🏦 XYZ MultiFinance (BNPL / Consumer Finance API)

A highly secure, dockerized transaction engine built in Go (Golang) using the Gin Web Framework, GORM, MySQL, and Redis. The system simulates a Buy Now Pay Later (BNPL) / Consumer Credit platform with multi-month credit limit tracking, automated contract installment generation, and payment processing.

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🚀 Key Features

• Credit Limit Engine: Tracks multi-tenor credit limits (1, 2, 3, and 6 months) for each registered user profile.
• Idempotency & Double-Request Prevention: Uses validation for External ID (validated via Redis) to prevent duplicate transactions or double charges.
• Signature-Based Authentication: High-security API validation using request signature checks to verify authorization headers.
• Unique API Call ID Logging: Features a transaction logger where every single request/response is annotated with a unique Api Call ID to simplify database auditing and log tracing.
• Nginx Load Balancing & Docker Compose: Ready for containerized deployment, pre-bundled with an Nginx reverse proxy/load balancer, a MySQL instance, and automated container shell scripts (install.sh, rebuild.sh).

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🧠 Database Schema

The GORM database models the credit lifecycle from user verification to bill payment:

erDiagram
    User {
        string user_id PK
        string user_status
        string nik UK
        string full_name
        string legal_name
        string tempat_lahir
        string tanggal_lahir
        double gaji
        string foto_ktp
        string foto_selfie
    }
    UserLimitBalance {
        string user_id PK "FK references User"
        double limit_one_month
        double limit_two_month
        double limit_three_month
        double limit_sixth
    }
    Transaction {
        string nomor_kontrak PK
        string user_id "FK references User"
        timestamp tanggal_kontrak
        timestamp tanggal_update
        double otr
        double admin_fee
        double harga_aset
        double jumlah_cicilan
        double jumlah_bunga
        double tenor
        double cicilan_bulanan
        string nama_aset
    }
    MonthlyBilling {
        string billing_id PK
        string user_id "FK references User"
        string nama_aset
        double tenor
        double harga_cicilan
        timestamp tanggal_tagihan
    }
    Bill {
        string kode_bayar PK
        string user_id "FK references User"
        double terbayarkan
        double total_tagihan
        double sisa_tagihan_bulanan
        timestamp tanggal_bayar
        double sisa_limit
        string status_bayar
    }

    User ||--|| UserLimitBalance : holds
    User ||--o{ Transaction : executes
    User ||--o{ MonthlyBilling : receives
    User ||--o{ Bill : pays

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🛡️ Transaction Security Pipeline

Every API call to /transaction passes through multiple layers of middleware to guarantee safety and compliance:

graph TD
    A[Client Request] --> B[Signature Verification]
    B -- Invalid Signature --> C[401 Unauthorized]
    B -- Valid Signature --> D[Idempotency Check: External ID]
    D -- Duplicate Found --> E[409 Conflict]
    D -- New Transaction --> F[Limit Availability Check]
    F -- Insufficient Limit --> G[400 Bad Request]
    F -- Approved --> H[Generate Contract Installments & Save to DB]
    H --> I[Log API Call ID & Return Success]

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🛣️ API Endpoints

Category	Method	Endpoint	Description	Security Required
User	POST	/create-user	Create new BNPL user profile	Valid Signature
Transaction	POST	/transaction	Initiate contract transaction & generate installment billings	Signature & Idempotency Key (External ID)

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⚙️ Setup & Installation

1. Pre-requisites

• Docker and Docker Compose installed
• Bash-capable terminal (Linux/macOS or Git Bash/WSL on Windows)

2. Auto-Deployment (Docker Compose)

Use the automated installation script to pull dependencies, set up the Nginx proxy, initialize the MySQL database, and run GORM migrations:

# Run the installation shell script
sh ./install.sh

If you modify the Go source code inside the XYZ-MultiFinance directory, rebuild the app container using:

sh ./rebuild.sh

3. Database Initial Setup & Permissions

1. Access the MySQL container instance:

   docker exec -it mysql mysql -u root -p

   (Password: Password123)
2. Set up the access privilege for the app runner:

   GRANT ALL PRIVILEGES ON *.* TO 'xyz'@'%' IDENTIFIED BY 'Password123' WITH GRANT OPTION;
   FLUSH PRIVILEGES;

4. Postman Collection

For testing the endpoint schemas, request signatures, and payloads, import the Postman collection file included in the root directory: XYZ-MultiFinance.postman_collection.json

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🛡️ License

Distributed under the MIT License. See LICENSE for more information.