What data pipelines and infrastructure feed a viral user acquisition model?
Tests causal attribution architecture. Great answers cover invite instrumentation with identity resolution, streaming pipelines that split organic and viral signups, and feature stores for network-state features.
Architect real-time usage-based billing for a PLG company
This tests event-driven metering, idempotent aggregation, and pricing decoupling at scale. A strong answer outlines real-time ingestion, stream processing for micro-events, a rules-based pricing engine, and dashboards with reconciliation.
Design a multivariate experimentation platform with collision-free concurrent bucketing and cross-device consistency
Tests orthogonal layers and cross-session assignment persistence. Cover: deterministic hashing per layer, a user profile service for sticky bucketing, and stable ID resolution across devices. Red flag: random bucketing or local storage breaking consistency.
Walk me through a magic link login system and its security considerations
Tests auth architecture and threat modeling for passwordless flows. Strong answers map request-token-email-verify-session, then harden with short expiry, single-use tokens, rate limits, and device binding.
How would you enforce a 3-project freemium limit and handle upgrades?
Tests entitlement and growth tradeoffs. Strong answers use API-level enforcement, atomic checks to prevent concurrent overages, soft upsell prompts preserving context, and async billing webhooks. Red flag: UI checks or limits in the projects table.
Design referral tracking from invite to conversion
Tests data modeling for multi-stage conversion tracking. Strong answers separate codes from conversion events, model status transitions, and enforce unique constraints. Weak answers merge invite and reward into one table or store denormalized counts on users.
Implement a 10% onboarding tutorial feature flag and track success
Tests lightweight experiment design and metric selection beyond vanity numbers. Strong answers cover user-ID bucketing, conversion tracking, and guardrail metrics like drop-off. Red flag: manual toggles or click-through without downstream outcomes.
How would you instrument a key activation milestone event?
Tests schema design beyond a bare event name. A strong answer includes the event key, user ID, timestamp, and properties like project_id and is_first_project. Red flag: omitting the user ID or sending only a human-readable label without structured context.
Design a highly available entitlements service with caching
This tests balancing read performance with consistency in access control. A strong answer proposes tiered caching with proactive invalidation, read-optimized hot paths, and event-sourced temporary grants.
What are the key architectural differences between freemium and free trial models?
WHAT IT TESTS: Feature-gating versus time-bomb architecture and infrastructure cost. ANSWER OUTLINE: Contrast tiered entitlements against trial timers and revocation; discuss free-user overhead and conversion tracking.
Design a system to handle subscription renewals
WHAT IT TESTS: Modeling a subscription state machine that decouples billing from access. ANSWER OUTLINE: States include active, past_due, and cancelled; transitions are payment success, dunning exhaustion, and grace expiry; use idempotent webhooks.
Model a 3-month 20% promo discount and apply it at billing
This tests separating coupon rules from per-user redemptions. Good answers use a coupons table for the 20%/3-month rule, a redemptions table for usage, and apply the discount to the first three invoices. A red flag is hard-coding the discount on the user row.
What schema changes are needed to add a Pro subscription tier?
This tests normalization of billing data versus hardcoding tiers. Add a plans table with integer cents pricing, link subscriptions via plan_id, and leave users untouched. Red flag: adding a tier string column to users or storing prices in subscriptions.
Design a real-time personalized notification trigger system
WHAT IT TESTS: Event-driven scheduling with backpressure. ANSWER OUTLINE: Stream events to a delayed queue, expose a rule UI to non-technical users, and deliver idempotently. RED FLAG: Cron polling a database without ordering or rate limits.
Compare fan-out-on-write vs fan-out-on-read for an activity feed
Tests whether you tie feed architecture to read/write ratios and follower distribution. Strong answers contrast push O(1) reads with celebrity storms against pull O(1) writes with read amplification, then propose a hybrid threshold.
Outline architecture for a weekly email digest of unread notifications
This tests batch processing and scheduled delivery at scale. Pre-aggregate unread counts, shard digest jobs across a distributed scheduler, and cache unsubscribes for fast filtering. Never scan the notifications table at send time for millions of users.
Design an A/B test for a Buy Now button
This tests experiment plumbing: deterministic bucketing, sticky storage, and logging. A strong answer covers user-ID hashing, cookie persistence, and impression-plus-conversion events.
Describe the data model and backend logic for a daily login bonus.
This tests streak state machines and calendar edge cases. A strong answer stores last_login_utc and streak_count, uses UTC day buckets, resolves timezones per user tz, and needs no leap-year logic. RED FLAG: Naive 24-hour windows break during DST shifts.
Describe the end-to-end data flow for tracking a 'Share' button click
WHAT IT TESTS: Full analytics pipeline design. ANSWER OUTLINE: Payload carries event type, user ID, timestamp, device, content; client batches with retry; backend validates and lands in a partitioned store. RED FLAG: Raw PII, no timestamps, no dedup.
Design a role-based personalized onboarding system
Tests separation of content and logic for scalable personalization. Strong answer: CMS-backed rule engine, multi-channel delivery, event-driven triggers, and per-segment metrics. Red flag: hardcoding role-specific UI components in the client.