Preview — Pro guide
You are seeing a portion of this guide. Sign in and upgrade to unlock the full article, quizzes, and interview answers.
Sections
Related Guides
Movie & Event Ticket Booking — LLD (Seat Map, Holds, Concurrency)
Low-Level Design
Hotel Booking System — LLD Deep Dive
Low-Level Design
Online Shopping Cart & Checkout — LLD (Lines, Idempotency, Order Boundaries)
Low-Level Design
SOLID Principles in Practice
Low-Level Design
LLD Interview Framework & Walkthrough
Low-Level Design
Airline Reservation & Seating — LLD (PNR, Segments, Overbooking Policy)
LLD for airline reservations: PNR, segments per FlightInstance, seat holds, FareRule/Strategy, overbooking policy, and TOCTOU-safe seat commit. Ties to hotel/movie-ticket holds; names DCS check-in and standby as extensions.
What Interviewers Are Actually Testing
Airline questions blend LLD and domain nouns from real ops: PNR (passenger name record) as the booking aggregate, segment-scoped inventory, cabin classes, and overbooking as a policy object — not a hardcoded 105% in Aircraft. They probe whether you conflate a Route (marketing) with a FlightInstance (one tail on one day) — the same show vs room mistake in ticketing. A staff answer names DCS (departure control) and no-show statistics as the business justification for overbooking, then returns to the class diagram. Weak answers model Flight as a string and ignore connections.
Clarifying Questions
Multi-segment itineraries (connecting flights)?
PNR has ordered Segments; a failure on leg 2 may require rollback of hold on leg 1 — or separate PNRs — ask.
Seat assignment at booking or at check-in?
Some airlines sell seats, some auto-assign. Model optional SeatAssignment per segment with status OPEN|ASSIGNED|SWAPPED.
Overbooking allowed?
Policy object OverbookingPolicy on route/cabin with max oversell; legal exposure if you do not state volunt/invol rules — LLD: interface + one impl.
Standby and upgrades?
Separate Waitlist queue per FlightInstance with priority rules — not List on Flight only.
Payment timing?
Hold → confirm like tickets; 24h risk-free in some jurisdictions — a FareRule strategy.
Codeshare and marketing carrier?
Below line for many interviews; if raised, add OperatingCarrier vs MarketingCarrier on segment.