atm project

A simple project imitating a backend REST API for an ATM.

How to run

Clone the repo
npm install
npm run db (requires Docker, leave terminal open)
npm run start (in a new Terminal window)

The DB_INIT_DUMMY_DATA flag is set to true by default when running locally. It will initialise the DB to the folowwing available cash entries:

$1000 x 1
$500 x 2
$200 x 10
$100 x 0
$50 x 0
$20 x 10
$10 x 10
$5 x 0
$2 x 10
$1 x 0

Technologies used

This project is a standalone REST API server built using Node + Typescript. It connects to an PostgreSQL database. Some of the 3rd party libraries used:

koa - as an API framework
slonik - PostgreSQL client
big.js - For hanling decimal precision numbers
a few more small library and helpers

Concurrency safety

The project ensures concurrency safety (i.e. when multiple payout operations are performed concurrently and compete for the limited amount of bills/coins). The project ensures there are no unexpected results (e.g. negative amount of bills/coins left, etc.) by relying on PostgreSQL function's transactional nature.

The payout operation consists of 3 steps:

get available coins/bills amounts from the DB;
in code, find the desired combination of coins&bills to withdraw;
transactionally substract the amounts from the DB. If concurrent operation modified them to the point where there is no enough coins/bills to satisfy the payout, transaction is reverted and exception raised.

Tests, linting & prettier

This project is set up with Unit Tests using jest, Linting with eslint and prettier for formatting the code.

To run all the checks:

npm run build
npm run lint
npm run test

Exposed endpoints

Request:

POST localhost:5000/payouts
{
     "value": "2000"
}

Response:

{
    "selectedEntries": [
        {
            "id": "1000",
            "singleUnitValue": "1000",
            "count": 1
        },
        {
            "id": "500",
            "singleUnitValue": "500",
            "count": 2
        }
    ]
}

Notes

I have realised an edge case when the algorithm fails to select the optimal solution (the least number of banknotes/coins). I have expressed it as a Unit test that fails right now Can give up on some big value bills to achieve overall optimaly low number of bills/coins.

I have found descriptions of this problem with more description and example algorithms, e.g. here:

https://en.wikipedia.org/wiki/Change-making_problem

Given limited time resources I didn't have a chance to make the fixes yet.

radoslaw-medryk / atm-backend