# multiple datasources using Spring Boot

This Spring Boot application demonstrates how to work with multiple datasources and implement efficient parallel data fetching.

***

## Features

* Multiple database connections (PostgreSQL and MySQL)
* Parallel data fetching using CompletableFuture
* Database migrations using both Liquibase and Flyway
* Exception handling with Problem Details
* Virtual Threads support
* Swagger API documentation

***

## Architecture

### Database Setup

* **PostgreSQL Database**: Stores member information
  * Managed by Liquibase migrations
  * Uses sequence-based ID generation
* **MySQL Database**: Stores cardholder information
  * Managed by Flyway migrations
  * Uses identity-based ID generation

### Key Components

* **DetailsService**: Implements parallel data fetching
  * Uses CompletableFuture for asynchronous operations
  * Implements timeout handling (5 seconds)
  * Provides comprehensive error handling
* **Data Models**:
  * `Member`: Core member information (PostgreSQL)
  * `CardHolder`: Card-related information (MySQL)
* **Exception Handling**:
  * `CustomServiceException`: For service-layer errors
  * `MemberNotFoundException`: For missing member scenarios
  * Problem Details support for standardized error responses

## Configuration

### PostgreSQL Configuration

```properties
spring.datasource.url=jdbc:postgresql://localhost:5432/memberdb
spring.datasource.username=postgres
spring.datasource.password=postgres
```

### MySQL Configuration

```properties
app.datasource.cardholder.url=jdbc:mysql://localhost:3306/cardholderdb
app.datasource.cardholder.username=user
app.datasource.cardholder.password=password
```

## Getting Started

1. Start PostgreSQL and MySQL databases
2. Configure database connections in `application.properties`
3. Run the application: The databases will be automatically initialized
4. Access Swagger UI: <http://localhost:8080/swagger-ui.html>

## Implementation Notes

* Database migrations run automatically on startup
* Data bootstrapping occurs via `ApplicationReadyEvent`
* Parallel data fetching implemented using Spring's @Qualifier("applicationTaskExecutor")
* Virtual Threads enabled for improved scalability
* HikariCP connection pooling with optimized settings
* Automatic commit disabled for better transaction control

## API Endpoints

* `GET /api/details/{memberId}`: Fetches member details from both databases in parallel
  * Returns combined information from both datasource's
  * Implements timeout handling (5 seconds)
  * Returns standardized error responses using Problem Details

## Testing

* Integration tests demonstrate concurrent request handling
* MockMvcTester used for API testing
* Testcontainers with parallel startup for database testing


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