thiagoxvo/antaeus

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 for the different markets in which we operate. Your task is to build the logic that will schedule payment of 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.

Instructions

Fork this repo with your solution. Ideally, we'd like to see your progression through commits, and don't forget to update the README.md to explain your thought process.

Please let us know how long the challenge takes you. We're not looking for how speedy or lengthy you are. It's just really to give us a clearer idea of what you've produced in the time you decided to take. Feel free to go as big or as small as you want.

Developing

Requirements:

>= Java 11 environment

Open the project using your favorite text editor. If you are using IntelliJ, you can open the build.gradle.kts file and it is gonna setup the project in the IDE for you.

Building

./gradlew build

Running

There are 2 options for running Anteus. You either need libsqlite3 or docker. Docker is easier but requires some docker knowledge. We do recommend docker though.

Running Natively

Native java with sqlite (requires libsqlite3):

If you use homebrew on MacOS brew install sqlite.

./gradlew run

Running through docker

Install docker for your platform

docker build -t antaeus .
docker run antaeus

App Structure

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

├── buildSrc
|  | gradle build scripts and project wide dependency declarations
|  └ src/main/kotlin/utils.kt 
|      Dependencies
|
├── pleo-antaeus-app
|       main() & initialization
|
├── pleo-antaeus-core
|       This is probably 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 database 
|       models, mappings and access layer.
|
├── pleo-antaeus-models
|       Definition of the Internal and API models used throughout the
|       application.
|
└── pleo-antaeus-rest
        Entry point for HTTP REST API. This is where the routes are defined.

Main Libraries and dependencies

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
Sqlite3 - Database storage engine

Happy hacking 😁!

Solution

The implementation followed the instructions and will run a background job on every 10 seconds, but will only process invoices on the configured date (first of the month default)
As the scheduler keeps running, we got a retry when failed for free, when the application is started, it will run the scheduler until the point it will process all payments
The billing service was implemented with a lot of naive approaches (all documented how it could be improved)

Improvements

Improve resilience on billing service
- retries to recover from network glitches or payment provider unstable
- backoff retry to recover from insufficient balance
- a queueing system to handle better the payments that need to be processed or not, dead letter queue for things that need manual intervention
Improve scheduling on billing scheduler
- Use something more robust than a regular Timer.schedule

Disclaimer: That was the first time I wrote Kotlin

thiagoxvo / antaeus