Cracking the Senior System Design Interview: RADIO, NFRs, Scale and CAP

Senior system design interviews don't test whether you know what a load balancer is. They test how you think under pressure: how you handle ambiguity, weigh brutal trade-offs and defend every box you draw. In this deep dive we unpack the framework used to evaluate senior engineers, architects and principal candidates. You'll learn: \- The RADIO framework (Requirements, API, Data model, Infrastructure, Optimize) and how to split your 45 minutes \- The one question that instantly signals seniority \- REST vs gRPC, API versioning, and why offset pagination breaks at scale \- Choosing a database by access pattern, and justifying every component with the NFRs \- Why skipping non-functional requirements is the number one reason senior candidates fail \- Peak vs average TPS, P99 tail latency, latency budgets, RPO and RTO, and compliance \- Sticky sessions, sharding, hot shards, read replicas, CQRS and event streaming \- The math of the nines, active-active vs active-passive, circuit breakers, bulkheads and graceful degradation \- Exponential backoff with jitter to survive the thundering herd \- The CAP theorem, the consistency spectrum, and why banks must choose CP to prevent double spending Chapters 00:00 Introduction 00:49 They test judgement, not definitions 02:22 The roadmap 03:36 The RADIO framework 04:47 Requirements in five minutes 06:00 The seniority signal 06:47 API design and versioning 08:20 Offset vs cursor pagination 09:05 Choosing the data model 10:18 Justifying infrastructure with NFRs 11:28 Network boundaries 11:50 Attacking your own design 12:39 Observability and cost 14:37 The URL shortener trap 15:23 The NFR cheat sheet 15:46 Peak vs average traffic 16:33 Tail latency and fan-out 17:44 Latency budgets 19:16 RPO and RTO 20:02 Compliance: GDPR, PCI DSS, SOX 21:37 Scaling out and sticky sessions 23:13 Sharding and the hot shard 24:49 Cross-shard query trade-offs 25:58 Read replicas and replication lag 27:08 CQRS 28:41 Event streaming and idempotency 31:02 The nines of availability 33:04 Active-active vs active-passive 34:37 Circuit breakers 36:09 Bulkheads 37:18 Graceful degradation 38:26 Thundering herd, backoff and jitter 40:25 The CAP theorem 43:09 The consistency spectrum 45:32 The FinTech exception: double spending 47:05 Why a rejected transaction beats a duplicate 48:13 Recap: every design is a trade-off 49:23 Final thought #SystemDesign #SoftwareEngineering #DistributedSystems #InterviewPrep #TechInterviews #SoftwareArchitecture Interview Prep Podcast