feature: Add Monolithic Architecture pattern (#2664)

monolithic-architecture/README.md

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+---
+
+title: Monolithic Architecture 
+category: Architectural  
+language: en  
+tag:
+- monolithic
+
+---
+
+## Intent
+
+In a monolithic architecture, services are interconnected and share data within the same system boundary. This approach centralizes the e-commerce operations, ensuring data integrity and simplifying cross-service updates.
+
+## Explanation
+
+Real world example
+
+> Consider our e-commerce application which encompasses two primary functionalities: managing orders and customer information. Specifically, the `Order module` is responsible for storing and processing order details while the `Customer module` keeps track of customer data.  
+> When a customer places a new order, it becomes crucial for the `Order module` to communicate with the `Customer module`. This is to check if the order being placed doesn't exceed the customer's credit limit.  
+> Instead of relying on distributed data storage or external calls, our monolithic architecture enables the `Order Service` and `Customer module` to seamlessly interact with each other's data directly, ensuring the order's adherence to the customer's credit limit.
+
+In plain words
+
+> Monolithic architecture bundles different modules together under one system, allowing them to easily access and modify shared data, ensuring data consistency.
+
+Wikipedia says
+
+> In software engineering, a monolithic application describes a software application that is designed as a single service.
+
+**Programmatic Example**
+
+Let's start from the layout of the shared database. Here's our `localdb.txt`.
+
+```txt
+CUSTOMERS  
+ID, CREDIT_LIMIT  
+234, 60000.0  
+235, 1000.0  
+
+ORDERS  
+ID, CUSTOMER_ID, STATUS, TOTAL  
+4567, 234, ACCEPTED, 54044.30
+```  
+
+Next we can introduce our `CustomerModule`. It contains `getCustomer`, `updateCreditLimit`   and `createCustomer`  for calling customer module.
+
+```java  
+/**
+ * Module interface defining the operations related to customers.
+ */
+public interface CustomerModuleInterface {
+
+    /** Path to the database file where customer data is stored. */
+    String DB_FILE = "monolithic-architecture/etc/localdb.txt";
+
+    /**
+     * Retrieves the customer details for the given customer ID.
+     *
+     * @param customerId The ID of the customer whose details are to be fetched.
+     * @return An array containing the customer details.
+     * @throws Exception If any error occurs while fetching the data.
+     */
+    String[] getCustomerById(int customerId) throws Exception;
+
+    /**
+     * Updates the credit limit for the customer with the specified ID.
+     *
+     * @param customerId The ID of the customer whose credit limit needs to be updated.
+     * @param newCreditLimit The new credit limit value to be set for the customer.
+     * @throws Exception If any error occurs while updating the credit limit.
+     */
+    void updateCreditLimit(int customerId, double newCreditLimit) throws Exception;
+
+    /**
+     * Creates a new customer with the specified initial credit limit.
+     *
+     * @param creditLimit The initial credit limit for the new customer.
+     * @return The ID of the newly created customer.
+     * @throws Exception If any error occurs while creating a new customer.
+     */
+    String newCustomer(double creditLimit) throws Exception;
+}
+
+```  
+
+Then we can introduce our `OrderModule`. It contains find method `getOrderTotalByCustomerId` and  create method `makeOrder`  for calling order module.
+
+```java  
+/**
+ * Module interface defining the operations related to orders.
+ */
+public interface OrderModuleInterface {
+
+    ICustomeModule customerModule = new CustomerModule();
+    String DB_FILE = "monolithic-architecture/etc/localdb.txt";
+
+    /**
+     * Create a new order for a given customer with a specified total.
+     *
+     * @param customerId The ID of the customer making the order.
+     * @param total The total amount for the order.
+     * @return The ID of the newly created order.
+     * @throws Exception If there's an error during order creation.
+     */
+    String makeOrder(int customerId, double total) throws Exception;
+
+    /**
+     * Fetches the total of all orders made by a specific customer.
+     *
+     * @param customerId The ID of the customer whose order totals are to be fetched.
+     * @return An Optional containing the order totals in String array format.
+     * @throws Exception If there's an error during data retrieval.
+     */
+    String[] getOrderTotalByCustomerId(int customerId) throws Exception;
+}
+```  
+
+Now running the `App` for interactive functionalities calling :
+
+```bash
+Choose an option:
+1. Get Customer by ID
+2. Update Customer Credit Limit
+3. Create New Customer
+4. Make an Order
+5. Get Order Total by Customer ID
+Q. Quit
+Your choice: 1
+Enter Customer ID: 234
+Customer Info: 234, 60000.0
+
+Choose an option:
+1. Get Customer by ID
+2. Update Customer Credit Limit
+3. Create New Customer
+4. Make an Order
+5. Get Order Total by Customer ID
+Q. Quit
+Your choice: 2
+Enter Customer ID: 234
+Enter New Credit Limit: 60000.0
+Credit Limit Updated Successfully!
+
+Choose an option:
+1. Get Customer by ID
+2. Update Customer Credit Limit
+3. Create New Customer
+4. Make an Order
+5. Get Order Total by Customer ID
+Q. Quit
+Your choice: 
+```
+
+
+## Class diagram
+
+![alt text](./etc/monolithic-architecture.png "Microservices : shared-database")
+
+## Applicability
+
+Use the Monolithic Architecture pattern when you want to ensure data consistency and require high application performance.
+
+## Credits
+
+* [Wikipedia - monolithic architecture](https://en.wikipedia.org/wiki/Monolithic_application)
+* [Monolithic architecture (microservices.io)](https://microservices.io/patterns/monolithic.html)

