Tasks 2–5: Kafka ingestion, H2/JPA persistence, Incentive API integration, and /balance endpoint

[muqtadir66]

Implements Tasks 2–5 of the Forage J.P. Morgan SE simulation by building out Midas Core—a Spring Boot service that ingests, validates, persists, and enriches transactions, and exposes balances via a small read API.

Task 2 – Kafka Integration

• Added a Spring Kafka listener to consume Transaction messages from the configured topic.
• Introduced Kafka configuration (consumer/producer factories, JSON (de)serializers, and a ConcurrentKafkaListenerContainerFactory) so the listener receives Transaction objects directly.
• Verified with Embedded Kafka (TaskTwoTests) and used breakpoints to capture the first four transaction amounts.

Task 3 – H2/JPA Persistence

• Validated senderId/recipientId and sufficient funds.
• Applied balance updates atomically in a single DB transaction.
• Persisted each accepted transaction as a TransactionRecord with many-to-one links to UserRecord (sender/recipient).

Task 4 – Incentive REST API

• Posted validated Transaction to the external Incentive API; parsed Incentive { amount }.
• Credited recipients with amount + incentive while senders pay amount only.
• Stored incentive on each TransactionRecord.

Task 5 – Read API (/balance)

• Exposed GET /balance?userId=... returning JSON Balance.
• Returns 0 for unknown users.

Changes (by area):

Kafka (Task 2):

• src/main/java/com/jpmc/midascore/config/KafkaConfig.java
  Consumer/producer factories, JSON (de)serializers, and ConcurrentKafkaListenerContainerFactory (topic from application.yml).

• src/main/java/com/jpmc/midascore/component/TransactionListener.java
  @KafkaListener that receives Transaction and delegates to the service layer (used for TaskTwoTests debugging).

Domain & Persistence (Task 3):

• src/main/java/com/jpmc/midascore/service/TransactionService.java
  Validation → balance updates → persistence (later extended for Task 4).

• src/main/java/com/jpmc/midascore/entity/TransactionRecord.java
  JPA entity: sender, recipient (many-to-one), amount, incentive.

• src/main/java/com/jpmc/midascore/repository/TransactionRepository.java
  Repository for TransactionRecord.

External REST (Task 4):

• src/main/java/com/jpmc/midascore/config/RestConfig.java
  RestTemplate bean (short timeouts).

• src/main/java/com/jpmc/midascore/foundation/Incentive.java
  DTO for Incentive API response.

Read API (Task 5):

• src/main/java/com/jpmc/midascore/controller/BalanceController.java
  GET /balance?userId= → returns Balance { amount } JSON.

Config:

• src/main/resources/application.yml
  Kafka topic property; incentive API URL; JPA/H2 settings; server port for tests.

(Existing scaffold files such as foundation/Transaction.java, foundation/Balance.java, entity/UserRecord.java, repository/UserRepository.java, and MidasCoreApplication.java remain in place.)

Notes:

• Kafka gives decoupling and burst tolerance; the listener consumes JSON → Transaction via configured deserializers.
• JPA/H2 ensures transactional integrity for balance updates and keeps the DB backend swappable.
• Incentive API failures default to incentive 0 (no extra debit to sender).
• /balance returns 0 for unknown users and preserves Balance.toString() format required by tests.