Money Transfer API

This project implements a RESTful API (including data model and the backing implementation) for money transfers between accounts.

Summary
Goals
Dependencies
Data Model
Usage
Limitations
TODOs

Summary

Money transfer API with simple and extensible data model.
Supports transfer in one currency.
Transfer is idempotent even in the case of concurrent requests.
3 main APIs: a) Create Account: POST /api/1.0/account, b) Get Account: GET /api/1.0/account?id=<account_id>, and c) Transfer: POST /api/1.0/transfer
Minimal use frameworks/libraries.

Goals

Keep it simple and to the point (e.g. no authentication support).
The datastore should run in-memory for the sake of this test.
The transfer API should be able to handle concurrency and be idempotent.
The final result should be executable as a standalone program (should not require a pre-installed container/server). So, I have not used jetty/tomcat server, simply implemented with basic jdk11 httpserver module.
Demonstrate with tests that the API works as expected.

Dependencies

jdk11
Jackson for JSON (de)serialization
Lombok to simplify POJO classes
vavr for easy functional programming

Data Model

There are only two models: Account and Transaction

@Value
@Builder(toBuilder = true)
public class Account
{
    String id;
    String name;
    Double balance;
    String currency;
    Boolean state;
    String createdAt;
    String updatedAt;
}

@Value
@Builder(toBuilder = true)
public class Transaction
{
    String id;
    String type;
    String requestID;
    TransactionStatus status;
    String createdAt;
    String updatedAt;
    String completedAt;
    String reference;
    Handshake[] handshakes;

    @Value
    @Builder
    public static class Handshake
    {
        String id;
        String accountID;
        Double amount;
        String currency;
    }
}

Simple ConcurrentHashMap is used to store the data and access concurrently. The transfer is designed as a handshake between two account; one handhsake being a debit from one account, and the other being a credit to another account.

Usage

Ensure environment has jdk11 setup. The project has been tested with jdk 11.0.5 on macOS version 10.13.6. But it should work on Linux or Windows environment too.
Clone the repo.
Fire the main class (com.revolut.backend.App), which should start the httpserver.
APIs:

curl -X POST localhost:8000/api/1.0/account -d '{"name": "sagar" , "balance" : 200.0, "currency" : "EUR"}'
Note that balance parameter is optional and if no balance is provided then account starts with a minimum balance of 100.
Response will be a json containing the account id, e.g. {"id": "f0462e0d-ecba-4391-b4ca-971b102a0922"}
To get an account:
curl -X GET localhost:8000/api/1.0/account?id=f0462e0d-ecba-4391-b4ca-971b102a0922
Get Account response will be like:

{
    "id": "f0462e0d-ecba-4391-b4ca-971b102a0922",
    "name": "sagar",
    "balance": 150.5,
    "currency": "EUR"
}

Transfer Request:

curl -X POST localhost:8000/api/1.0/transfer -d 
'{
    "requestID": "req1" , 
    "sourceAccountID" : "f0462e0d-ecba-4391-b4ca-971b102a0922", 
    "targetAccountID" : "57278ec7-8587-4554-ac76-c285adeb640c", 
    "amount" : 50.50,  
    "currency" : "EUR", 
    "reference" : "test transfer"
}'

requestID is a string parameter to ensure that the request is idempotent. sourceAccountID is the UUID of the account from which the money is to be transferred. targetAccountID is the UUID of the account to which the money is to be transferred. amount is a Double value, upto two decimal places, indicating the amount to transfer. currency is the 3-letter ISO code of the currency in which transfer is to happen. Currently the API does not support cross-currency transfer, however the logic can be easily extended to support that without making any change in the data model. reference is the string description of the transfer.

Limitations

As mentioned before that the API currently supports transfer in one currency. But the logic can be easily extended.
Tried to package jar with dependencies using maven-assembly-plugin and maven-shade-plugin but still the creation of fat jar was failing. Hence, there is no executable. But, the application runs from the main class.

TODOs

Build an executable jar with all the dependencies.
Add more logging.

codope / revolut-backend