System Design and Modeling

Design Goals

System design converts requirements into a blueprint for implementation. A good design is modular, maintainable, scalable, secure, and meets performance targets.

Logical vs Physical Design

Logical design describes what the system does — inputs, outputs, data stores, and processes — independent of technology. Physical design describes how it will be realized with specific hardware, software, database products, and interfaces.

Data Flow Diagram (DFD)

A DFD models the flow of data through processes. It uses four symbols:

• External entity (square) — source or sink.
• Process (rounded rectangle or circle) — transforms input to output.
• Data store (open rectangle) — file or database.
• Data flow (arrow) — movement of data.

DFDs are drawn in levels: Level 0 is the context diagram showing the system as one process; Level 1 decomposes it into major sub-processes; Level 2 decomposes each further.

DFD Construction Rules

1. Every process must have at least one input and one output.
2. Data stores can only be connected to processes, never directly to entities or other stores.
3. Names must be nouns for entities/stores and verbs for processes.
4. Inputs and outputs must balance between parent and child diagrams.

Entity-Relationship (E-R) Diagram

An E-R diagram models data in terms of entities, attributes, and relationships. It is the foundation of relational database design.

• Entity — a thing (Student, Course).
• Attribute — a property (roll_no, name).
• Relationship — how entities relate (Student enrolls in Course).
• Cardinality — 1:1, 1:N, M:N.
• Key — primary key uniquely identifies an entity.

Data Dictionary

The data dictionary is a central catalog listing every data element in the system, its type, length, constraints, default value, and where it is used. It eliminates ambiguity and supports impact analysis.

Structure Charts

A structure chart shows the hierarchical decomposition of a program into modules and the data passed between them. It emphasizes cohesion (do one thing) and coupling (minimize interactions).

UML Basics

The Unified Modeling Language is the de facto standard for object-oriented modeling. Key diagrams include:

• Use-case diagram — who does what.
• Class diagram — static structure of classes and relationships.
• Sequence diagram — interactions over time.
• Activity diagram — workflows.
• State diagram — object lifecycle.

Input/Output and Interface Design

Form design follows principles of simplicity, consistency, and error prevention. Group fields logically, validate inputs, give immediate feedback, and provide defaults.

Summary

System design is a set of complementary models. DFDs describe processes; E-R diagrams describe data; UML adds behavior and interaction. Data dictionaries tie them together. A well-modeled system is easier to build, test, and evolve.

Important Questions

1. Differentiate logical and physical design.
2. Explain DFD symbols with an example.
3. What is a context diagram? How is it different from a Level-1 DFD?
4. List the rules for drawing DFDs.
5. Define entity, attribute, and relationship. Draw an E-R diagram for a library.
6. What is a data dictionary? Why is it useful?
7. Explain cohesion and coupling with examples.
8. List five UML diagrams and their purposes.