Master Software Architecture: Module 4 – Application Design Fundamentals | Complete Guide with C#, ASP.NET & SQL Server Examples

 

📑 Table of Contents

1. Introduction to Application Design Fundamentals

2. Requirements Analysis & Functional Decomposition
   2.1 Business Requirements vs Technical Requirements
   2.2 Functional Decomposition Techniques
   2.3 Example: E-Commerce Order Management in ASP.NET & SQL Server

3. UML Diagrams in Application Design
   3.1 Class Diagrams
   3.2 Sequence Diagrams
   3.3 Use Case Diagrams
   3.4 Component Diagrams
   3.5 Deployment Diagrams
   3.6 Example: UML for Online Banking System

4. Domain-Driven Design (DDD) Basics
   4.1 What is DDD?
   4.2 Entities, Value Objects, Aggregates
   4.3 Repositories & Services
   4.4 Example: Customer & Invoice Domain in ASP.NET

5. Modeling Entities, Relationships & Aggregates
   5.1 Entity Relationship Modeling
   5.2 Aggregate Root & Boundaries
   5.3 Example: Sales Order Aggregate in ERP
   5.4 Best Practices & Pitfalls

6. Data Flow & Control Flow Modeling
   6.1 Data Flow Diagrams (DFD)
   6.2 Control Flow Diagrams (CFD)
   6.3 Example: Payment Gateway Workflow
   6.4 Exception Handling & Alternatives

7. Best Practices in Application Design
   7.1 Layered Architecture (N-tier)
   7.2 SOLID Principles
   7.3 Error Handling & Logging
   7.4 Security Considerations

8. Pros, Cons & Alternatives of Application Design Approaches
   8.1 UML vs Code-First Design
   8.2 DDD vs Traditional ER Modeling
   8.3 Centralized vs Decentralized Architectures

9. Conclusion & Key Takeaways

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1. Introduction to Application Design Fundamentals

Application design is the bridge between business requirements and working software. It ensures that applications are scalable, maintainable, and aligned with business goals. In this module, we cover the foundations that every software architect and senior developer must master.

Example:

• An ERP System must handle sales, purchase, inventory, accounting, and reporting. Without proper design fundamentals, such systems become complex, slow, and error-prone.

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2. Requirements Analysis & Functional Decomposition

2.1 Business vs Technical Requirements

• Business Requirements: What stakeholders want (e.g., “System should allow customers to place an order”).

• Technical Requirements: How the system achieves it (e.g., “Use SQL Server stored procedures to handle order transactions”).

2.2 Functional Decomposition

Breaking down complex requirements into smaller, manageable functions.

Example Breakdown – Online Order System:

• Place Order

  • Select Product

  • Add to Cart

  • Checkout

• Payment

  • Apply Discounts

  • Process Payment

  • Generate Invoice

2.3 Example: E-Commerce Order Management (ASP.NET + SQL Server)

SQL Table Example – Orders & OrderItems

CREATE TABLE Orders (
    OrderId INT PRIMARY KEY IDENTITY,
    CustomerId INT,
    OrderDate DATETIME DEFAULT GETDATE(),
    TotalAmount DECIMAL(18,2)
);

CREATE TABLE OrderItems (
    OrderItemId INT PRIMARY KEY IDENTITY,
    OrderId INT FOREIGN KEY REFERENCES Orders(OrderId),
    ProductId INT,
    Quantity INT,
    Price DECIMAL(18,2)
);

ASP.NET C# Example – Saving Order

public void SaveOrder(Order order)
{
    using(SqlConnection conn = new SqlConnection(connectionString))
    {
        conn.Open();
        SqlTransaction transaction = conn.BeginTransaction();
        try
        {
            // Insert Order
            SqlCommand cmd = new SqlCommand("INSERT INTO Orders (CustomerId, TotalAmount) OUTPUT INSERTED.OrderId VALUES (@CustomerId, @TotalAmount)", conn, transaction);
            cmd.Parameters.AddWithValue("@CustomerId", order.CustomerId);
            cmd.Parameters.AddWithValue("@TotalAmount", order.TotalAmount);
            int orderId = (int)cmd.ExecuteScalar();

            // Insert Items
            foreach(var item in order.Items)
            {
                SqlCommand itemCmd = new SqlCommand("INSERT INTO OrderItems (OrderId, ProductId, Quantity, Price) VALUES (@OrderId, @ProductId, @Quantity, @Price)", conn, transaction);
                itemCmd.Parameters.AddWithValue("@OrderId", orderId);
                itemCmd.Parameters.AddWithValue("@ProductId", item.ProductId);
                itemCmd.Parameters.AddWithValue("@Quantity", item.Quantity);
                itemCmd.Parameters.AddWithValue("@Price", item.Price);
                itemCmd.ExecuteNonQuery();
            }

            transaction.Commit();
        }
        catch (Exception ex)
        {
            transaction.Rollback();
            throw new ApplicationException("Order saving failed", ex);
        }
    }
}

👉 This shows functional decomposition applied to real ERP design.

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3. UML Diagrams in Application Design

UML helps visualize complex systems.

3.1 Class Diagram Example (Customer-Order System)

• Customer

• Order

• OrderItem

Customer 1 --- * Order
Order 1 --- * OrderItem

3.2 Sequence Diagram – Place Order

1. Customer → WebApp: PlaceOrder()

2. WebApp → OrderService: Validate()

3. OrderService → DB: SaveOrder()

4. DB → WebApp: Success

5. WebApp → Customer: Confirmation

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4. Domain-Driven Design (DDD) Basics

4.1 What is DDD?

DDD connects business concepts with software design using domains, aggregates, and repositories.

4.2 Example – Invoice Domain in ASP.NET

public class Invoice
{
    public int InvoiceId { get; private set; }
    public int CustomerId { get; private set; }
    public decimal Amount { get; private set; }
    public List<InvoiceLine> Lines { get; private set; } = new();

    public void AddLine(int productId, int qty, decimal price)
    {
        Lines.Add(new InvoiceLine(productId, qty, price));
    }
}

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5. Modeling Entities, Relationships & Aggregates

ERP Example – Sales Order Aggregate

• Order (Aggregate Root)

• OrderItems (Child Entities)

Best Practice:

• Only aggregate root should handle updates.

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6. Data Flow & Control Flow Modeling

Example – Payment Gateway

• Data Flow: Customer → Checkout → Payment Gateway → Bank → Response

• Control Flow: If success → Generate Invoice; else → Rollback

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7. Best Practices

• Apply SOLID principles

• Use Layered Architecture (UI, Business, Data Access)

• Log all exceptions

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8. Pros, Cons & Alternatives

• UML Pros: Clear visualization | Cons: Time-consuming

• DDD Pros: Aligns with business | Cons: Complex for small projects

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9. Conclusion

Application Design Fundamentals transform requirements into structured, scalable solutions. By mastering UML, DDD, entity modeling, and flow diagrams, you can design robust enterprise applications with C#, ASP.NET, and SQL Server.

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