Parking Lot · Analysis

01 · Frame the problem

Understand the central challenge

Design the objects and contracts for a multi-floor parking lot. Motorcycles, cars and trucks need compatible spaces. Concurrent arrivals must never receive the same spot.

Separate spot-selection preferences from exclusive ownership and ticket/payment lifecycle. The hardest cases are competing arrivals and ambiguous payment outcomes, where a locally convenient action can violate another operation's ownership.

02 · Find the design pressure

Start simple, then identify the gap

Begin with spots and active tickets behind one service. A check-then-assign implementation races, while unconditional exit release is unsafe. Move claims into an atomic repository operation and make payment/receipt identity part of ticket state.

Define park(vehicle): Ticket and exit(ticketId, paymentKey): Receipt. Use VehicleType, SpotType and TicketStatus values. Repository.tryClaim enforces exclusive ownership; pricing calculates a quote; a payment adapter handles side effects. Test full capacity, incompatible vehicles and duplicate payment callbacks.

03 · Define success

Requirements to preserve

Allocate a compatible available spot and issue a ticket.
Release a spot exactly once after successful payment.
Swap pricing policies without changing allocation logic.
Define behavior for full lots, lost tickets and repeated exit requests.

04 · Identify the correctness boundary

Invariants, representation and mutation boundaries

Selection ranks candidates; only the repository's conditional claim owns a spot. Keep claim and ticket creation atomic, and freeze the exit quote before calling payment outside the transaction. Payment identity belongs to the ticket workflow, while release verifies that the spot is still owned by that ticket.

05 · Make it concrete

Reason through a worked example

Two cars choose the last car-sized spot. One claim succeeds; the other retries and returns LotFull. The winner exits, but payment times out. Its spot remains occupied until reconciliation confirms the same payment. Repeating exit then returns the stored receipt and cannot release another driver's spot.