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-5th Edition,Aug 22,2005. 22.12 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 22.12 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. 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-5th Edition,Aug 22,2005. 22.13 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 22.13 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. 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-5th Edition,Aug 22,2005. 22.14 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 22.14 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. 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-5th Edition,Aug 22,2005. 22.15 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 22.15 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. 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 Transaction may access data at several sites. Each site has a local transaction manager responsible for: Maintaining a log for recovery purposes Participating in coordinating the concurrent execution of the transactions executing at that site. Each site has a transaction coordinator,which is responsible for: Starting the execution of transactions that originate at the site. Distributing subtransactions at appropriate sites for execution. Coordinating the termination of each transaction that originates at the site,which may result in the transaction being committed at all sites or aborted at all sites. Database System Concepts-5th Edition,Aug 22,2005. 22.16 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 22.16 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. Distributed Transactions Transaction may access data at several sites. Each site has a local transaction manager responsible for: Maintaining a log for recovery purposes Participating in coordinating the concurrent execution of the transactions executing at that site. Each site has a transaction coordinator, which is responsible for: Starting the execution of transactions that originate at the site. Distributing subtransactions at appropriate sites for execution. Coordinating the termination of each transaction that originates at the site, which may result in the transaction being committed at all sites or aborted at all sites