kreativecoder / antaeus

🚀 The Pleo Backend Challenge

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 😁!

Thoughts

"Make it work, make it right, make it fast" has been one of my guiding principles of development, it allows me focus on solving the problem and building my optimizations on top of that. Following this, the first solution I thought of was how to charge a single invoice, charge multiple and make this efficient?

Assumptions

• Only one instance of the app will be used in billing the invoices, hence concurrency and related problems were not put into consideration.
• There'll only be a few hundred invoices to process, anything more than this, then the fetching will be batched not to overload the system.

Improvements

• I probably didn't write idiomatic kotlin code, this is something I'm getting better in as I do more Kotlin projects.
• Testing, I added a few unit tests, but no integration tests to confirm all the part of the system are working as expected.
• The scheduler for the billing is in the core, I'll separate that into its own module and eventually a service to separate concerns and finer grain of control.
• The cron is static and whilst it can be changed via environment variable to any time, this would not work should we decide to have different schedule for payment for customers.
• Currency converter to reduce the rate of CurrencyMismatchException, implement a callback from the provider for currency change events
• Retrying is instant, this will not play out in a prod environment, a cool off period to allow systems to recover will be ideal.

Time Spent

About 6hrs spread over 3 days due to availability.

Libraries

• Quartz-Scheduler - Job scheduling library