Distributed Database Management Systems: Lecture 4

Distributed Database 
Management System 
Lecture 04
Virtual University of Pakistan


Fragmentation 
Transparency
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Responsibility of Transparency
Transparency is desirable but there 
is a compromise between level of 
Transparency and difficulty/cost
The language/Compiler: to provide 
uniform method of manipulating 
data. Avoid connectivity details. 
Operating System: already 
provides in form of Device Drivers
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Responsibility of Transparency
Can also provide network Trns. However 
not all provide complete.
Third layer is DBMS. Uses features of 

Operating System in particular for 
network transparency.
Practically, we get combination of all 
three.
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Lan T
R/F T
NwT
DI
Data

Layers of Transparency
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Reliability in DDBS
Reliability through Distributed 
Transactions: Avoids simple point of failure
Concurrency Issues: Transaction involving 
multiple records
Failure Recovery: Involving multiple sites
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Performance Improvement
Through Data Localization
– Each site handles a portion of data, so 
contention of CPU or I/O is relatively less
– Reduces remote access delays; no matter 
how fast networks are latency delays are 
there and may be unacceptable in certain 
case

Inter­query and Intra­query 
parallelism
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Complicating Factors

In case of Replication:
–Choose the appropriate copy
–Update impact reflected on all 
replicas

Failure Recovery
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Complication Factors
Complexity
Cost: hardware and also the 
duplication of manpower
Distribution of Control may cause 
problems if not used cautiously 
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The Problem Areas

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Database Design
Query Processing
Concurrency Control
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OS Support
Heterogeneity 
Relationship among 

them 
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Directory
Management
Query
Processing

Distributed
DB Design

Reliability

Concurrency
Control
Deadlock
Management

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Thanks

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