volkodava/transfer-api

A simple money transfer REST API implementation with concurrency

transfer-api

A simple money transfer REST API implementation with concurrency

Features

Account API

• Create account
• View account details

Transfer API

• Create transfer
• View transfer details
• List all transfers

Notes

Assumption: solution operates only with debit accounts.

The API designed as a REST APIs with JSON payload.

Transfer processing consists of the following steps (see picture below):

• register transfer (single thread, thread-1)
• validate transfer (single thread, thread-1)
• withdraw money from the source account (single thread, thread-1)
• (partition into blocks of threads)
  • deposit money to the target account (single thread, thread-N)
  • complete transfer, save transfer to in-memory storage (single thread, thread-N)

All these steps implemented as pipeline using rxjava library (see: PipelineExecutor.java#L67).

Java ConcurrentHashMap used as in-memory storage. Withdraw and deposit operations on accounts performed atomically.

Throttling of transfer requests implemented using Java BlockingQueue, which is also used as communication channel between service (accept transfer requests) and pipeline executor (process transfer requests).

Mission critical pieces of application covered with tests.

Tools, Frameworks, Libraries

• Java 11+
• Maven build-automation tool
• javalin lightweight web framework for REST APIs
• guice lightweight dependency injection framework
• rxjava library for asynchronous programming
• Swagger UI library for REST APIs documentation

How to run the demo

• Clone the project from GitHub: git clone git@github.com:volkodava/transfer-api.git

• Build the project: mvn clean install

• Run application: java -jar target/transfer-api-1.0.jar

  • Configuration options can be overridden using environment variables: PORT=8080 BUFFER_SIZE=1000 MAX_THREADS=100 java -jar target/transfer-api-1.0.jar

• Execute unit tests: mvn test

• Execute unit + integration tests: mvn integration-test

Examples

• To create account with initial balance of 1000 EUR:

curl -X POST "http://localhost:8080/accounts" -H  "accept: application/json" -H  "Content-Type: application/json" -d "{\"initialBalance\":1000}"

account id will be returned in the JSON payload, Location header will contain link to account information API.

• To request account information:

curl -X GET "http://localhost:8080/accounts/de07e939-55dd-4086-b559-86db399e51d5" -H  "accept: application/json"

• To transfer 10 EUR from account de07e939-55dd-4086-b559-86db399e51d5 to account 8810b77a-f326-4e27-8e48-4e77a7f27e05:

curl -X POST "http://localhost:8080/transfers" -H  "accept: application/json" -H  "Content-Type: application/json" -d "{\"sourceAccountId\":\"de07e939-55dd-4086-b559-86db399e51d5\",\"targetAccountId\":\"8810b77a-f326-4e27-8e48-4e77a7f27e05\",\"amount\":10}"

account id will be returned in the JSON payload, Location header will contain link to transfer information API.

• To request transfer information:

curl -X GET "http://localhost:8080/transfers/3d98e966-2a39-46e2-9afc-d2b7cf2285d4" -H  "accept: application/json"

CI builds

Link: https://github.com/volkodava/transfer-api/actions

API Documentation

API documentation available after application starts at http://localhost:${PORT}/swagger-ui.

Project Structure

• the code of solution (src/main/java/com/demo):

├── api                        (API implementation)
│   ├── account                (Account API)
│   │   ├── controller         (REST API controller)
│   │   │   └── validator      (User requests validator)
│   │   ├── dto                (Data transfer object(s))
│   │   ├── model              (Business model)
│   │   ├── repository         (Data persistence logic)
│   │   ├── service            (Business logic)
│   │   └── validator          (Data validator(s), used in business logic)
│   ├── common                 (API-level common classes both for account and transfer APIs)
│   ├── exception              (API exceptions)
│   ├── model                  (Shared business model)
│   └── transfer               (Transfer API)
│       ├── controller         ...
│       │   └── validator      ...
│       ├── dto                ...
│       ├── manager            (Business logic to process events, pipeline executor)
│       ├── model              ...
│       ├── repository         ...
│       ├── service            ...
│       ├── store              (Simple event store implementation)
│       └── validator          ...
├── common                     (Application-level shared classes)
└── util                       (Utility classes)

• unit tests (src/test/java/com/demo):

└── api                        (API tests)
    ├── account                (Account API)
    │   └── service            (Business logic tests)
    └── transfer               (Transfer API)
        ├── manager            (Event-processing tests)
        └── service            ...

• integration tests (src/integrationTest/java/com/demo):

└── api                        (API tests)
    ├── account                (Account API)
    │   └── controller         (REST API controller tests)
    ├── common                 (API-level shared classes)
    └── transfer               (Transfer API)
        └── controller         ...