MVC Design Pattern: The Essential Guide

So, you have completed the initial steps of gathering requirements, designing, and getting your team’s nod, you’re ready to dive into coding. But this task goes beyond simple coding challenges. You’re about to build a service that interacts with databases, APIs, and other services.

So, How will you effectively organize your code?

Let me introduce you to the Model-View-Controller (MVC) design pattern. This design pattern offers a systematic approach to organizing code in a way that separates concerns, simplifies development, and enhances collaboration among teams.

In this article, we will understand the MVC pattern, its components, and their interaction, showcasing how MVC can revolutionize your projects. Whether you’re new to coding or just want to learn more about MVC, we’re going to explore why it’s such a helpful tool for making great software.

Let’s dive in!

What is Model View Controller?

Model-View-Controller (MVC) is a software architectural pattern commonly used for development that divides an application into three interconnected components. This separation helps in managing complex applications, as it separates the internal representations of information from the ways that information is presented to and accepted by the user. The three components of the MVC pattern are:

1. Model: The Model component corresponds to all the data-related logic that the user works with. This can represent either the data that is being transferred between the View and Controller components or any other business logic-related data.
   For example, a Model might represent a character in a game, a product in an inventory, or a record in a database.
2. View: The View component is used for all the UI logic of the application. It presents the model data to the user and specifies exactly how that data should be displayed. If the Model data changes, the View must update its presentation as needed. This can involve a simple refresh of part of the UI or a complete change in how the data is displayed.
3. Controller: The Controller acts as an interface between the Model and View components to process all the business logic and incoming requests, manipulate data using the Model component, and interact with the Views to render the final output. The Controller receives input from users via the View, then processes the user’s data with the help of Model components and passes the results back to the View.

Why Should You Use MVC?

The MVC pattern decouples these major components, allowing efficient code reuse and parallel development. For example, developers can work on the View, while others can work on the Controller logic at the same time, potentially on different workstations. This separation of concerns provides a flexible approach to developing web applications, making it easier to update or modify either the visual appearance or the backend database without affecting the other.

Extended MVC architecture

An extended MVC architecture often includes additional layers or components beyond the traditional Model, View, and Controller to handle specific aspects of a web application more efficiently. These extensions can include Services (or Service Layer) and Repositories, among others.

Components of the Extended MVC pattern:

1. Model: The model component will consist of the data structure of the table entity or any other object.
2. Repository: Abstracts the logic for accessing the database. It provides methods for fetching, updating, and storing user data without exposing the database details to other components.
3. Service: Contains the business logic. It acts as a mediator between the Controller and the Repository, ensuring that the data is validated, processed, and passed to the repository layer correctly.
   For instance, before updating details in the database, the Service might validate request details.
4. Controller: Handles HTTP requests from the user’s browser, interacting with the Service’s to retrieve or update details.
   For example, when a user submits a form to update their profile, the UserController receives this request.
5. View: Displays information on a webpage. This could include forms for entering or updating details and sections for displaying information.

This extended MVC architecture allows for a clear separation of concerns, making it easier to manage complex application logic, maintain the codebase, and scale the application. Each component has a specific responsibility, promoting cleaner, more modular, and more testable code.

MVC Implementation

Let's understand MVC with the help of an example. For the sake of this example, we will create a product listing API. For the purpose of this example, we will use Java as our language of choice.

Model

The Product class represents the data model in the MVC pattern. It encapsulates the properties of a product, such as id, name, count, price, and description, along with getters and setters. This class is responsible for holding the product data but not for business logic or data access.

public class Product {
    String id;
    String name;
    int count;
    double price;
    String description;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getCount() {
        return count;
    }

    public void setCount(int count) {
        this.count = count;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

Repository

The ProductRepository acts as a data access layer. It's not a part of the classic MVC but is used here to abstract the details of data access from the model. It provides methods to retrieve and save Product objects to a database. For instance, getProduct fetches a product by it’s id, and createOrUpdateProduct saves or updates a product in the database.

public class ProductRepository {

  /**
   * Retrieves a product based on its ID.
   *
   * @param productId The ID of the product to be retrieved.
   * @return The product corresponding to the product ID.
   * @throws ProductNotFoundException If the product does not exist for the given ID.
   */
  public Product getProduct(final String productId) {}

  /**
   * Creates or updates a product. Generates an ID if the product does not have one.
   *
   * @param product The product to be created or updated.
   * @return The created or updated product.
   */
  public Product createOrUpdateProduct(Product product) {}
}

Service

The ProductService sits between the controller and the repository (and indirectly, the model). It contains business logic that operates on the data model. In this example, getProduct retrieves a product by ID, handling any exceptions, and updateProductQuantity updates a product's quantity, including logic to check for sufficient quantity before proceeding with the update. This layer ensures that the business rules are encapsulated and separated from the web layer and data access layer.

