1. Distributed Database Management Systems
Lecture 12

2. Global Schema Architecture
G External
Schema
G External
Schema
Global
Schema
Schema Integration
L External
Schema
Component
Schema
Component
Schema
L External
Schema
• • •
Schema Translation
Local
Schema
Local
Schema
• • •
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GCSs in MDBS and logically integrated DDBS is different
Design process moves in inverse directions, MDBS design process is bottom up, where as in logically integrated DDBS it is top-down
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4. Virtual University of Pakistan
Multidatabase Systems: Architectures
External
Schema
Global
Component
• • •
Local
Global Schema Architecture
Federated
Schema
Export
Component
• • •
Local
External
Federated Database Architecture
Schema Integration
Schema Translation
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5. Global Data Dictionary
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A directory is a database that contains data about data (meta-data)
Called global directory in case of a DDBS
Contains mappings
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A single large or multiple for different sites
Hierarchies of DD can be built
Location: whether to keep at a single site or distributed
Single site increases load on that site
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Single copy or replication
Replication increases availability and reliability
All three issues are orthogonal to each other
That concludes our Architecture discussion
Lets summarize
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9. Virtual University of Pakistan
Introduction
Schema Design
The DDB Design concerns two major steps, Schema Design and Schema Distribution
Schema Distribution
Distribution of Schema decides the placement of data as well
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10. Virtual University of Pakistan
DDB Design
Top-Down Design Process
Bottom-Up Design Process
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11. System Requirements (Objectives)
Requirement Analysis
System Requirements (Objectives)
User Input
Conceptual Design
View Design
View Integration
Global Conceptual Schema
External Schema Def.
Access In formation
Distributed Design
User Input
Local Conceptual Schemas
Physical Design
Physical Schema
Feedback
Feedback
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Observation and Monitoring

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Conceptual Design is Global Conceptual Schema
View Design results in individual External Schema Definition (Global users)
Activities so far are similar to Centralized DB Design
Then start the distribution activity
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Bottom-Up Design Process
External
Schema
Global
Component
• • •
Local
Multidatabase Architecture
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14. Distribution Design Issues
Why fragments
How should we
How much should be fragmented
Any way to test correctness
Allocation Strategy
Required Information
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Disadvantages:
Difficult to manage in case of non-exclusive Fragmentation (replication)
Maintenance of Integrity constraints
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16. Thanks