System Design — Junior Interview Question Bank¶

A complete, ordered junior-level question bank for system design interviews — 42 sections, ~15,800 lines. Each section file follows the same shape: every question lists what the interviewer is probing, a concrete model answer (with real products and back-of-envelope math), and the likely follow-up. Every file ends with a Rapid-Fire Self-Check and a Next step: link, so you can read straight through as one path.

Level: Junior (0–2 years). Goal: correct, concrete, honest answers — the vocabulary and core-component reasoning, not deep internals. Middle / Senior / Professional banks live alongside this folder.

Start here → 01 · Introduction

Foundations¶

• 01 · Introduction — what system design is, how to approach it, FRs vs NFRs, key characteristics, numbers to know
• 02 · Trade-offs Framework — CAP, PACELC, consistency vs availability
• 03 · Capacity Estimation — QPS, storage, bandwidth, latency budgets
• 04 · Back-of-Envelope — number tables, Fermi estimation

Networking¶

• 05 · Networking & Protocols — OSI/TCP-IP, TCP vs UDP, TLS, HTTP/1-2-3, WebSockets, SSE
• 06 · Domain Name System — resolution flow, record types, TTL, GeoDNS, Anycast
• 07 · Content Delivery Networks — pull/push, invalidation, edge, security
• 08 · Load Balancers — L4/L7, algorithms, health checks, GSLB

Compute & API¶

• 09 · Communication — HTTP, RPC, gRPC, REST, GraphQL, idempotency
• 10 · Application Layer — microservices, discovery, stateless, service mesh
• 11 · API Design at Scale — gateway, federation, versioning, pagination, webhooks, BFF

Data¶

• 12 · Databases — RDBMS, KV, document, wide-column, OLAP, graph, vector, NewSQL, replication, sharding
• 13 · Storage Systems — object/block/file, GFS/HDFS, LSM vs B-tree, lake vs warehouse
• 14 · Caching — cache-aside, write-through/behind, eviction, invalidation, stampede
• 15 · Data Streaming & Big Data — batch/stream, Kafka, Lambda vs Kappa, CDC, ETL/ELT

Async & Coordination¶

• 16 · Asynchronism — message/task queues, back pressure, DLQ, delivery guarantees
• 17 · Background Jobs — event/schedule-driven, returning results, retries
• 18 · Concurrency & Coordination — idempotency keys, leases/fencing, locking, coordination services

Building Blocks & Patterns¶

• 19 · Building Blocks — rate limiter, consistent hashing, unique IDs, distributed lock/cache/queue, typeahead
• 20 · Reliability Patterns — circuit breaker, bulkhead, retry, throttling, leader election
• 21 · Cloud Design Patterns — strangler, sidecar, CQRS, event sourcing, claim check
• 22 · Performance Antipatterns — chatty I/O, busy DB, retry storm, no caching

Reliability & Ops¶

• 23 · Monitoring — health/availability/performance, instrumentation, alerts
• 24 · Observability — logs/metrics/traces, SLO/SLI, RED/USE, distributed tracing
• 25 · Chaos Engineering — failure modes, fault injection, game days, blast radius
• 26 · Deployment & Infrastructure — Docker, Kubernetes, blue-green/canary, CI/CD, IaC, DR
• 27 · Security at Scale — authn/authz, OAuth2/OIDC, JWT, encryption, DDoS, WAF
• 28 · Data Privacy & Compliance — PII, GDPR, residency, audit logging, key lifecycle

Specialized¶

• 29 · Multi-Tenancy & SaaS — isolation models, per-tenant partitioning, noisy neighbor
• 30 · Geospatial Systems — geohashing, quadtrees, S2/H3, proximity search
• 31 · ML & Recommendation Systems — retrieve/rank funnel, feature store, online vs offline

Capstone¶

• 32 · Classic Problems — URL shortener, rate limiter, news feed, chat, typeahead, crawler
• 33 · Real-World Architectures — Kafka, Cassandra, Redis, Spanner, S3, DynamoDB
• 34 · Interview Playbook — RESHADED, requirements, estimation, deep dives, wrap-up

Staff-Track Awareness¶

• 35 · Architecture Decision-Making — ADRs, RFCs, fitness functions, build vs buy
• 36 · Large-Scale Migrations — strangler fig, zero-downtime, expand-contract, dual-write
• 37 · Sociotechnical & Org Design — Conway's Law, Team Topologies, cognitive load
• 38 · Cost & Efficiency (FinOps) — cost modeling, capacity planning, performance economics
• 39 · Global / Multi-Region — active-active, residency, geo-routing, conflict resolution
• 40 · SRE & Reliability Engineering — error budgets, incident mgmt, postmortems, load shedding
• 41 · Performance Engineering & Tail Latency — p99/p999, hedged requests, Little's Law, Amdahl
• 42 · Data Governance & Contracts — schema registry, data contracts, lineage, MDM

Part of the System Design interview collection.