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 spendingChapters00:00 Introduction00:49 They test judgement, not definitions02:22 The roadmap03:36 The RADIO framework04:47 Requirements in five minutes06:00 The seniority signal06:47 API design and versioning08:20 Offset vs cursor pagination09:05 Choosing the data model10:18 Justifying infrastructure with NFRs11:28 Network boundaries11:50 Attacking your own design12:39 Observability and cost14:37 The URL shortener trap15:23 The NFR cheat sheet15:46 Peak vs average traffic16:33 Tail latency and fan-out17:44 Latency budgets19:16 RPO and RTO20:02 Compliance: GDPR, PCI DSS, SOX21:37 Scaling out and sticky sessions23:13 Sharding and the hot shard24:49 Cross-shard query trade-offs25:58 Read replicas and replication lag27:08 CQRS28:41 Event streaming and idempotency31:02 The nines of availability33:04 Active-active vs active-passive34:37 Circuit breakers36:09 Bulkheads37:18 Graceful degradation38:26 Thundering herd, backoff and jitter40:25 The CAP theorem43:09 The consistency spectrum45:32 The FinTech exception: double spending47:05 Why a rejected transaction beats a duplicate48:13 Recap: every design is a trade-off49:23 Final thought#SystemDesign #SoftwareEngineering #DistributedSystems #InterviewPrep #TechInterviews #SoftwareArchitectureInterview Prep Podcast