Backend Test

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

1. How to start?

1. In project directory run gradlew clean build on Windows or ./gradlew clean build on UN*X to compile, run tests and package the application.
2. Go to build/libs and run the application via java -jar transfer-service.jar. It will start server on 8080 port (if you want to change the port, you should run application via java -Dhttp.port=other_port_number -jar transfer-service.jar, where other_port_number - number of http port which you prefer).

2. Implementation details

Third-party libraries

1. Javalin - A simple web framework
2. H2 - Java SQL database
3. Flyway - A database migration tool
4. Guice - DI framework
5. Lombok - An annotation-based code generator

Problems

1. H2 is used as data storage. This causes a file system to become clogged with database stuff (doesn't take up much space, because when the application starts and stops, flyway is clearing the data).
2. The application supports transactions only on DAO-level.

3. API

All requests to the API are relative http://localhost:port

Endpoints

Endpoint	Description	Parameters	Success Response/Status Code
POST /account/:balance	Creates new account with specified balance	:balance - balance on account	{ "id": id, "balance": balance } 201 CREATED
GET /account/:id	Returns account by id	:id - account's id	{ "id": id, "balance": balance } 200 OK
DELETE /account/:id	Deletes account by id	:id - account's id	204 NO CONTENT
POST /account/:fromId/transfer/:toId/:amount	Transfers specified amount of money from account to another account	:fromId - account's id that balance will be reduced :toId - account's id that balance will be increased :amount - amount of money which will be transferred	204 NO CONTENT

Possible Errors

Status Code	Error
404 NOT FOUND	Account was not found.
400 BAD REQUEST	The balance value must not be negative.
400 BAD REQUEST	The transfer of money to the same account is not allowed.
400 BAD REQUEST	Not enough money on balance.
4**, 5**	Other unhandled errors.