Sheet 18

FAANG Mock Interview Prep & Timer Mode

10 high-yield system design interview scenarios with expected answers, red flags, bonus points, and a 60-minute mock timer.

Mock Interview Timer60:00

60-Minute Interview Stage Progression & Time Budgets

0:00 - 5:001. Requirements & Clarifications

Scope functional vs non-functional, DAU, read/write ratio, latency SLA.

5:00 - 15:002. Back-of-the-Envelope Math

State 10M MAU → 2M DAU → 400k PCU → 6.6k Peak RPS anchor chain.

15:00 - 30:003. High-Level Architecture

Draw Client → CDN → NLB/ALB → Gateway → Microservices → Kafka → DB/Cache.

30:00 - 50:004. Deep Dives & Bottlenecks

Idempotency keys, saga outbox, partition keys, DB connection limits, cache invalidation.

50:00 - 60:005. Failure Modes & Economics

DDoS defense, load shedding priority ladder, DLQ retry, cost estimation.

10 Master FAANG Scenario Flashcards

Scenario #1

1. Design an e-commerce checkout flow at 10M DAU

Expected Answer (Whiteboard Talk-Track):

Walk client → CDN → gateway → order svc → payment svc (idempotent) → Kafka outbox → inventory → confirmation. Anchor everything on Peak_RPS (6.6k+).

Referenced Sheets

02_User_Metrics, 06_Backend_Services, 07_Network_Latency

Red Flags to Avoid

Jumping straight into microservice diagrams without stating scale numbers and latency budget first.

Bonus Points (Staff+)

Mention Saga pattern with transactional outbox for dual-write avoidance and exactly-once publish.

Scenario #2

2. Add real-time order tracking & delivery updates

Scenario #3

3. Handle a Black Friday 10x traffic surge without crashing

Scenario #4

4. Prevent double-charging during payment network retries

Scenario #5

5. Diagnose a sudden Kafka consumer lag spike of 100k messages

Scenario #6

6. Design a Dead-Letter Queue (DLQ) & replay mechanism for bad deploys

Scenario #7

7. Scale database storage when primary disk exceeds 20 TB

Scenario #8

8. Reduce P95 latency from 500ms down to 150ms

Scenario #9

9. Cut cloud infrastructure costs by 40% without SLA risk

Scenario #10

10. Migrate from Apache ZooKeeper to KRaft with zero downtime