Data Replication (Cont.) Advantages of Replication Availability:failure of site containing relation r does not result in unavailability of r is replicas exist. Parallelism:queries on r may be processed by several nodes in parallel Reduced data transfer:relation r is available locally at each site containing a replica of r. Disadvantages of Replication Increased cost of updates:each replica of relation r must be updated. Increased complexity of concurrency control:concurrent updates to distinct replicas may lead to inconsistent data unless special concurrency control mechanisms are implemented. One solution:choose one copy as primary copy and apply concurrency control operations on primary copy Database System Concepts-5th Edition,Aug 22,2005. 22.7 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 22.7 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. Data Replication (Cont.) Advantages of Replication Availability: failure of site containing relation r does not result in unavailability of r is replicas exist. Parallelism: queries on r may be processed by several nodes in parallel. Reduced data transfer: relation r is available locally at each site containing a replica of r. Disadvantages of Replication Increased cost of updates: each replica of relation r must be updated. Increased complexity of concurrency control: concurrent updates to distinct replicas may lead to inconsistent data unless special concurrency control mechanisms are implemented. One solution: choose one copy as primary copy and apply concurrency control operations on primary copy
Data Fragmentation Division of relation r into fragments n,r2,...,r which contain sufficient information to reconstruct relation r. Horizontal fragmentation:each tuple of r is assigned to one or more fragments Vertical fragmentation:the schema for relation r is split into several smaller schemas All schemas must contain a common candidate key(or superkey)to ensure lossless join property. A special attribute,the tuple-id attribute may be added to each schema to serve as a candidate key. Example relation account with following schema Account =(branch_name,account_number,balance Database System Concepts-5th Edition,Aug 22,2005. 22.8 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 22.8 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. Data Fragmentation Division of relation r into fragments r1 , r2 , …, rn which contain sufficient information to reconstruct relation r. Horizontal fragmentation: each tuple of r is assigned to one or more fragments Vertical fragmentation: the schema for relation r is split into several smaller schemas All schemas must contain a common candidate key (or superkey) to ensure lossless join property. A special attribute, the tuple-id attribute may be added to each schema to serve as a candidate key. Example : relation account with following schema Account = (branch_name, account_number, balance )
Horizontal Fragmentation of account Relation branch name account number balance Hillside A-305 500 Hillside A-226 336 Hillside A-155 62 account=Opranch_name="Hillside(account) branch_name account number balance Valleyview A-177 205 Valleyview A-402 10000 Valleyview A-408 1123 Valleyview A-639 750 account2=Obranch_name="Vallewiew(account) Database System Concepts-5th Edition,Aug 22,2005. 22.9 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 22.9 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. Horizontal Fragmentation of account Relation branch_name account_number balance Hillside Hillside Hillside A-305 A-226 A-155 500 336 62 account1 = branch_name=“Hillside” (account ) branch_name account_number balance Valleyview Valleyview Valleyview Valleyview A-177 A-402 A-408 A-639 205 10000 1123 750 account2 = branch_name=“Valleyview” (account )
Vertical Fragmentation of employee_info Relation branch name customer name tuple id Hillside Lowman 1 Hillside Camp 2 Valleyview Camp 3 Valleyview Kahn 4 Hillside Kahn 5 Valleyview Kahn 6 Valleyview Green 7 deposit=Iranch name.customer name.tuple_id(employee_info) account number balance tuple id A-305 500 1 A-226 336 2 A-177 205 3 A-402 10000 4 A-155 62 5 A-408 1123 6 A-639 750 7 deposit2=Iaccount number.balance.tuple_id(employee_info) Database System Concepts-5thEdition,Aug 22,2005. 22.10 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 22.10 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. Vertical Fragmentation of employee_info Relation branch_name customer_name tuple_id Hillside Hillside Valleyview Valleyview Hillside Valleyview Valleyview Lowman Camp Camp Kahn Kahn Kahn Green deposit1 = branch_name, customer_name, tuple_id (employee_info ) 1 2 3 4 5 6 7 account_number balance tuple_id 500 336 205 10000 62 1123 750 1 2 3 4 5 6 7 A-305 A-226 A-177 A-402 A-155 A-408 A-639 deposit2 = account_number, balance, tuple_id (employee_info )
Advantages of Fragmentation Horizontal: allows parallel processing on fragments of a relation allows a relation to be split so that tuples are located where they are most frequently accessed Vertical: allows tuples to be split so that each part of the tuple is stored where it is most frequently accessed tuple-id attribute allows efficient joining of vertical fragments allows parallel processing on a relation Vertical and horizontal fragmentation can be mixed. Fragments may be successively fragmented to an arbitrary depth. Database System Concepts-5th Edition,Aug 22,2005. 22.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 22.11 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. Advantages of Fragmentation Horizontal: allows parallel processing on fragments of a relation allows a relation to be split so that tuples are located where they are most frequently accessed Vertical: allows tuples to be split so that each part of the tuple is stored where it is most frequently accessed tuple-id attribute allows efficient joining of vertical fragments allows parallel processing on a relation Vertical and horizontal fragmentation can be mixed. Fragments may be successively fragmented to an arbitrary depth