Appointment Booking System

Interview-Ready Approach

1) Clarify Scope and SLOs

• •Problem statement: Design an appointment scheduling platform with availability management, booking, reminders, and waitlists for service businesses.
• •Design for a peak load target around 100 RPS (including burst headroom).
• •Businesses: ~10,000
• •Providers per business: ~10 avg
• •Bookings per day: ~500,000
• •Double-booking tolerance: Zero
• •Booking confirmation latency: < 1 second

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.
• •Caching: Put cache on hot read paths first and pick cache-aside or write-through explicitly.
• •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.
• •Bound staleness with TTL + invalidation hooks for critical entities.
• •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.
• •Caching: Higher hit rate cuts latency/cost, but stale data and invalidation bugs become primary risks.
• •Notifications: Multi-channel coverage increases reach but adds per-channel failure modes and policy complexity.

Practical Notes

• •Use pessimistic locking (SELECT FOR UPDATE on the slot row) to prevent concurrent users from booking the same slot.
• •Store availability as a list of time ranges per provider per day. When a booking is made, subtract from available ranges.
• •Reminders: a scheduled job scans for appointments due for reminders in the next few minutes. Enqueue messages to an SMS/email queue.

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 -> Load Balancer -> API Gateway -> API Service -> Primary NoSQL DB -> Redis Cache -> Message Queue -> Background Workers

Design strengths

• •Cache sits on the read path to absorb repeated queries and keep DB pressure stable.
• •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.