or Kafka and heard "exactly-once" promised; everything from offsets to transactions is defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Kafka and SQS behaviour is from the Apache Kafka KIPs and documentation and the AWS SQS Developer Guide, checked in October 2026.

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

Question 4

How would you design ad click event aggregation?

Accepted Answer

**Target Audience:** Intermediate engineers preparing for system design rounds — you know what a queue and a database are; every streaming term is defined when it first appears.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Traffic figures are stated assumptions. Apache Flink windowing behaviour (allowed lateness, late-data side outputs) is from the Flink windows documentation, checked October 2026.

> **How to read this d

SYSTEM DESIGNDECODED.

Hard

How would you design a distributed job scheduler?

How would you design a notification system?

Exactly-once processing — can Kafka or SQS guarantee it?

How would you design ad click event aggregation?