Money Transfer REST API

The following project is a simple REST API for money transferring between accounts. The application is designed to be a standalone executable program that runs without any additional setup. The application supports creation and retrieval of user accounts. When creating an account, user selects currency in which money will be stored. Additionally, users are allowed to deposit, withdraw or transfer money between two accounts. It is possible to deposit or withdraw money from the user account in a different currency. It is also possible to transfer money between two accounts even if the accounts have different currencies.

Assumptions

• The application is developed without use of heavy frameworks, such as Spring or Hibernate. A micro REST framework Spark was selected because of it's simplicity and support for both Java and Kotlin. The Norm was selected to reduce the ORM overhead and have a thin layer over JDBC instead.
• Money modifications happen synchronously for the sake of testing. In production version the communication would become asynchronous given the high rate of transaction operations. The endpoint would product a Kafka message to the topic, which would have a number of consumers for processing the message.
• All money related operations, including the currency conversion are performed by moneta. The currency conversion works only if the ECB provider or it's substitute is accessible over the network.
• Move resource tests to the integration tests stage as a part of CI/CD pipeline using the Robot framework.
• Endpoints for listing the entities exists for testing purposes only. The production version would include pagination, filtering, etc.

Requirements

Technologies

• Java 11
• Maven 3
• JaCoCo

Frameworks

• Spark
• Gson
• Guice
• Moneta
• H2
• Norm
• SLF4J
• JUnit 4
• Mockito
• Unirest

Data Model

The data model is designed around two entities: accounts and transactions.

Accounts represent the user account in the system that can hold money in the pre-selected currency. Accounts are non-modifiable, except for the amount. The amount is always tight to the account currency.

Transactions can be split into three types: payables, receivables and transfers.

Paybles occur whenever the user withdraws the money from an account or sends the money to another account via transfers.

Receivables occur whenever the user deposits the money to an account or receives the money to another account via transfers.

Transfers occur only as a result of transferring money from one account to another. If the transfer is present, that means there is a corresponding payables and receivables operations from that transfer.

All transactions are immutable and are used as events storage.

The following ERM diagram illustrates the entities and their relationships.

REST API

Postman Collection for Money Transfer REST API

Swagger API Documentation

Account

Method	Path	Intention
POST	/account	Create an account
GET	/account	List all accounts
GET	/account/:id	Get account by id

Transaction

Method	Path	Intention
POST	/transaction/payables	Withdraw the money from an account
POST	/transaction/receivables	Deposit the money to an account
POST	/transaction/transfers	Transfer the money between two accounts
GET	/transaction/payables	List all payables
GET	/transaction/receivables	List all receivables
GET	/transaction/transfers	List all transfers

Build

mvn clean install

Run

java -jar target/moneytransfer-0.1-jar-with-dependencies.jar

The server is running on: http://localhost:4567/