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:

• low latency - the responses should be returned in less than 200ms
• scalability - it should be able to scale out in case of high demand
• availability - should have an up time of at least 99.99%
• consistency - strong consistrency is needed so in different threads we dont have different parking lots statuses (eg in one thread park lot x is occupied but in a different thread it is free and a car can be parked there)

API Design

db:

model parking:{

status: reserved | free | taken

reservedFrom?: Date

reservedUntil?: Date

paidAt?: Date

leftAt?:Date

carId:String

}

GET /parkings - shows all parkings details

POST /reserve/parking/{parkingId} body:{ reservedFrom: Date, reservedUntil: Date} - used for reserving a parking lot

POST /pay/parking/{parkingId} body:{ carId:string; cardDetails:JSON } - used for paying a parking lot

POST /park/parking/{parkingId}?is_parking={is_parking}&leaving_at={leaving_at} - used for parking/leaving parking earlier

POST /parking/{parkingId}/status/{status} - used by admins to reset the status of a parking in case of no show

High-Level Design

reserving, parking, leaving parking, updating parking status flow:

1. client makes call to DNS to fetch the IP of domain.
2. DNS responds with IP
3. client makes API call to IP address
4. call hits load balancer - we need one in case we must scale out and add multiple instances of services.
5. load balancer routes to parking service which fetches parking cache and in case of updates it also mutates the cache and parking db or in case cache is missing on read, it fetches data from db directly
6. response is returned to client

payment flow:

1. same 4 steps as above
2. load balancer routes to payment service which call payment API and updates parking db with paidAt: Date.now() in case of succesful payment or doesnt update the parking status and user must retry payment process
3. response is returned to client

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.