Background

This is my first backend project using Kotlin. I have played around with Kotlin for Android development and I must confess, I had some challenges setting this up locally and I had to depend on a lot of googling and documentation help since my autocomplete feature didn't work(maybe by Antaeus himself :D) and syntax highlighting to some extent didn't.

Nevertheless, I have found the project totally worth my time!

Summary of Solution

The billing service is started once the app runs and checks if it's the first day of the month to charge invoices. To charge the invoices, it runs them with coroutines with the intention of executing this as fast as possible with high concurrency.

Detailed Outline of Solution

1. Have a schedule that's triggered on the first of the month at a hour that then runs a long running service to charge invoices with PENDING status.

2. If the date current invocation time is not the first which shouldn't, it delays the run until the next 1st of the month.

3. For each pending invoice, attempt a charge for the invoice with:

   • Throw CustomerNotFoundException when no customer with id is found.

     • Escalate this to an admin who can manually rectify this but I've decided to mark the invoice as invalid.

   • Throw CurrencyMismatchException when the currencies from invoice and customers don't match.

     • Escalate this to an admin would be ideal for further investigation. However, I've decided to assign the currency on the customer table to the invoice currency.

   • Throw NetworkException when a network error occurs.

     • Add a retry mechanism for one more time before escalating to an agent.
     • Ideally, this should go into a messaging queue, AMQP since the messages are important and the cost of losing any of messages is far higher that not achieving an optimal throughput

   • True if the account was successfully charged.

     • Update the invoice status as PAID

   • False if the account balance of the customer is not up to to the invoice amount.

     • Leave invoice status as PENDING

4. For each invoice charge process, run in a separate coroutines which are cheap to spin and doesn't hurt performance that much.

5. Additional URL endpoint $ curl http://localhost:7000/rest/v1/invoices/update to manually test out the charging of all due invoices.

Changes Made

I added a column to the invoice table to track validity of each invoice:

• Add valid to Invoice table which defaults to true for a new invoice. This field shows which invoice we've marked as invalid and should be dropped subsequently.

Possible Improvements

• Integrate a service that converts currency which accepts (value, currency, newCurrency) and returns (value, currency) . This will increase the robustness against value losses.
• Increase the parallelism of the invoice jobs. There's a room for improvement here as more customers get added, there's a potential bottleneck lurking.
• Add a proper job queue which allows for retries of failed jobs (displayable via UI) using some exponential backoff or even manual. An overkill here might be using AirFlow :)

Antaeus

Antaeus (/ænˈtiːəs/), in Greek mythology, a giant of Libya, the son of the sea god Poseidon and the Earth goddess Gaia. He compelled all strangers who were passing through the country to wrestle with him. Whenever Antaeus touched the Earth (his mother), his strength was renewed, so that even if thrown to the ground, he was invincible. Heracles, in combat with him, discovered the source of his strength and, lifting him up from Earth, crushed him to death.

Welcome to our challenge.

The challenge

As most "Software as a Service" (SaaS) companies, Pleo needs to charge a subscription fee every month. Our database contains a few invoices in the different markets we operate in. Your task is to build the logic that will pay those invoices on the first of the month. While this may seem simple, there is space for some decisions to be taken and you will be expected to justify them.

Structure

The code given is structured as follows. Feel free however to modify the structure to fit your needs.

├── pleo-antaeus-app
|
|       Packages containing the main() application. 
|       This is where all the dependencies are instantiated.
|
├── pleo-antaeus-core
|
|       This is where you will introduce most of your new code.
|       Pay attention to the PaymentProvider and BillingService class.
|
├── pleo-antaeus-data
|
|       Module interfacing with the database. Contains the models, mappings and access layer.
|
├── pleo-antaeus-models
|
|       Definition of models used throughout the application.
|
├── pleo-antaeus-rest
|
|        Entry point for REST API. This is where the routes are defined.
└──

Instructions

Fork this repo with your solution. We want to see your progression through commits (don’t commit the entire solution in 1 step) and don't forget to create a README.md to explain your thought process.

Happy hacking 😁!

How to run

./docker-start.sh

Libraries currently in use

• Exposed - DSL for type-safe SQL
• Javalin - Simple web framework (for REST)
• kotlin-logging - Simple logging framework for Kotlin
• JUnit 5 - Testing framework
• Mockk - Mocking library