IoT Platform 1 - Smart Home Hub

Interview-Ready Approach

1) Clarify Scope and SLOs

• •Problem statement: Build a smart home platform connecting 100 k devices with real-time control and status updates.
• •Design for a peak load target around 15,000 RPS (including burst headroom).
• •Total devices: ~600,000
• •Concurrent online devices: ~100,000
• •Command latency: < 1 second
• •Status update frequency: Every 30 seconds
• •Households: ~50,000

2) Capacity Planning Method

• •Convert traffic and growth constraints into request rate, storage growth, and concurrency budgets.
• •Keep at least 2-3x safety margin per tier (ingress, compute, storage, async workers).
• •Reserve explicit latency budgets per hop so p95 can be defended in review.

3) Architecture Decisions

• •Databases: Define a clear system-of-record and design read/write paths separately before adding optimizations.
• •API Design: Standardize API boundaries, idempotency keys, pagination, and error contracts first.
• •WebSockets: Use persistent connection gateways and decouple fanout via pub/sub or queues.
• •Auth: Centralize identity verification and keep authorization checks close to domain resources.

4) Reliability and Failure Strategy

• •Use strong write constraints (transactions or conditional writes) and explicit backup/restore strategy.
• •Apply strict input validation and backward-compatible versioning.
• •Track connection churn, backpressure, and session resumption behavior.
• •Use short-lived tokens and secure key rotation workflows.

5) Validation Plan

• •Run one peak-load test, one dependency-degradation test, and one failover test.
• •Verify idempotency for all retried writes and async consumers.
• •Track user-facing SLOs first: p95 latency, error rate, and successful throughput.

6) Trade-offs to Call Out in Interviews

• •Databases: SQL gives stronger transactional guarantees; NoSQL often gives better write scaling and flexibility.
• •API Design: Rich APIs improve developer speed but can create long-term compatibility burden.
• •WebSockets: WebSockets reduce interaction latency but complicate scaling and state management.
• •Auth: Central auth simplifies policy, but makes auth service availability/security critical.

Practical Notes

• •MQTT is the standard protocol for IoT - lightweight, pub/sub, supports QoS levels.
• •Group devices by household; a message broker topic per household keeps traffic isolated.
• •Device shadows (AWS IoT pattern) store the last-known state so the app always has data even if the device is offline.

Why This Solution Works

Request path: The solution keeps ingress, service logic, and stateful dependencies separated so each layer can scale independently.

Reference flow: IoT Devices -> API Gateway -> API Service -> Auth Service -> Primary NoSQL DB -> Realtime Bus

Design strengths

• •Security controls are enforced at ingress to protect downstream capacity.

Interview defense

• •This design makes bottlenecks explicit (ingress, core compute, persistence, async workers).
• •It supports progressive scaling without re-architecting the core request path.
• •It keeps correctness-sensitive state changes in durable systems while offloading background work asynchronously.