monolithic-architecture/etc/localdb.txt

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+CUSTOMERS
+ID, CREDIT_LIMIT
+234, 60000.0
+235, 1000.0
+
+ORDERS
+ID, CUSTOMER_ID, STATUS, TOTAL
+4567, 234, ACCEPTED, 54044.30

monolithic-architecture/etc/outputUML.puml

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+@startuml
+interface com.iluwater.monolithic.architecture.ICustomerService {
+~ {static} String DB_FILE
+~ String[] getCustomerById(int)
+~ void updateCreditLimit(int,double)
+~ String newCustomer(double)
+}
+class com.iluwater.monolithic.architecture.App {
+- {static} ICustomerService customerService
+- {static} IOrderService orderService
++ {static} void main(String[])
+- {static} void displayMenu()
+}
+interface com.iluwater.monolithic.architecture.IOrderService {
+~ {static} ICustomerService customerService
+~ {static} String DB_FILE
+~ String makeOrder(int,double)
+~ String[] getOrderTotalByCustomerId(int)
+}
+class com.iluwater.monolithic.architecture.CustomerService {
+- String[] findCustomerById(int)
+- void setCreditLimit(int,double)
+- String createCustomer(double)
++ String[] getCustomerById(int)
++ void updateCreditLimit(int,double)
++ String newCustomer(double)
+}
+class com.iluwater.monolithic.architecture.OrderService {
+- String[] findOrderTotalByCustomerId(int)
+- double calculateAllOrders(String[])
+- String createOrder(int,double)
+- int insertOrder(List<String>)
++ String makeOrder(int,double)
++ String[] getOrderTotalByCustomerId(int)
+}
+
+
+com.iluwater.monolithic.architecture.ICustomerService <|.. com.iluwater.monolithic.architecture.CustomerService
+com.iluwater.monolithic.architecture.IOrderService <|.. com.iluwater.monolithic.architecture.OrderService
+
+com.iluwater.monolithic.architecture.IOrderService --> com.iluwater.monolithic.architecture.ICustomerService : uses
+@enduml

monolithic-architecture/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>monolithic-architecture</artifactId>
+
+    <properties>
+        <maven.compiler.source>17</maven.compiler.source>
+        <maven.compiler.target>17</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-inline</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+    </dependencies>
+
+</project>

monolithic-architecture/src/main/java/com/iluwater/monolithic/architecture/App.java

