# Reel ke poore notes

> Detailed notes for every InternHack reel: real-life system design, mock interview answers, diagrams and ready-to-use interview answers. With sources.

Canonical: https://www.internhack.xyz/notes
Last updated: 2026-11-01

Detailed notes for every InternHack reel: real-life system design, mock interview answers, diagrams and ready-to-use interview answers. With sources.

- **[How FASTag works: why your car doesn't stop at the toll (System Design Notes)](https://www.internhack.xyz/notes/fastag)**
  System Design · 2026-11-01 · 11 min read
  The full FASTag system design: what's actually on the sticker, how the barrier opens without the bank, the NPCI and bank flow, failure cases, and how to explain it in an interview.
- **[Never charge twice: Idempotency keys (Mock Interview Notes)](https://www.internhack.xyz/notes/payment)**
  Interview Prep · 2026-11-01 · 8 min read
  How to prevent payment double charges: why a timeout doesn't mean failure, why the common answers fail, the full idempotency key design, 409 vs 422, and the answer to the server-crash question.

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