ch25-Distributed Databases and Client-Server Architectures

ch25-Distributed Databases and Client-Server Architectures...

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Slide 25- 1 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe
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Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Chapter 25 Distributed Databases and Client-Server Architectures
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Slide 25- 3 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Chapter 25 Outline 1. Distributed Database Concepts 2. Data Fragmentation, Replication and Allocation 3. Types of Distributed Database Systems 4. Query Processing 5. Concurrency Control and Recovery 6. 3-Tier Client-Server Architecture
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Slide 25- 4 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Distributed Database Concepts A transaction can be executed by multiple networked computers in a unified manner. A distributed database (DDB) processes Unit of execution (a transaction) in a distributed manner. A distributed database (DDB) can be defined as A distributed database (DDB) is a collection of multiple logically related database distributed over a computer network, and a distributed database management system as a software system that manages a distributed database while making the distribution transparent to the user.
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Slide 25- 5 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Distributed Database System Advantages Management of distributed data with different levels of transparency : This refers to the physical placement of data (files, relations, etc.) which is not known to the user (distribution transparency).
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Slide 25- 6 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Distributed Database System Advantages (transparency, contd.) The EMPLOYEE, PROJECT, and WORKS_ON tables may be fragmented horizontally and stored with possible replication as shown below.
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Slide 25- 7 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Distributed Database System Advantages (transparency, contd.) Distribution and Network transparency : Users do not have to worry about operational details of the network. There is Location transparency, which refers to freedom of issuing command from any location without affecting its working. Then there is Naming transparency, which allows access to any names object (files, relations, etc.) from any location.
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Slide 25- 8 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Distributed Database System Advantages (transparency, contd.) Replication transparency : It allows to store copies of a data at multiple sites as shown in the above diagram. This is done to minimize access time to the required data. Fragmentation transparency : Allows to fragment a relation horizontally (create a subset of tuples of a relation) or vertically (create a subset of columns of a relation).
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Slide 25- 9 Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Distributed Database System Other Advantages Increased reliability and availability : Reliability refers to system live time, that is, system is running efficiently most of the time. Availability is the probability that the system is continuously available (usable or accessible) during a time interval.
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