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+package com.iluwater.monolithic.architecture;
+
+import java.util.Scanner;
+
+
+/**
+ * The main application class providing a command-line interface to interact
+ * with customer and order modules.
+ * <p>
+ * The application provides menu-driven options to the user for performing various
+ * operations like fetching customer details by ID, updating credit limits,
+ * creating new customers, making orders, and fetching order totals for a given customer.
+ * </p>
+ * <p>
+ * The underlying data operations are performed through {@link CustomerModuleInterface} and
+ * {@link OrderModuleInterface} implementations.
+ * </p>
+ */
+public class App {
+
+  /**
+   * An instance of the customer module for performing customer-related operations.
+   */
+  private static final CustomerModuleInterface customerModule = new CustomerModule();
+
+  /**
+   * An instance of the order module for performing order-related operations.
+   */
+  private static final OrderModuleInterface orderModule = new OrderModule();
+
+  /**
+   * The main entry point of the application.
+   *
+   * @param args Command-line arguments (not used in the current implementation).
+   */
+  public static void main(String[] args) {
+    Scanner scanner = new Scanner(System.in);
+    while (true) {
+      displayMenu();
+      String choice = scanner.nextLine();
+
+      try {
+        switch (choice) {
+          case "1":
+            System.out.print("Enter Customer ID: ");
+            int customerId = scanner.nextInt();
+            scanner.nextLine(); // Consume newline
+            String[] customer = customerModule.getCustomerById(customerId);
+            System.out.println("Customer Info: " + String.join(", ", customer));
+            break;
+
+          case "2":
+            System.out.print("Enter Customer ID: ");
+            customerId = scanner.nextInt();
+            System.out.print("Enter New Credit Limit: ");
+            double creditLimit = scanner.nextDouble();
+            scanner.nextLine(); // Consume newline
+            customerModule.updateCreditLimit(customerId, creditLimit);
+            System.out.println("Credit Limit Updated Successfully!");
+            break;
+
+          case "3":
+            System.out.print("Enter Initial Credit Limit: ");
+            creditLimit = scanner.nextDouble();
+            scanner.nextLine(); // Consume newline
+            String newCustomerId = customerModule.newCustomer(creditLimit);
+            System.out.println("New Customer Created with ID: " + newCustomerId);
+            break;
+
+          case "4":
+            System.out.print("Enter Customer ID: ");
+            customerId = scanner.nextInt();
+            System.out.print("Enter Order Amount: ");
+            double amount = scanner.nextDouble();
+            scanner.nextLine(); // Consume newline
+            String orderId = orderModule.makeOrder(customerId, amount);
+            System.out.println("Order Created Successfully with ID: " + orderId);
+            break;
+
+          case "5":
+            System.out.print("Enter Customer ID: ");
+            customerId = scanner.nextInt();
+            scanner.nextLine(); // Consume newline
+            String[] orderTotal = orderModule.getOrderTotalByCustomerId(customerId);
+            System.out.println("Order Total: " + String.join(", ", orderTotal));
+            break;
+
+          case "Q":
+            System.out.println("Exiting the application.");
+            scanner.close();
+            return;
+
+          default:
+            System.out.println("Invalid choice. Please select a valid option.");
+        }
+      } catch (Exception e) {
+        System.out.println("An error occurred: " + e.getMessage());
+      }
+    }
+  }
+
+  /**
+   * Displays the main menu options to the user.
+   */
+  private static void displayMenu() {
+    System.out.println("\nChoose an option:");
+    System.out.println("1. Get Customer by ID");
+    System.out.println("2. Update Customer Credit Limit");
+    System.out.println("3. Create New Customer");
+    System.out.println("4. Make an Order");
+    System.out.println("5. Get Order Total by Customer ID");
+    System.out.println("Q. Quit");
+    System.out.print("Your choice: ");
+  }
+
+}