Q14Software Engineering

A Data Flow Diagram (DFD) is a highly rigorous, two-dimensional graphical modeling technique utilized extensively during the Systems Analysis phase of the SDLC. A DFD strictly maps the exact mathematical flow of raw data through a software system. Crucially, a DFD completely ignores control logic (loops, if/then statements) and strictly focuses on where data originates, how it is algorithmically transformed by processes, and where it is permanently stored.

The Four Mathematical Symbols of DFD (Gane & Sarson or Yourdon)

• 1. External Entity (Square): Represents a physical person, department, or external system (e.g., "Student", "Bank") that actively pushes raw data into our system or receives data from it.
• 2. Process (Circle or Rounded Rectangle): The absolute core of the system. It represents an algorithm or function that violently transforms incoming data into outgoing data (e.g., "Calculate Fees").
• 3. Data Store (Open-ended Rectangle): Represents a physical database, file cabinet, or hard drive where data is permanently frozen and stored for future retrieval (e.g., "Student Database").
• 4. Data Flow (Arrow): The physical pipeline. It mathematically shows the exact direction data is moving between entities, processes, and stores. It MUST be labeled with the exact name of the data (e.g., "Application Form").

Level-0 DFD (Context Diagram) for a College Registration System

The Level-0 DFD is the absolute highest altitude view. It violently compresses the ENTIRE complex software system into exactly ONE massive process bubble (labeled 0). It shows how the system interacts with the outside world, completely hiding internal databases.


• External Entity 1: Student. Data Flow IN: "Registration Request, Personal Info". Data Flow OUT: "Fee Receipt, Class Schedule".
• Central Process (0): College Registration Management System.
• External Entity 2: Accounts Department. Data Flow OUT: "Daily Fee Collection Report".

Level-1 DFD for a College Registration System

The Level-1 DFD shatters the massive Level-0 process into its core internal sub-processes, revealing the internal databases and how data is actually mathematically routed.

• Process 1.0 (Verify Application): Receives "Application Form" from Student. Verifies it. If valid, sends "Verified Data" to Process 2.0.
• Process 2.0 (Calculate Fees): Receives "Verified Data". Calculates required tuition. Sends "Fee Invoice" back to Student.
• Process 3.0 (Process Payment): Student submits "Payment". Process 3.0 verifies payment. It pushes the permanent record into the Fee Database (Data Store D1). It generates a "Fee Receipt".
• Process 4.0 (Generate Schedule): Once payment is verified in D1, this process triggers. It reads available classes from the Course Database (Data Store D2). It assigns classes, stores the result in Student Database (Data Store D3), and outputs the "Class Schedule" to the Student.