Chapter 12 - Distributed Database Management Systems

🎯 Learning Outcomes

Define distributed database and DDBMS
Explain data and process distribution levels
Describe DDBMS components and architecture
Understand transparency features in distributed databases
Apply fragmentation, replication, and allocation strategies
Evaluate distributed query optimization and concurrency control

📘 Key Concepts

DDBMS: A Distributed Database Management System governs storage and processing of logically related data across multiple sites connected via a network.

Distributed Processing: Logical operations are shared across independent systems.

Distributed Database: Data is stored across multiple physical locations but appears unified to users.

🧩 Transparency Features

Distribution Transparency: Users access data without knowing its physical location
Transaction Transparency: Ensures consistency across distributed transactions
Failure Transparency: System recovers from site failures without user impact
Performance Transparency: Optimizes queries across sites
Heterogeneity Transparency: Supports different DBMSs and platforms

📦 Data Distribution Strategies

Fragmentation: Divides tables into subsets (horizontal, vertical, mixed)
Replication: Stores copies of data at multiple sites
Allocation: Determines where fragments and replicas reside

⚙️ Query Optimization

Distributed query optimization minimizes total cost using access time, communication cost, and CPU time. Techniques include:

Static vs. dynamic optimization
Rule-based vs. cost-based algorithms
Replica transparency to hide multiple copies

🔐 Concurrency & Two-Phase Commit

Concurrency Control: Prevents inconsistencies in multi-site transactions
Two-Phase Commit: Ensures all sites commit before final COMMIT
DO-UNDO-REDO Protocol: Supports recovery and consistency

🧪 Laboratory Activity: Designing a Distributed Database

💡 Objectives

Apply fragmentation and replication strategies
Simulate distributed query execution
Evaluate performance and transparency features

📋 Instructions

Choose a domain (e.g., e-commerce, university, logistics)
Design a centralized schema
Apply horizontal and vertical fragmentation
Replicate key fragments across two simulated sites
Write distributed queries using simulated remote access
Document transparency features and optimization results

📖 Glossary

The ability of a DDBMS to hide the fact that a database is fragmented across multiple sites.

A protocol that ensures distributed transactions are committed only when all participating sites agree.