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 parking reservation

< 10 ms

• two users should not be able to book the same parking lot at the same time.
• scalable upto 1000 parking lots
• parking lot should not be blocked on no shows.
• strong consistency for booking parking slots
• eventual consistency for showing available parking slots
• fault tolerant
• highly available

scale

1000 spots

10000 users

Read vs reserve request 10:1

10 get requests per second

1 parking request per second

API Design

get /parkingspots?type={} -> [parkingspot]

post /parking/{parkingspotid}/reserve

-> ReservationId & statuscode

201 for created or 404 not found

// webhook

post /paymentdone/

{

reservationid

}

get /reservationstatus/{reservationId}

-> Succeeded | Failed| Cancelled

get /booking/{reservationId}-> reservation

patch /parkingspot/{id}

{

status - occupied | Available

} -> HttpStatusCode

High-Level Design

Entities

User

ParkingSpot

Reservation

parking service will be used for managing vehicles that enter the parking spot.

Postgres will be used as database

Reservation cleanup janitor service will cleanup old reservations

Parking reservation service will handle reservations

Parking fetch service will handle fetch of the available parking spots

Detailed Component Design

parking reservation service

• 3 node deployment will be done with 1 active node

parking reservation flow

• User gets the list of available parking spots according to vehcile type
• user chooses a parking spot and gives a call to reserve the parking spot
• A distributed lock is created for the parking spot. If already the lock exists we return error "spot already taken" to the user

Distributed lock will be taken on the node that is handliing all the requests. We dont' need a separate redis instance for this as the volume is low

• Distributed lock ttl expiry is 5 minutes. User is expected to complete payment in 5 minutes
• A reservation entry is made in the reservation table. Its status is set to InProgress
• user is responsed with reservationId
• user initiates a payment a webhook is registered that stripe calls after payment is done.
• when the webhook arrives, we know payment is completed for the reservationId. We run a transaction on the database, we see if there is already a reservation at an overlapping time which is completed for this parking spot, we mark the status of the reservation as failed.
• Otherwise, we change the status of the reservation entry to completed.
• user will keep querying the status of the reservation every 3 seconds and we will return the status

parking service entry

• vehicle enters with a reservationId
• parking service verifies that reservationId exists and time of entry matches, if time doesn't match or reservationid doesn't exist entry is denied.
• Reservation entry is marked as VehicleParked
• A vehicle with a valid reservationid is not allowed past the startTime.

parking service exit

• vehicle exits with a reservationid
• parking service marks the reservation entry as Expired so that others can book.
• Charges extra if time is exceeded

Parking slot fetch service

• We will query the database to check if a reservation already exists that overlaps with the time customer specified and if that reservation status is Completed .

If the reservation stastus is completed but the current Time is greater than the reservation time, that means no o ne has come for the erservation and we can give the resrvation of the parking slot to someone else.

Database design

User - userId(PK), name

ParkingSlot - id(PK), Type(secondary index)

Reservation- resevationId(Pk), userId,

parkingSlotId, startTime, endTime, Status

Query to see if there are overlapping reservations

select * from Reservation where parkingSlotId=id && !(startTime >= currentEndTime || endTime<= currentStartTime) & Status == Completed &&

( currentTime < startTime )-> Overlapping reservations

project count(*)

if count > 0 -> can't do reservation

query to get free parking slots

select * from Resevation where && !(startTime >= currentEndTime || endTime<= currentStartTime) & Status == Completed &&

( currentTime < startTime )

| project parkingSlot

| get all parking slots from parkingslot table which are not in parkingSlot table and of type specified

Reservation cleanup service

This service wil lcleanup old reservations