Event RSVP Platform

Interview-Ready Approach

1) Clarify Scope and SLOs

• •Problem statement: Design an event creation and RSVP platform where organizers create events and guests respond.
• •Design for a peak load target around 500 RPS (including burst headroom).
• •Events created/month: ~10,000
• •Avg guests per event: ~50
• •RSVP response time: < 300 ms
• •Reminder delivery: Within 5 minutes of scheduled time
• •Max event capacity: 1,000

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.
• •Notifications: Model notifications as event-driven fanout with per-channel workers (email/push/webhook).

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 delivery state machine and dead-letter undeliverable events.

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.
• •Notifications: Multi-channel coverage increases reach but adds per-channel failure modes and policy complexity.

Practical Notes

• •Use a database transaction to atomically check capacity and insert the RSVP - prevents overbooking.
• •Waitlist promotion: when a guest cancels, a background job promotes the next waitlisted guest and sends them an email.
• •QR code check-in: encode the RSVP ID in a QR code. The organizer's app calls an API to mark the guest as checked in.

Why This Solution Works

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

Reference flow: Web Clients -> API Gateway -> API Service -> Primary NoSQL DB -> Message Queue -> Background Workers -> Notification Fanout

Design strengths

• •Async queue/event bus isolates bursty workloads and supports retries without blocking synchronous requests.

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.