Master Concept Checklist

Every concept, topic, and subtopic worth knowing for a strong backend engineering / system design interview in 2026 — organized into 16 stages, from internet fundamentals through AI-era system design. Use it as a checklist: tick off what you've mastered, and use the status column to see what's already covered on this site versus what you'll need to study elsewhere.

Legend: ✅ Covered in depth on this site  ·  🟡 Partially covered  ·  ⬜ Not yet covered on this site

Stage 0: Prerequisites ⬜ Not yet covered

How the Internet Works

• Client-Server Architecture
• Request-Response Model
• HTTP/HTTPS Protocol
• HTTP Methods (GET, POST, PUT, PATCH, DELETE)
• HTTP Status Codes (2xx, 3xx, 4xx, 5xx)
• HTTP/1.1 vs HTTP/2 vs HTTP/3
• Cookies vs Sessions vs JWT
• TLS/SSL Handshake
• WebSocket Protocol
• Server-Sent Events (SSE)

Networking Fundamentals

• OSI Model (7 Layers)
• TCP vs UDP
• TCP Three-Way Handshake
• IP Addressing (IPv4, IPv6)
• DNS (Domain Name System)
• DNS Resolution Process
• DNS Caching and TTL
• DNS Load Balancing
• Ports and Sockets
• NAT (Network Address Translation)
• CDN Basics

Back-of-Envelope Math

• Latency Numbers Every Engineer Should Know
• Throughput Calculations
• Storage Estimation
• Bandwidth Estimation
• QPS (Queries Per Second) Estimation
• DAU/MAU to RPS Conversion
• Power of Two Table for Capacity Planning

Stage 1: Core Building Blocks ✅ Covered in depth

Fully covered in Core Building Blocks.

• 1.1 APIs ✅ — REST, GraphQL, gRPC, API Gateway, API Design Patterns → APIs
• 1.2 Databases ✅ — Relational, NoSQL, Database Internals (B-Tree, LSM, WAL) → Databases
• 1.3 Caching ✅ — Strategies, Levels, Invalidation, Eviction, Distributed Caching, CDN → Caching
• 1.4 Load Balancing ✅ — Types, Algorithms, High Availability → Load Balancing
• 1.5 Proxies ✅ — Forward, Reverse, Transparent, Anonymous, SOCKS → Proxies

Stage 2: Scalability & Performance ✅ Covered in depth

Fully covered in Scalability & Performance.

• 2.1 Horizontal vs Vertical Scaling ✅ → Horizontal vs Vertical
• 2.2 Database Scaling ✅ → Database Scaling
• 2.3 Sharding ✅ → Sharding
• 2.4 Database Replication ✅ → Replication
• 2.5 Partitioning ✅ → Partitioning
• 2.6 Consistent Hashing ✅ → Consistent Hashing
• 2.7 Denormalization ✅ → Denormalization
• 2.8 Rate Limiting ✅ → Rate Limiting

Stage 3: Distributed Systems ✅ Covered in depth

Fully covered in Distributed Systems.

• 3.1 CAP Theorem ✅ → CAP Theorem
• 3.2 PACELC Theorem ✅ → PACELC Theorem
• 3.3 Consistency Models ✅ → Consistency Models
• 3.4 Consensus Algorithms (Paxos, Raft, ZAB) ✅ → Consensus Algorithms
• 3.5 Distributed Transactions (2PC, 3PC, Saga, Outbox, Event Sourcing, CQRS) ✅ → Distributed Transactions
• 3.6 Distributed Systems Patterns (Circuit Breaker, Bulkhead, Retry, Sidecar) ✅ → Distributed Patterns
• 3.7 Distributed Data Structures (CRDTs, Vector Clocks, Gossip) ✅ → Distributed Data Structures
• 3.8 Service Discovery ✅ → Service Discovery
• 3.9 Configuration Management ✅ → Configuration Management

Stage 4: Messaging & Async 🟡 Partially covered

Covered in Advanced Topics and Architecture Patterns.

• 4.1 Message Queues (Point-to-Point, Pub-Sub, DLQ, Visibility Timeout) ✅ → Message Queues
• 4.2 Delivery Guarantees (At-Most/Least/Exactly-Once, Idempotent Consumers) ✅ → covered within Message Queues
• 4.3 Message Queue Technologies (Kafka, RabbitMQ, SQS/SNS, Pulsar, NATS, Redis Streams) ✅ → covered within Message Queues
• 4.4 Event-Driven Architecture (Events vs Commands, Event Bus, Schema Registry) ✅ → Event-Driven and Event Sourcing & CQRS
• 4.5 Stream Processing (Kafka Streams, Flink, Spark Streaming, Windowing, Watermarks) ⬜ — not covered anywhere on the site
• 4.6 CQRS (Command/Query separation, Projections, Read/Write models) ✅ → Event Sourcing & CQRS

Stage 5: Architecture Patterns ✅ Covered in depth

Fully covered in Architecture Patterns.

