tailieunhanh - FUNDAMENTALS OF DATABASE SYSTEMS Fourth Edition phần 9

Như chúng ta đã thấy trong trường hợp của hai ORDBMSs, nhiều chức năng được yêu cầu để hoạt động trên hai hoặc dữ liệu không gian ba chiều, hình ảnh, văn bản, và như vậy. Với một liên kết tĩnh của tất cả các thư viện chức năng, không gian địa chỉ DBMS có thể tăng bởi một thứ tự cường độ. | 830 Chapter 25 Distributed Databases and Client-Server Architectures Distributed Databases in Oracle In the client-server architecture the Oracle database system is divided into two parts 1 a front-end as the client portion and 2 a back-end as the server portion. The client portion is the front-end database application that interacts with the user. The client has no data access responsibility and merely handles the requesting processing and presentation of data managed by the server. The server portion runs Oracle and handles the functions related to concurrent shared access. It accepts SQL and PL SQL statements originating from client applications processes them and sends the results back to the client. Oracle client-server applications provide location transparency by making location of data transparent to users several features like views synonyms and procedures contribute to this. Global naming is achieved by using TABLENAME.@ DATABASENAME to refer to tables uniquely. Oracle uses a two-phase commit protocol to deal with concurrent distributed transactions. The COMMIT statement triggers the two-phase commit mechanism. The RECO recoverer background process automatically resolves the outcome of those distributed transactions in which the commit was interrupted. The RECO of each local Oracle Server automatically commits or rolls back any in-doubt distributed transactions consistently on all involved nodes. For long-term failures Oracle allows each local DBA to manually commit or roll back any in-doubt transactions and free up resources. Global consistency can be maintained by restoring the database at each site to a predetermined fixed point in the past. Oracle s distributed database architecture is shown in Figure . A node in a distributed database system can act as a client as a server or both depending on the situation. The figure shows two sites where databases called HQ headquarters and Sales are kept. For example in the application shown running at .