Requirements

Functional Requirement

1. Allow reservation of a parking spot.
2. Process payment for the reservation.
3. Enable parking of a car in the reserved spot.
4. Support early departure before reservation time expires.
5. Gate check-in/out.
6. User gets notified on booking confirmation, booking reminder, booking cancellation, delay arrival, booking end time.
7. Handle no show.
   1. Giving a grace period for user if did not arrive on the booking time. After the grace time release the booking.
   2. Cancellation policy where user has to cancel the booking in a configured time before else user has to pay a penalty. 

Non-Functional Requirements:

1. Availability: The system should be highly available.
2. Consistency:
   1. Tow user trying to book and parking spot should not allocate the same spot.
   2. No two user should get the same parking spot
3. Durability: Once spot is created needs to ensure that data never lost
4. Reliability
   1. The System should behave correctly and deliver the functional requirement even in case of failure, request spikes and other outage.

API Design

1. User Can book the parking spot
   1. HTTP Method: POST
   2. Endpoint: /api/v1/booking
   3. Request body: User Id, Vehicle Number, booking date, start time, end time
   4. Response: Booking Confirmation Id, Parking Spot number
2. User Can get the details of the parking spot
   1. HTTP Method: GET
   2. Endpoint: /api/v1/booking/{vehicle-number}/{booking date}
   3. Request Path Variable: Vehicle Number and booking date
   4. Response: Booking Details such as booking spot, booking date, start and end time.
3. User Can cancel the parking
   1. HTTP Method: PATCH
   2. Endpoint: /api/v1/booking/{booking-id}
   3. Request Path Variable: booking id
   4. Response: Cancellation confirmation details.
4. User Can get the Available parking spot
   1. HTTP Method: GET
   2. Endpoint: /api/v1/booking/availability
   3. Request Body: Vehicle Type, Booking Date or booking start and end time.
   4. Response: List of all available parking spots for the given criteria
5. Check in/check out
   1. HTTP Methos: PATCH
   2. Endpoint: /api/v1/booking/{booking-id}/{status}
   3. Request Path Variable: booking id
   4. Response: Cancellation confirmation details.

High-Level Design

1. User sent a request for different operations to the server it first go through the API gateway.
2. API Gateway route the request to the parking service. where api gateway route the request using round robin technique
3. Parking service performs the operations requested by user.
4. For the Parking reservation request once user select the parking slot it will be locked for a user for a configurable time default 5 minutes. if all the processing between the given time is completed the unique parking id will be generated else use transaction will be marked as cancelled and the parking slot will be make available to the other user. this technique also make help to maintain the consistency.
5. Once Parking slot is reserved for the user the event will be generated and sent the messaging broker from where Notification service will consume it and make the notification for the user and sent as a mail or a phone message.
6. The Batch Processing service will execute a batch for every five minutes and load all the reserved slot details (Before and after reservation time) and cache into memory, another batch process executes every minute and check for a slot in memory which start time about start or passed the start time and notify the user accordingly.

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.