Design a Real-Time EV Charging Station Reservation System for a City

Design a reservation system for the electric-vehicle charging stations of a city. Drivers see which chargers are available and reserve one in real time, then drive there and charge. The report states only this goal, so the number of stations, how a reservation is held and released, and how the system learns the real state of each charger are open: settle them with the interviewer before designing. ```hint Two drivers, one charger Think about what guarantees that two drivers who press "reserve" on the same charger at the same moment cannot both succeed. ``` ```hint The map can be wrong A charger can be taken by a driver without a reservation, go offline, or be abandoned by a no-show. Think about where the truth about availability lives and how quickly the app learns about a change. ``` ### Clarifying Questions - Does a driver reserve a specific charger for a time slot later in the day, or only hold an available charger until arrival? - How many stations and chargers does the city have, and how many drivers search and reserve at peak? - How long is a reservation held, and what happens when a driver does not arrive or stays plugged in too long? - Can chargers report their state (free, charging, faulted) to the system, and can a driver without a reservation plug in? - Do chargers differ in connector type and power, so that search must filter on them? - Is payment part of this design or handled by another service? ### What a Strong Answer Covers - Requirements and an estimate of stations, chargers and request rates - A data model of stations, chargers and reservations, and the life cycle of a reservation - Concurrency control that prevents double booking, such as a conditional update, a unique constraint or a lock with a lease - A search path that finds nearby available chargers quickly, separate from the booking path - Real-time availability from charger reports, pushed to the app, with rules for when the reservation state and the charger disagree - Expiry of holds, no-shows, failure handling and monitoring ### Follow-up Questions - A charger goes offline while a reservation on it is active. What do the system and the driver see? - How would you let drivers book a slot hours in advance while drivers without a reservation can still use idle chargers? - A popular station receives far more requests than it has chargers at rush hour. How would you allocate them fairly? - How do you keep the availability shown in the app consistent with what the chargers report?