Question 1

What is idempotency and why does it matter?

Accepted Answer

**Target Audience:** Beginners who have built or called a web API and want to understand why retries are dangerous and how real systems make them safe.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). HTTP method semantics are cited from RFC 9110; Stripe behaviour from the Stripe API reference ("Idempotent requests"); the Idempotency-Key header from the IETF httpapi working-group Internet-Draft.

> **How to read this doc:** Every idea is explain

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

How do you make a payment system reliable against retries?

Accepted Answer

**Target Audience:** Engineers who know what an API and a database transaction are, and want to design the money path so that retries and crashes can never double-charge a customer.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Stripe idempotency behaviour is cited from the Stripe API reference ("Idempotent requests"). Capacity numbers are worked arithmetic from stated assumptions, recomputed in Python.

> **How to read this doc:** Every idea i

Question 4

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 5

How would you design a payment system?

Accepted Answer

**Target Audience:** Backend engineers preparing for system design rounds at fintech, marketplace and e-commerce companies, and anyone who wants to understand where money actually lives inside a payment system.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (identical output). Stripe behaviour (amounts in minor units, a 7-day authorization window for most online card payments, shortened to 5 days for Visa online merchant-initiated transactions from April 2024) is from the Str

SYSTEM DESIGNDECODED.

Medium

What is idempotency and why does it matter?

Hard

How would you design a notification system?

How do you make a payment system reliable against retries?

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

How would you design a payment system?