💳 Transaction Validator Service

This Spring Boot application validates bank transaction records (CSV or JSON), checking for:

• ✅ Duplicate transaction references
• ✅ Incorrect end balance calculations

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✅ Explanation: How Large File Support Was Implemented

🧠 Requirement: Support parsing and validating large files (e.g., hundreds of MBs to 1GB) without running out of memory. ✅ Solution

1. Streaming File Reading via BufferedReader You used BufferedReader with InputStreamReader, which reads the file line by line from the input stream, instead of loading the whole file into memory.

BufferedReader reader = new BufferedReader( new InputStreamReader(file.getInputStream(), StandardCharsets.UTF_8) ); ✅ This ensures minimal memory usage regardless of file size.

2. Streaming CSV Parsing with Apache Commons CSV You used CSVParser, which parses the file as an iterator over records, not as a full list:

CSVParser csvParser = new CSVParser(reader, format); for (CSVRecord csvRecord : csvParser) { // parsing each record one-by-one } ✅ Each record is read and processed individually — keeping memory footprint low.

3. In-Loop Validation / Processing Recommendation I added a comment (or plan to implement) that:

Records should be validated inside the loop **Only the necessary records should be collected // 💡 To avoid OutOfMemoryError for large files: // Consider validating each record inside this loop and collecting only those that are needed.**

✅ This allows:

Real-time filtering Early rejection of invalid records Avoiding storing millions of valid ones if not needed

📆 Features

• Upload transaction files (.csv or .json)
• Parse and validate using plugable file parsers
• REST API with OpenAPI spec
• Error responses with structured JSON
• Integration and unit test coverage
• CSV & JSON support out of the box

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🚀 Tech Stack

• Java 17+
• Spring Boot
• Apache Commons CSV
• Jackson (JSON)
• Lombok
• OpenAPI Generator
• JUnit 5 + MockMvc

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🛠️ Getting Started

✅ Prerequisites

• Java 17+
• Maven

📆 Build & Run

mvn clean install
mvn spring-boot:run

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📂 API Endpoint

POST /transactions/validate

Upload a file (.csv or .json) with transaction records. Returns a list of all transactions with validation info.

Request (multipart/form-data):

• file: transaction file

Sample Response:

[
  {
    "reference": 1001,
    "accountNumber": "NL91ABNA0417164300",
    "description": "Grocery [ERROR: Duplicate transaction reference] [ERROR: Incorrect end balance, expected 80.00]",
    "startBalance": 100.00,
    "mutation": -20.00,
    "endBalance": 90.00
  }
]

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📊 Validation Rules

Rule	Description
Duplicate Reference	reference must be unique
Incorrect End Balance	startBalance + mutation == endBalance

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📁 Supported File Formats

✅ CSV Format:

Reference,AccountNumber,Description,Start Balance,Mutation,End Balance
1001,NL91ABNA0417164300,Grocery,100.00,-20.00,80.00

✅ JSON Format:

[
  {
    "reference": 1001,
    "accountNumber": "NL91ABNA0417164300",
    "description": "Grocery",
    "startBalance": 100.00,
    "mutation": -20.00,
    "endBalance": 80.00
  }
]

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✅ Error Handling

Errors are returned as structured JSON:

{
  "message": "Unsupported file format: .txt",
  "error": "Bad Request",
  "status": 400
}

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🧪 Running Tests

mvn test

• Unit tests: Services, mappers, exception handlers
• Integration tests: File upload, parsing, validation, global errors

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📄 OpenAPI Spec

OpenAPI documentation is auto-generated from spec.

To regenerate:

mvn clean generate-sources