19 - Application Design & Development

🎯 Learning Objectives

• Describe the 3-tier architecture (presentation, business logic, data)
• Explain how applications connect to databases (ODBC, JDBC, Python DB-API)
• Understand cookies and sessions for state management
• List RAD frameworks and their advantages

📖 Core Content

19.1 3-Tier Architecture

• Modularity: Each tier can be developed independently
• Scalability: Scale each tier separately
• Security: Data tier is never exposed to clients

19.2 Database Connectivity

ODBC (Open Database Connectivity)

• Standard API for accessing databases from C/C++
• Database-independent: same code works for different DBMS
• Uses a driver manager to translate calls

JDBC (Java Database Connectivity)

• Java equivalent of ODBC
• Four types of drivers (Type 1-4), with Type 4 (pure Java) being most common

Python DB-API

python
Run Inline
import psycopg2  # PostgreSQL adapter
conn = psycopg2.connect(
    host="localhost",
    database="university",
    user="postgres",
    password="secret"
)
cur = conn.cursor()
cur.execute("SELECT name, salary FROM instructor WHERE dept_name = %s", ('CS',))
results = cur.fetchall()
for row in results:
    print(f"Name: {row[0]}, Salary: {row[1]}")
cur.close()
conn.close()

19.3 Cookies and Sessions

1. User logs in → Server creates a session, sends a session cookie
2. Browser sends cookie with each request → Server looks up session
3. User logs out → Session destroyed, cookie expires

19.4 WSGI (Web Server Gateway Interface)

19.5 RAD Frameworks

19.6 Embedded SQL

c
EXEC SQL SELECT name, salary
    INTO :name_var, :sal_var
    FROM instructor
    WHERE ID = :id_var;

📝 Practice Questions

Q1. Describe the 3-tier architecture and its advantages.

Answer

Tiers: Presentation (UI), Business Logic (application server), Data Access (database server)

Advantages: Modularity, independent scalability, security (data never exposed to clients), reusability of components.

Q2. What is the difference between cookies and sessions?

Answer

Cookies are stored client-side; sessions are stored server-side. Sessions are more secure (data not exposed to the client). Cookies are typically used to store the session ID.

Q3. List two Python drivers for PostgreSQL.

Answer

psycopg2, pg8000, ocpgdp, PyGreSQL (any two).

🔗 Cross-References

• Related: BSCS2003 (MAD 1) — Web application development
• Related: 08 - SQL Joins (used in application queries)
• Textbook: Chapter 9 (Application Design) Join Discord Previous18 - DecompositionNext20 - Storage Systems