• Right click on PaymentGateway.WebApi
• Select Set as StartUp project
• Select PaymentGateway.WebApi
  

• Go to Run and Debug
• Select .NET Core Launch (web) from the drop-down at the top
• Press the debug button
  

Test Credit card numbers that will pass validation.

See e2e\readme.md for more information on running the Cypress tests.

• Format of the Merchant ID
• Format of the Terminal ID
• Only handling card payments. Other payment methods are "phase 2" of the project
• If a payment fails, the payment needs to be submitted again, with a new Transaction Date

I decided to test the API using Cypress because it's quick to configure and easy to use. Another benefit is that testing an API in 2 languages can highlight issues in the usage of the RESTful API (unit tests in c# and e2e in Javascript). If I used C# all the way through, I wouldn't see the pain of the different date formats as easily, because I would likely be using strongly-typed properties all the way through. Likely the payment gateway would be consumed by multiple different tech-stacks if is used widely enough.

I used fluent validation because it has some useful inbuilt validators such as the card number validator. I could have also used value types for each of the fields in the request to ensure they are valid values.

I used the MediatR library because I think that the mediator pattern helps create a clean architecture within the app.

I have tried to optimise for readibility of the code, and tried to not prematurely refactor. As such decisions like splitting the logic into multiple projects, or splitting a function out into another class have been deferred until there's more information eg way more logic, way more objects.

I didn't unit test the MakeAPaymentRequestHandler class because each branch in the logic is tested by the Cypress tests. Given more time I might consider useful tests for this class.

• Instead of relying on datetime, I might use a library such as noda time for more reliable dates.
• I would containerise this API. My PC is "Windows 10 Home" so it's a bit of a pain to install docker on.
• I may get rid of the magic strings in the validation logic. I held off for the time being because I think that some of the messages needed to be customised according to which validation rule triggered the error.
• If I were working with others to get this project completed, I would most likely use short-lived branches and create PRs for review. In this case it seemed overkill to branch and merge all the time.
• I might implement the decorator pattern for logging and other things such as telemetry and caching.
• I would implement persistent storage. ATM I'm storing in memory, but lots of storage options would be easier to implement with an instance already up-and running or docker instaled on my PC. I would likely keep the in memory implementation for local testing.