Money Transfer Service

Original assignment

Design and implement a RESTful API (including data model and the backing implementation) for money transfers between accounts. Explicit requirements:

You can use Java or Kotlin.
Keep it simple and to the point (e.g. no need to implement any authentication).
Assume the API is invoked by multiple systems and services on behalf of end users.
You can use frameworks/libraries if you like (except Spring), but don't forget about requirement #2 and keep it simple and avoid heavy frameworks.
The datastore should run in-memory for the sake of this test.
The final result should be executable as a standalone program (should not require a pre-installed container/server).
Demonstrate with tests that the API works as expected.

Implicit requirements:

The code produced by you is expected to be of high quality.
There are no detailed requirements, use common sense.

Please put your work on github or bitbucket.

About the project

There was intention to keep the minimal list of dependencies. Eventually, this project got only some dependencies for logging, parsing JSON and embedded relational database
The embedded database is MariaDB (MySQL compatible). There is no need to install dependencies. The daemon spins up automatically on application startup.
A tiny HTTP framework was written for this project. It uses only what really needed - e.g. it does not even know about the full list of HTTP methods and status codes. See the sources in src/main/java/com/revolut/mts/http, usage in Application.java.
API is minimalistic as well.

API

Create user

 POST /users => 201
 
 Request object:
 {
     "name": "alice"
 } 
 
 Response object:
 {
     "id": 1,
     "name": "alice"
 }

Deposit money to user account

 POST /deposits
 
 Request object:
 {
     "username": "alice",
     "amount": {
        "currency" : "USD",
        "amount": 1.0
     }
 }
 
 Response is empty

Get user and their balance

GET /users/{user-name} => 201

Response object:
{
   "id": 1,
   "name": "alice",
   "wallet": [
       {
          "currency": "USD",
          "amount": 122.0
       },
       ...
   ]
}

Create transaction

POST /transactions => 201

Request object:
{
    "sender": "alice",
    "receiver": "bob",
    "target_currency": "USD",
    "source_money": {
        "amount": 1.0,
        "currency": "BTC"
    }
}

Response object:
{
    "id": 123
}

Commit transaction

 PUT /transcation/{txid} => 200
 
 Respose is empty

Design decisions

How to store currency codes?

Where was 2 options on how to represent the codes in SQL schema:

Store values as ENUM
Store them in a separate EAV table
Store its in the application layer The 3rd option might lead to inconsistency of the data due to potential bugs in application logic. Use of the 1st option would lead to a lot of ALTER TYPE operations in future which involves high risks and reduces flexibility.

Due to a few amount of currently supported currencies the hybrid of 1st and 2nd ways has been considered to be more appropriate. Currently, MySQL MEMORY engine does not support foreign keys checks on inserts and updates so the testing for existence of a currency is moved on application layer.

Also, consider the similar implementation of the operation in PostgreSQL:

The problem is that changing the member list for an ENUM column restructures the entire table with ALTER TABLE, which can be very expensive on resources and time. If you have ENUM('red', 'blue', 'black') but need to change it to ENUM('red', 'blue', 'white'), MySQL needs to rebuild your table and look through every record to check for the now-invalid value 'black'. MySQL is literally dumb and will even do this when all you did was add a new value to the end of the member list! (It is rumored that appending an ENUM member list will be handled better in the future, but I doubt that this is a high priority feature.)

Source

xotonic/mts

Money Transfer Service

Original assignment

About the project

API

Design decisions

How to store currency codes?