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

<p>Senior system design interviews don&#39;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.</p><p><br></p><p>You&#39;ll learn:</p><p>- The RADIO framework (Requirements, API, Data model, Infrastructure, Optimize) and how to split your 45 minutes</p><p>- The one question that instantly signals seniority</p><p>- REST vs gRPC, API versioning, and why offset pagination breaks at scale</p><p>- Choosing a database by access pattern, and justifying every component with the NFRs</p><p>- Why skipping non-functional requirements is the number one reason senior candidates fail</p><p>- Peak vs average TPS, P99 tail latency, latency budgets, RPO and RTO, and compliance</p><p>- Sticky sessions, sharding, hot shards, read replicas, CQRS and event streaming</p><p>- The math of the nines, active-active vs active-passive, circuit breakers, bulkheads and graceful degradation</p><p>- Exponential backoff with jitter to survive the thundering herd</p><p>- The CAP theorem, the consistency spectrum, and why banks must choose CP to prevent double spending</p><p><br></p><p>Chapters</p><p>00:00 Introduction</p><p>00:49 They test judgement, not definitions</p><p>02:22 The roadmap</p><p>03:36 The RADIO framework</p><p>04:47 Requirements in five minutes</p><p>06:00 The seniority signal</p><p>06:47 API design and versioning</p><p>08:20 Offset vs cursor pagination</p><p>09:05 Choosing the data model</p><p>10:18 Justifying infrastructure with NFRs</p><p>11:28 Network boundaries</p><p>11:50 Attacking your own design</p><p>12:39 Observability and cost</p><p>14:37 The URL shortener trap</p><p>15:23 The NFR cheat sheet</p><p>15:46 Peak vs average traffic</p><p>16:33 Tail latency and fan-out</p><p>17:44 Latency budgets</p><p>19:16 RPO and RTO</p><p>20:02 Compliance: GDPR, PCI DSS, SOX</p><p>21:37 Scaling out and sticky sessions</p><p>23:13 Sharding and the hot shard</p><p>24:49 Cross-shard query trade-offs</p><p>25:58 Read replicas and replication lag</p><p>27:08 CQRS</p><p>28:41 Event streaming and idempotency</p><p>31:02 The nines of availability</p><p>33:04 Active-active vs active-passive</p><p>34:37 Circuit breakers</p><p>36:09 Bulkheads</p><p>37:18 Graceful degradation</p><p>38:26 Thundering herd, backoff and jitter</p><p>40:25 The CAP theorem</p><p>43:09 Th