Question 1

What is two-phase commit and its drawbacks?

Accepted Answer

**Target Audience:** Developers who know what a database transaction is and now meet data spread over several databases or services. No distributed-systems background 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. All timings and counts come from running the examp

Question 2

Kafka vs SQS — how do you choose for an event-driven system in 2025?

Accepted Answer

**Target Audience:** Beginners to messaging — you know what a server and a database are; queues, logs, offsets and consumer groups are defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product limits are from the AWS SQS Developer Guide and the Apache Kafka release announcements, checked in October 2026.

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

Question 3

What is the saga pattern for distributed transactions?

Accepted Answer

**Target Audience:** Developers who build or interview for microservice back ends. You know what a database transaction is; no distributed-systems background 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. All behaviour shown comes from running the example; the **V

Question 4

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 5

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 6

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