Question 1

How do you approach back-of-the-envelope capacity estimation?

Accepted Answer

**Target Audience:** Beginners to system design — you can multiply and divide; nothing else assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the calculator prints the same output in both).

> **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. Every number in the worked example was computed by the example program; the **Verified** section quotes its

Question 2

How would you design a ticket booking system like Ticketmaster?

Accepted Answer

**Target Audience:** Backend engineers preparing for the "design Ticketmaster" round, which is really a question about contention, holds and admission control.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (identical output). The Eras Tour figures are from Ticketmaster's statement of November 2022 as reported by CBS News and Cox Media Group stations.

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

Question 3

What are event sourcing and CQRS?

Accepted Answer

**Target Audience:** Back-end developers who have built CRUD services and keep hearing these two terms together. No prior knowledge of either assumed.
**Difficulty:** Hard
**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 **Verified** section

Question 4

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

How do you approach back-of-the-envelope capacity estimation?

Hard

How would you design a ticket booking system like Ticketmaster?

What are event sourcing and CQRS?

How would you design a payment system?