added legion project for flyweight design pattern

Author: Astrogeek77

Design Patterns/Chain of Responsibility/sentinel/HELP.md

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+# Getting Started
+
+### Reference Documentation
+For further reference, please consider the following sections:
+
+* [Official Apache Maven documentation](https://maven.apache.org/guides/index.html)
+* [Spring Boot Maven Plugin Reference Guide](https://docs.spring.io/spring-boot/4.0.1/maven-plugin)
+* [Create an OCI image](https://docs.spring.io/spring-boot/4.0.1/maven-plugin/build-image.html)
+* [Spring Web](https://docs.spring.io/spring-boot/4.0.1/reference/web/servlet.html)
+
+### Guides
+The following guides illustrate how to use some features concretely:
+
+* [Building a RESTful Web Service](https://spring.io/guides/gs/rest-service/)
+* [Serving Web Content with Spring MVC](https://spring.io/guides/gs/serving-web-content/)
+* [Building REST services with Spring](https://spring.io/guides/tutorials/rest/)
+
+### Maven Parent overrides
+
+Due to Maven's design, elements are inherited from the parent POM to the project POM.
+While most of the inheritance is fine, it also inherits unwanted elements like `<license>` and `<developers>` from the parent.
+To prevent this, the project POM contains empty overrides for these elements.
+If you manually switch to a different parent and actually want the inheritance, you need to remove those overrides.
+

Design Patterns/Chain of Responsibility/sentinel/README.md

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+# Sentinel: Chain of Responsibility Risk Engine 🛡️
+
+> A modular banking transaction validation system demonstrating the **Chain of Responsibility** design pattern to achieve loose coupling and fail-fast processing.
+
+## 📖 Project Overview
+**Sentinel** is a financial middleware service designed to validate transactions through a dynamic pipeline of security checks. 
+
+Instead of a monolithic `validateTransaction()` method with complex nested `if-else` logic, this project utilizes the **Chain of Responsibility** pattern. Each validation step (Sanity, Blacklist, Fraud) is an independent component that can be added, removed, or reordered without modifying the core service logic.
+
+## 🏗️ Design Pattern: Chain of Responsibility
+The application routes a request through a chain of handlers. If any handler fails the request, the process stops immediately (Fail-Fast).
+
+```mermaid
+graph LR
+    A[Request] --> B(Sanity Check)
+    B -- Pass --> C(Blacklist Check)
+    C -- Pass --> D(Fraud Check)
+    D -- Pass --> E[Approved ✅]
+    B -- Fail --> F[Rejected ❌]
+    C -- Fail --> F
+    D -- Fail --> F
+```
+## 🗝️ Key Components
+1.  **`RiskHandler` (Abstract Handler):** Defines the contract and manages the link to the `next` handler in the chain.
+2.  **`SanityCheckHandler`:** The first line of defense; validates basic constraints (e.g., Amount must be > 0).
+3.  **`BlacklistHandler`:** Simulates a security check against a database of banned users.
+4.  **`LargeTransactionHandler`:** Applies business logic to flag or reject transactions exceeding specific financial thresholds.
+5.  **`TransactionService`:** Orchestrates the chain creation using Spring's Dependency Injection (`@PostConstruct`).
+
+## 🛠️ Tech Stack
+* **Core:** Java 21, Spring Boot 3.x
+* **Architecture:** Chain of Responsibility Design Pattern
+* **Build Tool:** Maven
+* **Testing:** Postman / cURL
+
+## 🏃 How to Run
+
+1.  **Clone the repository:**
+    ```bash
+    git clone [https://github.com/yourusername/sentinel.git](https://github.com/yourusername/sentinel.git)
+    ```
+2.  **Navigate to the project directory:**
+    ```bash
+    cd sentinel
+    ```
+3.  **Run the application:**
+    ```bash
+    mvn spring-boot:run
+    ```
+4.  The server will start on `http://localhost:8080`.
+
+## 🔌 API Endpoints (Testing Guide)
+
+### 1. Successful Transaction (Happy Path)
+* **Request:** `POST /api/transactions/pay?user=John&amount=500`
+* **Result:** `Transaction Approved!`
+* **Console Logs:**
+    ```text
+    ✅ Sanity Check Passed
+    ✅ Blacklist Check Passed
+    ✅ Limit Check Passed
+    ```
+
+### 2. Validation Failure (Fail-Fast Demo)
+* **Request:** `POST /api/transactions/pay?user=John&amount=-100`
+* **Result:** `Transaction Rejected.`
+* **Console Logs:**
+    ```text
+    ❌ Validation Failed: Amount must be positive.
+    (Processing stopped here. Blacklist/Limit checks were NEVER executed)
+    ```
+
+### 3. Security Failure (Blacklist Demo)
+* **Request:** `POST /api/transactions/pay?user=BANNED_USER&amount=500`
+* **Result:** `Transaction Rejected.`
+* **Console Logs:**
+    ```text
+    ✅ Sanity Check Passed
+    ❌ Validation Failed: Account is blacklisted.
+    ```
+---
+Created by Gautam Jain to demonstrate Design Pattern implementation in Spring Boot.