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.

The server will have basically 3 components:

Cart: It will be responsible for Creating/Storing the ReservationCart and CheckoutCart.
- CreateReservationCart: When the user selects a reservation in the frontend, the FE will make a request for Cart to create a cart for that reservation, it will contain the reservation period, the userId linked to the reservation and the cost.
- CreateCheckoutCart: The same thing is for this endpoint. The user will have 5 minutes to finish the payments for each cart.
- MakeOrder will get the cart, validate whether the cart still applies to the current time and check with reservation component if the spot hasn't been filled yet. If the validations pass, it will go ahead to processing the payment.
Payments will communicate with the external PSP
- it will send a request to either process a new payment or refund an existing one.
- It will have a webhook to listen to the PSP responses (payment succeeded or not), it will signalize to Cart that the payment succeeded and that it can move forward to the reservation
Reservation: will receive the calls from Cart to verify whether spots are available, fill/empty a given spot when the cart payment succeeds
- Dealing with dual reservation: The design will not lock the spot when the user moves to payment, instead, it will keep it available and if 2 payments are processed for the same spot, whichever gets to the reservation component first is reserved. This is guaranteed to work due to levering ACID properties of the local DB.
- So, when the second payment arrives and the reservation sees that the spot is already filled, then it will return an error: `Error: spot already filled`, this will get back to cart that will trigger a refund for the payments component that will request the external PSP to refund it.

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