Data Transparency Data transparency:Degree to which system user may remain unaware of the details of how and where the data items are stored in a distributed system Consider transparency issues in relation to: Fragmentation transparency Replication transparency Location transparency Database System Concepts-6th Edition 19.12 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 19.12 ©Silberschatz, Korth and Sudarshan th Edition Data Transparency Data transparency: Degree to which system user may remain unaware of the details of how and where the data items are stored in a distributed system Consider transparency issues in relation to: Fragmentation transparency Replication transparency Location transparency
Naming of Data Items -Criteria 1.Every data item must have a system-wide unique name. 2.It should be possible to find the location of data items efficiently 3.It should be possible to change the location of data items transparently. 4.Each site should be able to create new data items autonomously. Database System Concepts-6th Edition 19.13 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 19.13 ©Silberschatz, Korth and Sudarshan th Edition Naming of Data Items - Criteria 1. Every data item must have a system-wide unique name. 2. It should be possible to find the location of data items efficiently. 3. It should be possible to change the location of data items transparently. 4. Each site should be able to create new data items autonomously
Centralized Scheme Name Server Structure: name server assigns all names each site maintains a record of local data items sites ask name server to locate non-local data items Advantages: satisfies naming criteria 1-3 Disadvantages: does not satisfy naming criterion 4 name server is a potential performance bottleneck name server is a single point of failure Database System Concepts-6th Edition 19.14 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 19.14 ©Silberschatz, Korth and Sudarshan th Edition Centralized Scheme - Name Server Structure: name server assigns all names each site maintains a record of local data items sites ask name server to locate non-local data items Advantages: satisfies naming criteria 1-3 Disadvantages: does not satisfy naming criterion 4 name server is a potential performance bottleneck name server is a single point of failure
Use of Aliases Alternative to centralized scheme:each site prefixes its own site identifier to any name that it generates i.e.,site 17.account. Fulfills having a unique identifier,and avoids problems associated with central control. However,fails to achieve network transparency. Solution:Create a set of aliases for data items;Store the mapping of aliases to the real names at each site. The user can be unaware of the physical location of a data item,and is unaffected if the data item is moved from one site to another. Database System Concepts-6th Edition 19.15 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 19.15 ©Silberschatz, Korth and Sudarshan th Edition Use of Aliases Alternative to centralized scheme: each site prefixes its own site identifier to any name that it generates i.e., site 17.account. Fulfills having a unique identifier, and avoids problems associated with central control. However, fails to achieve network transparency. Solution: Create a set of aliases for data items; Store the mapping of aliases to the real names at each site. The user can be unaware of the physical location of a data item, and is unaffected if the data item is moved from one site to another
Distributed Transactions and 2 Phase Commit Database System Concepts-6th Edition 19.16 ©Silberscha乜，Korth and Sudarshan
Database System Concepts - 6 19.16 ©Silberschatz, Korth and Sudarshan th Edition Distributed Transactions and 2 Phase Commit