畢業(yè)設計(論文)外文翻譯----分布式dbms的概念與設計-其他專業(yè)-在線瀏覽
2025-03-24 05:04本頁面
【正文】 steffective to add workstations to a work than to update a mainframe system. The second potential cost saving occurs where databases are geographically remote and the applications require access to distributed data. In such cases, owing to the relative expense of data being transmitted across the work as opposed to the cost of local access, it may be much more economical to partition the application and perform the processing locally at each site. Modular growth In a distributed environment, it is much easier to handle expansion. New sites can be added to the work without affecting the operations of other sites. This flexibility allows an anization to expand relatively easily. Increasing database size can usually be handled by adding processing and storage power to the work. In a centralized DBMS, a growth may 4 entail changes to both hardware and software. Complexity A distributed DBMS that hides the distributed nature form the user and provides an acceptable level of performance, reliability is inherently more plex than a centralized DBMS. The fact that data replication adequately, there will be degradation in availability, reliability, and performance pared with the centralized system, and the advantages we cited above will bee disvantages. Cost Increased plexity means that we can expect the procurement and maintenance costs for a DDBMS to be higher than those for a centralized DBMS. Furthermore, a distributed DBMS requires additional hardware to establish a work between sites. These are ongoing munication costs incurred with the use of this work. There are also additional labor costs to manage and maintain the local DBMSs and the underlying work. Security In a centralized system, access to the data can be easily controlled. However, in a distributed DBMS not only does access to replicated data have to be controlled in multiple locations, but the work itself has to be made secure. In the past, works were regarded as an insecure munication medium. Although this is still partially true, significant developments have been made to make work more secure. Integrity control more difficult Database integrity refers to the validity and consistency of stored data. Integrity is usually expressed in terms of constraints, which are consistency rules that the database is not permitted to violate. Enforcing integrity constraints generally requires access to a large amount of data that defines the constraint but which is not involved in the actual update operation itself. In a distributed DBMS, the munication and processing costs that are required to enforce integrity constraints may be prohibitive. We return to this problem in Section Lack of standards Alth
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