• 5.1 Monolithic Architecture ✅ → Monolith
• 5.2 Microservices Architecture ✅ → Microservices
• 5.3 Serverless Architecture ✅ → Serverless
• 5.4 Event-Driven Microservices ✅ → Event-Driven
• 5.5 Domain-Driven Design (DDD) ✅ → DDD
• 5.6 Twelve-Factor App ✅ → Twelve-Factor
• 5.7 Design Patterns (Creational, Structural, Behavioral, Saga, BFF) ✅ → Design Patterns

Stage 6: Reliability & Observability 🟡 Partially covered

Partially covered in Observability.

• 6.1 Reliability Engineering (SLA/SLI/SLO, Error Budgets, MTBF/MTTR) ✅ → Observability Fundamentals
• 6.2 Fault Tolerance (Chaos Engineering, Multi-Region, DR, RPO/RTO) ⬜ — not covered
• 6.3 Monitoring & Observability (Logs/Metrics/Traces, Golden Signals, RED/USE) ✅ → Observability Fundamentals
• 6.4 Performance Engineering (Load/Stress/Soak Testing, Capacity Planning) ⬜ — not covered
• 6.5 Incident Management (Postmortems, Root Cause Analysis, On-Call) ⬜ — not covered

Stage 7: Security 🟡 Partially covered

Covered in Security.

• 7.1 Authentication (Session, JWT, OAuth 2.0, OIDC, MFA) ✅ → Security Fundamentals
• 7.1 (cont.) SAML, Passwordless Authentication ⬜ — mentioned in passing, not covered in depth
• 7.2 Authorization (RBAC, ABAC, ACL, Least Privilege) 🟡 — RBAC/ABAC named, not deeply covered
• 7.3 Application Security (OWASP Top 10, SQLi, XSS, CSRF, SSRF, CORS) ✅ → Security Fundamentals
• 7.4 Data Security (Encryption at Rest/Transit, Key Management, Masking, GDPR) 🟡 — encryption covered, masking/tokenization/GDPR not
• 7.5 Infrastructure Security (Firewalls, DDoS, WAF, Zero Trust, Secrets Mgmt) 🟡 — DDoS and secrets management covered, Zero Trust and WAF internals are not

Stage 8: DevOps & Infrastructure ⬜ Not yet covered

8.1 Containers

• Docker Fundamentals
• Dockerfile
• Docker Compose
• Container Orchestration
• Container Registry

8.2 Kubernetes

• Pods, Deployments, Services
• ConfigMaps and Secrets
• Ingress Controllers
• Horizontal Pod Autoscaler (HPA)
• StatefulSets
• Persistent Volumes
• Service Mesh (Istio, Linkerd)
• Helm Charts

8.3 CI/CD

• Continuous Integration
• Continuous Delivery vs Continuous Deployment
• Pipeline Design
• GitOps
• Blue-Green Deployments
• Canary Releases
• Rolling Deployments
• Feature Flags

8.4 Infrastructure as Code

• Terraform
• CloudFormation
• Pulumi
• Ansible

8.5 Cloud Services

• Compute (EC2, Lambda, GCE, Azure VMs)
• Storage (S3, EBS, GCS, Blob Storage)
• Databases (RDS, DynamoDB, Cloud SQL, Cosmos DB)
• Managed Kafka (MSK, Confluent Cloud)
• Managed Queues (SQS, Pub/Sub)
• CDN (CloudFront, Cloud CDN, Azure CDN)
• DNS (Route 53, Cloud DNS)

Stage 9: Search & Data Systems 🟡 Partially covered

• 9.1 Full-Text Search (Inverted Index, Elasticsearch, Lucene, Autocomplete) ⬜ — not covered
• 9.2 Data Warehousing (OLTP vs OLAP, Star Schema, ETL/ELT, Data Lakes) ⬜ — not covered
• 9.3 Big Data Processing (MapReduce, Hadoop, Spark, Airflow) ⬜ — not covered
• 9.4 Graph Databases (Property Graph Model, Cypher/Gremlin, Traversal) 🟡 — mentioned as a database type in Databases, not covered as its own topic with query languages/traversal

Stage 10: Real-Time & Communication ✅ Covered in depth

Fully covered in Real-Time & Communication.