@AllArgsConstructor
public class ProductService {
  private static final Logger LOG = LogManager.getLogger(ProductService.class);
  private final ProductRepository productRepository;

  /**
   * Get product by ID.
   *
   * @param productId The ID of the product to retrieve.
   * @return The product with the specified ID.
   * @throws ProductNotFoundException if the product with the given ID is not found.
   */
  public Product getProduct(final String productId) throws ProductNotFoundException {
    try {
      return productRepository.getProduct(productId);
    } catch (ProductNotFoundException e) {
      LOG.error("Error fetching product with ID {}: {}", productId, e.getMessage());
      throw e;
    }
  }

  /**
   * Update product quantity based on consumed quantity.
   *
   * @param productId The ID of the product to update.
   * @param consumedQuantity The quantity to consume.
   * @return true if the update is successful, false otherwise.
   * @throws InsufficientQuantityException if the consumed quantity is greater than available
   *     quantity.
   * @throws ProductNotFoundException if the product with the given ID is not found.
   */
  public boolean updateProductQuantity(final String productId, final int consumedQuantity)
      throws InsufficientQuantityException, ProductNotFoundException {
    try {
      Product product = getProduct(productId);

      if (product.getCount() < consumedQuantity) {
        throw new InsufficientQuantityException(
            "Required Product quantity is not available for product " + product.getName());
      }

      product.setCount(product.getCount() - consumedQuantity);
      return true;
    } catch (ProductNotFoundException e) {
      LOG.error("Error updating product quantity for ID {}: {}", productId, e.getMessage());
      throw e;
    }
  }
}

Controller

The ProductController is part of the MVC pattern, acting as the intermediary between the view (not explicitly shown in the code but would be the user interface or API response structure in a web application) and the model/service layers. It processes user requests, interacts with them ProductService to retrieve or update data, and prepares the response. The getProduct method in the controller, for example, calls ProductService to fetch a product by its ID and returns this product to the view.

public class ProductController {
  private final ProductService productService;

  /**
   * Constructor for initializing the controller with necessary services.
   *
   * @param shoppingService The shopping service.
   * @param orderService The order service.
   * @param productService The product service.
   */
  public ProductController(ProductService productService) {
    this.productService = productService;
  }

  /**
   * Retrieves product information based on the product ID.
   *
   * @param productId The ID of the product.
   * @return The product information.
   * @throws Exception If there's an issue retrieving the product.
   */
  public Product getProduct(final String productId) throws Exception {
    return productService.getProduct(productId);
  }
}

View

In contemporary web applications, the View typically exists in a distinct layer, manifesting as a website, mobile application, or any user-facing interface. This layer consumes API responses provided by the Controller, enabling a dynamic presentation of data to the user. It leverages modern technologies and frameworks to render the user interface based on the data received, ensuring an engaging and interactive experience. The separation of the View into its own layer facilitates flexibility in development and design, allowing for seamless updates and enhancements to the user interface without directly impacting the backend logic.

Workflow Summary:

1. User Interaction: A user interacts with the view (e.g., a webpage or API endpoint) to request a specific product by its ID.
2. Controller Action: The request is routed to the ProductController, which call’s the ProductService to handle the request.
3. Business Logic: ProductService retrieves the product details by calling ProductRepository, applying any necessary business logic.
4. Data Access: ProductRepository interacts with the database to fetch the requested product.
5. Response Preparation: The product information is returned back up the chain: from ProductRepository to ProductService, then to ProductController, and finally to the user through the view.

To conclude, the MVC pattern champions the separation of concerns, rendering applications more maintainable, scalable, and comprehensible. Each component is tasked with a distinct responsibility: the Model outlines the data structure; the Repository orchestrates direct data transactions; the Service layer implements business logic; and the Controller directs the interaction between user requests, business logic, and subsequent responses.

Having smaller separate components helps to scale, maintain, and distribute workload within in team.

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