Final HLD Interview Case Study

Introduction

This capstone lesson walks a complete HLD interview for a fictional TechLearningPRO Learning Platform — courses, video lessons, payments, notifications, search, and analytics. You will apply requirements gathering, capacity estimation, architecture diagram, deep dives, and trade-off summary in one structured narrative.

Use this as a rehearsal script: practice aloud in 45 minutes with a timer. Interviewers score communication and decision quality more than memorized Netflix diagrams.

Every prior lesson — microservices, Kafka, Redis, CDN, sagas, observability — appears in a cohesive design.

Understanding the topic

Key concepts

• Scope MVP: browse catalog, watch video, purchase course, progress tracking, email receipt.
• Scale target: 5M DAU, 50k concurrent video streams peak, 10k checkout QPS peak sale.
• NFR: p99 API <200ms, video start <2s, 99.95% availability, PCI for payments.
• Bounded contexts: Catalog, Identity, Commerce, Playback, Progress, Notification, Search.
• Async everywhere non-critical: analytics, email, search index, recommendations.
• Evolution path: modular monolith → extract hot services under load.

flowchart TB
  Req --> Capacity
  Capacity --> API
  API --> Data
  Data --> Scale
  Scale --> Ops

Internal architecture

Architecture overview

flowchart TB
  Req --> Capacity
  Capacity --> API
  API --> Data
  Data --> Scale
  Scale --> Ops

Step-by-step explanation

1. Clients → CloudFront CDN (static + HLS) + API Gateway (JWT, rate limit).
2. Services: Catalog (PostgreSQL+Redis), Commerce saga (Order+Payment), Playback (signed URLs), Progress (Cassandra), Search (Elasticsearch via Kafka CDC).
3. Kafka backbone: OrderPlaced, VideoCompleted, ProgressUpdated events.
4. Redis: session, catalog cache, rate limits. S3+CDN video pipeline from prior lesson.
5. Observability: OpenTelemetry, SLO dashboards, PagerDuty on checkout errors.
6. DR: multi-AZ always; cross-region warm standby for tier-1 API RPO 5min.

Informative example

45-minute interview outline with sample commerce saga API surface:

# Minute 0-5: Requirements confirmed
# Minute 5-15: Diagram — CDN, Gateway, 6 services, Kafka, data stores
# Minute 15-30: Deep dives — video delivery, checkout saga, search CDC
# Minute 30-40: Bottlenecks — hot course flash sale, cache stampede, payment idempotency
# Minute 40-45: Summary trade-offs and evolution

apis:
  - POST /api/v1/courses/{id}/purchase   # Idempotency-Key, returns 202 + orderId
  - GET  /api/v1/courses?q=redis           # Elasticsearch
  - GET  /api/v1/lessons/{id}/manifest     # Signed CDN URL
  - POST /api/v1/progress                  # Async Kafka → Cassandra projection
events:
  - order.placed
  - payment.captured
  - lesson.completed
capacity:
  peak_checkout_qps: 10000
  concurrent_streams: 50000
  catalog_read_qps: 200000

Close every interview by stating what you'd build MVP week 1 vs phase 2. Shows pragmatism.

Real-world use

Real-world use cases

• Staff engineer loop combining product sense and architecture.
• Startup founder technical diligence presentation.
• Internal platform RFC for education vertical expansion.
• Interview prep mock with peer whiteboard review.

Best practices

• Drive the conversation — don't wait for prompts every step.
• Write assumptions on board; invite corrections early.
• Numbers before technologies: QPS then Redis not reverse.
• Call out single points of failure and mitigations proactively.
• End with monitoring, security, and DR — completeness signal.
• Reflect on trade-offs: 'We chose saga over 2PC because…'

Common mistakes

• Skipping requirements — design drifts from interviewer intent.
• No deep dive when asked 'pick one component' — stays shallow.
• Ignoring cost and operability — impractical over-engineering.
• Cannot adjust when interviewer changes DAU 10×.
• No closing summary — interviewer left confused about your design.

Advanced interview questions

Summary

Complete HLD interviews follow requirements → diagram → deep dive → bottlenecks → summary. TechLearningPRO platform exercises video, commerce, search, and notify patterns together. Start simple MVP; evolve services with measured bottlenecks. Kafka, Redis, CDN, sagas, and observability recur in every layer. Communication and structured trade-offs win as much as correct boxes. You have completed 42 HLD lessons — practice weekly mock designs to retain fluency.

HLD Interview Question Bank

Practice with 70 additional HLD interview questions covering scalability, databases, distributed systems, security, and classic case studies.