Data Replication (Cont.) Advantages of Replication Availability:failure of site containing relation r does not result in unavailability of ris 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-6th Edition 19.7 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 19.7 ©Silberschatz, Korth and Sudarshan th Edition 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