• 10.1 Real-Time Communication (WebSockets, SSE, Long/Short Polling, Socket.IO) ✅ → WebSockets, SSE, Socket.IO
• 10.2 Video & Streaming (Adaptive Bitrate, Transcoding, Live Streaming, DRM) ✅ → Video Streaming
• 10.3 Presence Systems (Online/Offline, Last Seen, Heartbeats) ✅ → Presence

Stage 11: Location & Geospatial ✅ Covered in depth

Fully covered in Location & Geospatial.

• 11.1 Geospatial Data (QuadTrees, R-Trees, Geohashing, H3, Geo-Fencing) ✅ → Spatial Indexes
• 11.2 Location-Based Services (Nearby Search, Routing, Tracking, Surge, ETA) ✅ → Nearby Search, Route Optimization, Real-Time Tracking

Stage 12: Classic System Design Problems ⬜ Not covered as worked examples

The site teaches every mechanism these problems draw on, but doesn't currently walk through any of the classic problems end-to-end as assembled solutions. See the Resources page for ranked question banks and full walkthroughs elsewhere.

Beginner: URL Shortener, Rate Limiter, Distributed Cache, Pastebin, Unique ID Generator, Notification System, Web Crawler, CDN, Key-Value Store

Intermediate: WhatsApp/Slack, News Feed, Search Autocomplete, YouTube/Netflix, Google Drive/Dropbox, Instagram, Uber/Lyft, Proximity Service, TikTok, Hotel Reservation, Reddit, Google Maps

Advanced: Google Docs, Payment System (Stripe), Stock Exchange, Distributed Message Queue, Zoom, S3-like Object Storage, DoorDash/Uber Eats, Code Deployment System, Distributed Locking Service, Google Search, Google Ads, Distributed Training System

Stage 13: AI-Era System Design (New in 2026) ⬜ Not covered

13.1 LLM Infrastructure

Transformer Architecture, Attention Mechanism, Tokenization, Embeddings, KV Cache, GPU Memory Management, Inference Optimization (Speculative Decoding, Continuous Batching), Model Serving (vLLM, TensorRT), Prompt Engineering, Chain-of-Thought, Tree-of-Thought

13.2 RAG (Retrieval-Augmented Generation)

Document Chunking Strategies, Embedding Models, Vector Similarity Search, Hybrid Search, Re-Ranking, Naive vs Agentic RAG, RAG Evaluation (RAGAS), Semantic Caching

13.3 Vector Databases

HNSW, IVF, pgvector, Pinecone, Weaviate, Qdrant, Milvus, Multi-Tenancy in Vector DBs

13.4 AI Agents

Tool Use / Function Calling, ReAct Pattern, Multi-Agent Orchestration, State Machine Agents, Agent Memory, Model Context Protocol (MCP), Agent-to-Agent Protocol (A2A), Guardrails and Safety

13.5 MLOps & AI Observability

Model Versioning, Prompt Versioning, LLM Tracing (LangSmith, Phoenix), Drift Detection, AI Cost Optimization, Guardrails AI

13.6 AI System Design Patterns

Orchestrator-Worker, Hierarchical Agent, Event-Driven Agent Trigger, Fan-Out/Fan-In, Circuit Breaker for LLM Calls, Fallback Chains, Semantic Caching Layer

Stage 14: Low-Level Design (LLD) 🟡 Partially covered

• 14.1 Object-Oriented Programming (SOLID, DRY/KISS/YAGNI, Composition vs Inheritance) ⬜ — not covered
• 14.2 Design Patterns (Creational, Structural, Behavioral — all 23 GoF patterns) ✅ → Design Patterns
• 14.3 UML Diagrams (Class, Sequence, Activity, State, Use Case) ⬜ — not covered
• 14.4 LLD Problems (Parking Lot, Elevator, Vending Machine, Chess, Hash Map, File System) ⬜ — not covered

Stage 15: Strategic Topics for 2026 🟡 Partially covered

For Backend Positioning

System Thinking, Trade-off Analysis, Failure Mode Analysis, Operational Maturity, Cost Awareness, Security Mindset, Product Impact, Communication, Incident Response, Code/Architecture Review

Interview Meta-Skills

Requirements Gathering, Scale Estimation, High-Level Design → Deep Dive Flow, Trade-off Discussion Framework, "It Depends" with Reasoning, Naming Your Assumptions, Time Management, Whiteboard/Diagramming Skills

Topic Count Summary

Use this as a living checklist — tick off what you've mastered, and treat the ⬜ stages as your external-reading list rather than assuming this site is a complete curriculum on its own.