Design a Parking Garage

Beginner Object-oriented design

The problem

Design the software for a multi-storey parking garage: drivers park, get a ticket, pay when they leave, and the garage knows what is free.

This is an object-oriented (low-level) design interview. The focus is classes, responsibilities and patterns, with a little concurrency.

You will sketch classes and method signatures. Real code is a bonus.

The garage has several floors and entrances.

Drive the conversation. Ask questions before drawing anything. State your assumptions out loud so they can be corrected early.
Use numbers. Turn the givens into requests per second, storage and bandwidth. Every later choice should point back to one of them.
Start simple, then evolve. Get a working version on the board first. Add caches, sharding and queues only when a number or a requirement asks for them.
Go deep on the hard parts. For this problem the hard parts are the class design, keeping pricing flexible, and avoiding two cars taking the same spot.
Name the trade-offs. Say what you chose, what you gave up, and what would make you change your mind.
Think about failure. Point out single points of failure and say what happens when a node, a cache or a whole zone goes down.

Single responsibility

A class should have one reason to change.

Strategy pattern

Put interchangeable algorithms (fee rules) behind one interface and pick one at runtime.

Open/closed

Open for extension, closed for modification: add a new class instead of editing old code.

Race condition

Two actions interleave badly, such as two cars both seeing a spot as free.

Atomic operation

Happens fully or not at all, with nothing else able to slip in between.

Decisions you confirm will collect here as you go.

Your design so far