chúng ta có thể có được một trong số họ từ khác bằng cách liên tục trao đổi cặp nonconflicting hành động, có nghĩa là, bằng cách thay đổi các cặp của các hành động có liên quan để không làm thay đổi kết quả. Lịch trình là tuần tự xung đột nếu nó là cuộc xung đột tương đương với một số lịch trình nối tiếp. | Parallel and Distributed Databases 635 8. In the Collaborating Servers architecture when a transaction is submitted to the DBMS briefly describe how its activities at various sites are coordinated. In particular describe the role of transaction managers at the different sites the concept of subtransactions and the concept of distributed transaction atomicity. Exercise Give brief answers to the following questions 1. Define the terms fragmentation and replication in terms of where data is stored. 2. What is the difference between synchronous and asynchronous replication 3. Define the term distributed data independence. Specifically what does this mean with respect to querying and with respect to updating data in the presence of data fragmentation and replication 4. Consider the voting and read-one write-all techniques for implementing synchronous replication. What are their respective pros and cons 5. Give an overview of how asynchronous replication can be implemented. In particular explain the terms capture and apply. 6. What is the difference between log-based and procedural approaches to implementing capture 7. Why is giving database objects unique names more complicated in a distributed DBMS 8. Describe a catalog organization that permits any replica of an entire relation or a fragment to be given a unique name and that provides the naming infrastructure required for ensuring distributed data independence. 9. If information from remote catalogs is cached at other sites what happens if the cached information becomes outdated How can this condition be detected and resolved Exercise Consider a parallel DBMS in which each relation is stored by horizontally partitioning its tuples across all disks. Employees eid integer did integer sal real Departments did integer mgrid integer budget integer The mgrid field of Departments is the eid of the manager. Each relation contains 20-byte tuples and the sal and budget fields both contain uniformly distributed values
