Smart parking system

Statement of the problem

Functional Requirements

Non-functional requirements

Core Entities

{
  id: UUID,
  gps_coordinates: {lat, lon},
  total_capacity: int,
  free_spots_counter: int, // semaphore!
  address: string
}

{
  id: UUID,
  user_id: UUID,
  parking_id: UUID,
  car_plate: string,
  created_at: timestamp,
  expires_at: timestamp,
  status: enum (pending, confirmed, cancelled, expired)
}

Atomic semaphore counter

The central technical challenge is to provide atomicity of decrement counter when booking. We need a compare-and-swap operation with a check: the counter should not fall below zero.

-- parking_book.lua
local counter = redis.call('GET', KEYS[1])
if tonumber(counter) > 0 then
  return redis.call('DECR', KEYS[1])
else
  return -1 -- No free seats
end

UPDATE parkings 
SET free_spots = free_spots - 1
WHERE id = $1 AND free_spots > 0;

-- affected_rows = 0 → no places available

Delayed Message Queues

If the user has reserved a place, but does not arrive for 15 minutes, the system should automatically release space and cancel the reservation.

# When booking
sqs.send_message(
    QueueUrl=timeout_queue,
    MessageBody={"booking_id": "...", "parking_id": "..."},
    DelaySeconds=900 # 15 minutes
)

# After 15 minutes the worker checks:
# - If the car does not arrive → INCR counter, cancel the reservation
# - If you arrived → do nothing

Geo-Sharding

Parking lots are naturally tied to geography, creating an ideal opportunity for location-based sharding.

Key Findings

Interviewer Rating