Snap (Snapchat) Software Engineer Case Interview: Designing a Camera-First, AR-Powered Social Feature at Scale

This case interview mirrors Snap’s camera-first culture and pragmatic, product-driven engineering style. You will collaboratively design an end-to-end solution for a new Snapchat feature, with emphasis on mobile constraints, AR (Lenses), ephemeral messaging semantics, real-time media delivery, privacy/safety for a young user base, and data-informed iteration. Sample prompt used by interviewers: “Design ‘Lens Party’: an ephemeral, invite-only group camera session where friends join live, apply a shared AR Lens, capture short clips, and auto-sync them into a private story that disappears after 24 hours.” What the interview covers at Snap: - Problem framing aligned to camera-first principles (how the camera opens fast, Lens selection/activation, and on-device vs server tradeoffs). - Functional and non-functional requirements typical to Snapchat: ephemeral retention policies; p95/p99 latency targets for capture, encode, upload, and playback; crash-free stability; battery/memory constraints on mid/low-tier Android devices; resiliency on flaky networks; cost awareness for media/CDN. - AR platform specifics: Lens runtime integration, model/texture packaging and delivery, cold-start/streaming strategies, and guardrails for device performance. - Real-time collaboration: session state sync, presence, voice/video/gesture signaling, and graceful degradation (fallbacks, retries, backpressure). - Messaging/media pipeline: media capture -> encode/transcode -> encrypt -> upload -> store -> CDN edge -> playback; idempotency, deduplication, and content addressing. - Privacy, safety, and trust: default-private design, location and minors’ protections, abuse/spam prevention, reporting flows, rate limits, and data minimization. - Measurement and iteration: success metrics (engagement, send/open rates, time-to-first-frame, crash-free rate), experiment design (A/B with holdouts), and rollout guardrails. Structure and timing (approx. 75 minutes): 1) Warm-up and role context (2m) 2) Problem framing and constraints (5m) 3) Requirements: functional + NFRs (8m) 4) High-level architecture (12m) 5) API & data modeling (8m) 6) Deep dive: AR & on-device performance (8m) 7) Deep dive: real-time session + media pipeline (10m) 8) Privacy, safety, abuse vectors (6m) 9) Metrics, experimentation, and cost tradeoffs (6m) 10) Testing, rollout, and observability (3m) 11) Risks, tradeoffs, and roadmap (4m) 12) Wrap-up and Q&A (3m) What interviewers look for (Snap-specific): - Kind, Smart, Creative collaboration: clear communication, considerate tradeoff discussion, and creative yet practical solutions. - Mobile-and-AR intuition: awareness of camera cold start, memory/battery limits, model/lens asset sizing, and network variability. - Systems thinking with pragmatism: simple, evolvable architecture that scales to DAU-level fanout and bursty traffic patterns. - Privacy-by-default mindset: principled handling of ephemeral data, encryption, access controls, and safety mechanisms. - Data-informed iteration: well-defined metrics, experiment plans, and rollback/guardrail criteria. Reusable template the candidate can follow: - Problem statement: user goals, constraints, and success criteria. - Requirements: functional flows; NFRs (latency, reliability, cost, privacy, battery/memory, offline resilience). - Architecture: client (camera pipeline, Lens runtime, capture/encode, background upload), services (API gateway/auth, session/presence, messaging, media store + CDN, realtime pub/sub, feature flags/experimentation, metrics/logging). - Data model & APIs: story/session entities, media manifests, access policies, idempotent upload/playback endpoints. - Scaling & reliability: fanout patterns, caching, backpressure, retries, circuit breakers, multi-region failover. - Privacy/safety: minimal data retention, abuse prevention, reporting, rate limiting, and visibility rules. - Metrics/experiments: p95/p99 latencies, crash-free %, engagement, experiment design and guardrails. - Testing & rollout: device matrix, soak tests, staged rollout, observability (dashboards/alerts), and rollback plan.