TopUp Feature

How to run:

• Install Docker and Docker Compose
• Browse to the project root directory
• Run docker-compose up -d
• Swagger (TopUpService): http://localhost:8080/swagger/index.html
• Swagger (AccountService): http://localhost:8082/swagger/index.html (just an bank account simulation)

Problem Solution

• Big Picture
• 

Design (TopUpService)

• A simple 3 layer archtecture inspired by the 4 tenets of Onion Archtecture (is not "by the book", it's just a 'inspiration')
• 
• Ref: https://jeffreypalermo.com/2013/08/onion-architecture-part-4-after-four-years/
• ps: I like this approach. It's simple but it's interface oriented and has great testability.

Disclaimers:

• I put more effort in top-up service. Account and user service are just simulations
• In real world, solutions like this need more unit tests and integration tests.
• There are comments in code (to explain some of my point of view about real world solutions)
• There are so many challenges in a real world solution like these:
  • Distributed transactions (Saga Pattern or something)
  • Distributed locks
  • Commit and Rollback actions
  • Retry policies
  • Events
  • Audit
  • Observability
  • So... none of these challenges were addressed here. But I'm aware of them.