Circle Interview Question｜Ticket Reservation API Design & Flight Booking System Design (High-Frequency System Design / Backend / API Design)

Problem 1 — Ticket Reservation API Design

Codewriting

How do you design the API/backend for a ticket reservation app?
Examples include movie tickets or concert tickets.
The app should allow users to hold their ticket(s) for 10 minutes before booking.

• Execution time limit: 2 seconds (plaintext)
• Memory limit: 1 GB

Problem 2 — Third-Party Airline Booking APIs

3-party APIs

Suppose we want to design an automated flight-booking service for our clients.
Our job is to book the cheapest flight for a given origin, destination, and departure date.
We can get flight data and create bookings using many different third-party Airline booking APIs.

Problem 1 — Ticket Reservation App (10-Minute Hold)

Key ideas:

1. Core entities
   • Seat → available/held/booked
   • Hold (expires after 10 minutes)
   • Reservation (after payment)
2. Architecture
   • Hold Manager with TTL
   • Reservation Manager
   • Atomic seat updates to prevent oversell
   • Background expiration cleaner
3. Important APIs
   • POST /hold → place a temporary hold
   • POST /reserve → convert hold to booking
   • DELETE /hold/{id} → release
   • Query APIs for seat availability and user state
4. Consistency
   • Use Redis/DynamoDB conditional updates
   • Ensure no overbooking even under concurrency

Problem 2 — Automated Flight Booking via Multiple Providers

Key ideas:

1. Layered system
   • Aggregator queries multiple providers concurrently
   • Normalization layer standardizes responses
   • Booking Orchestrator completes multi-step reservations
2. Search flow
   • Parallel API calls
   • Deduplicate & sort results
   • Pick cheapest flight
3. Booking flow
   • Hold seat
   • Confirm booking
   • Rollback if provider fails
4. Reliability
   • Retry, timeout, circuit breaker
   • Full logging + metrics

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