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High-Level Design (HLD) Series: From Basics to Millions of Users ​

High-Level Design (HLD) is at the heart of system design interviews and real-world architecture.
This series is structured to take you from fundamentals → advanced distributed systems → practical case studies, so you can use it as a course or jump into topics as needed.

Articles in This Series ​

1. Foundations ​

  1. System Design Mindset
    Clarifying requirements → constraints → bottlenecks → scaling.
    Always start simple (monolith) and scale step by step.
  2. Workload Estimation
    Requests/sec, QPS, throughput.
    Storage needs (GB → TB → PB).
    Network bandwidth & latency awareness.

2. Diagramming ​

  1. C4 Model

2. Databases & Storage ​

  1. Database for HLD

3. Caching ​

  1. Cache-aside, Write-through, Write-back
  2. TTLs & Eviction Policies
  3. CDN Caching
  4. Pitfalls: Invalidation & Hot Keys

4. Networking & Communication ​

  1. Protocols: HTTP/HTTPS, gRPC, WebSockets
  2. APIs: REST vs GraphQL
  3. Load Balancing (L4 vs L7, Algorithms)
  4. CDNs & Edge Computing

5. Scalability Patterns ​

  1. Horizontal vs Vertical Scaling
  2. Microservices vs Monoliths
  3. Event-driven Architectures: Queues, Pub/Sub, Retries, Backpressure

6. Distributed Systems Concepts ​

  1. CAP Theorem & PACELC
  2. Consensus Algorithms: Raft, Paxos
  3. Quorum Reads/Writes
  4. Leader Election, Heartbeats, Failover
  5. Eventual vs Strong Consistency

7. Reliability & Fault Tolerance ​

  1. Replication (Sync vs Async)
  2. Failover Strategies (Active-Passive, Active-Active)
  3. Geo-replication & Multi-region Systems
  4. Graceful Degradation
  5. Circuit Breakers, Retries, Timeouts

8. Security ​

  1. Authentication vs Authorization (OAuth2, JWT, RBAC)
  2. TLS & Encryption (At Rest vs In Transit)
  3. Rate Limiting & Throttling
  4. DDoS Protection

9. Observability ​

  1. Monitoring (Prometheus, Datadog)
  2. Centralized Logging (ELK, Splunk)
  3. Distributed Tracing (Jaeger, OpenTelemetry)
  4. Alerting Systems (PagerDuty, OpsGenie)

10. Common System Design Problems ​

  1. URL Shortener (TinyURL)
  2. News Feed (Facebook/Twitter)
  3. Chat System (WhatsApp, Slack)
  4. Search (Google/Elasticsearch)
  5. Video Streaming (YouTube/Netflix)
  6. E-commerce Checkout (Amazon)
  7. Ride Hailing (Uber)
  8. Payment System

11. Soft Skills for HLD Interviews ​

  1. Interview Strategy & Trade-offs

12. Scaling to Millions ​

  1. Scaling One-Pager

13. Frequently Asked Problems ​

  1. Index page

How to Use This Series ​

  • Beginner? Start with Foundations + Databases.
  • Interview Prep? Focus on Scalability, CAP, Reliability, and Common Problems.
  • Real-world Engineer? Deep dive into Distributed Systems, Observability, and Security.
  • Quick Review? Read the Scaling One-Pager before your interview.

Further Reading ​

  • Designing Data-Intensive Applications — Martin Kleppmann
  • System Design Interview — Alex Xu
  • Site Reliability Engineering (SRE) — Google
  • High Scalability Blog
  • Engineering blogs of Netflix, Uber, Airbnb, and Meta

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