Requirements

Functional Requirements:

• Allow reservation of a parking spot.
• Process payment for the reservation.
• Enable parking of a car in the reserved spot.
• Support early departure before reservation time expires.
• Gate check-in/out.
• Handle no show.

Non-Functional Requirements:

• System should be highly available
• System should be consistent so that 2 user trying to block same parking space only one will be able to book
• System should be secure. Since we are dealing with transactions (payment)
• TLS 1.3 all the communications are encrypted.

Capacity Estimation

Suppose we have 2000 parking space. this is just a table which will store the facts related to parking lot. parking lot id, parking lot number, parking lot size. Size of it is fixed since we have only 2000 parking space. So this table will have data of size considering each field is taking 4 bytes. So, size of 1 record will be 4 * 3 = 12 bytes. There are 2000 parking spots 12 * 2000 = 24000 bytes. Just for parking fact table

considering a slot can be booked for at-least 1 hour worst case scenario is we in an hour we will have number of booking = 1 * 2000, for the entire day it will be 24 * 2000 = 48000, now for entire year 48000 * 365 = 17, 52, 000 bookings.

If the bookings table contains booking id, parking lot id, start time, end time, transaction id, user id

considering each field takes 4 bytes, size of 1 record = 4 * 6 = 24 bytes

For a year = 24 * 17, 52, 000 = 4, 20, 48, 000 bytes

For n years n * 4, 20, 48, 000 bytes

API Design

1. Endpoint (POST) that will let you book based on parking lot selection - we will redis lock to handle concurrent booking. So, that only one user is allowed to book a parking spot at a given duration. Since we user request is able to acquire a lock he is given a window of 5 mins in order to make a payment and block the spot for the selected duration. Otherwise we will release the lock. For payment we will use API gateway.
2. We will give user an endpoint to report if the reserved spot is taken by someone so that the person taking care of parking lots get a notification to clear the parking lot or compensate the user with another space. Which brings us to the point based on role user who books the lot and admin role who manages the parking lot would be a good choice to implement. In backend as well as frontend
3. In case user want to depart early an endpoint would be given so that they release the spot. The spot then show available to other users.
4. Check In/Check Out time. This will be handled by a different service which tracks the time duration for which the vehicle was in parking lot. It will also handle early depart

High-Level Design

Database Design

Since 2 people cannot book same parking slot at same time the database should be consistent so we should choose a system which is ACID compliant not a db which is eventually consistent NoSQL. For our use case will use any SQL database. I will choose a PostgreSQL database.

bookings table

users table

parking lot table

transaction table

Detailed Component Design

Deep dive into 2-3 key components. Explain how they work, how they scale, discuss tradeoffs, capacity, and any relevant algorithms or data structures.