transactions-at-the-bank.md

Transactions at the bank

Context

You are working at a bank and are tasked to rewrite some of their APIs.

In particular, you will now need to write the code that transfers money between 2 accounts.

Structure

The bank employs this DB structure:

TABLE transactions
  - reference (unique)
  - amount
  - account nr

The Transactions table registers any transaction in an account (ie. today I paid 20$ for a movie with my credit card).

TABLE balances
  - account nr (unique)
  - balance

The Balances table represents the account balance of customers (ie. I have $5k in my bank account).

The bank uses a relational DB to store these info (let's say MySQL) and you are tasked to port the transfer API method from an old framework to NodeJS. Here's how your code starts to look like:

app.post('/transfer', (req, res) => {
  // here starts the fun
})

Requirements

• Implement the /transfer API which transfers X amount of money from account A to account B.
• You don't need working code but the more production-ready it is, the better.
• The request should be a POST request to the method /transfer with the payload: {from:account, to:account, amount:money}.
• The response should return the following payload: {id:transaction_id, from:{id:account, balance:current_balance},to:{id:account,balance:current_balance}, transfered:transfer_amount}
• You can change the request or the response payloads but, please, specify the reasons inside the README.md file.
• Remember that you are working at a bank, so you need to make sure the request and the data you save are always valid and consistent.
• You don't need to care about user authentication or authorization.
• Prevent that one customer might tap on the "pay/transfer" button twice by accident.
• Try not to use an ORM.

Considerations

• Focus your attention on how to manage the logic at the database level.
• Please think about all the different use cases:
  • What happens under high concurrency?
  • What happens if your database becomes unavailable in the middle of your logic?
  • What happens if 2 users (A, B) transfer money to user C at the same time?
  • And so forth.
• How do you handle and structure the errors that you return to the client?
• Please use async / await to handle async operations in NodeJS implementation.