DDB分布式数据库系统 第一讲 Ddb-L1 introduction

1、About the Instructor,Name: Gong Zhiguo Office: N415 Phone: 3974465 E-Mail: zggongumac.mo,From File Processing to DBMS,File of current accounts,File of saving accounts,File of customers,Program 4 customer information,Program 1 deposit withdraw,Program 2 transfer,Program 3 printing stmt,BANK DATA BASE

2、,Program 4 customer information,Program 1 deposit withdraw,Program 2 transfer,Program 3 printing stmt,DDB S = Database + Networking,The technology of computer networks, promotes a mode of work that goes against all centralization efforts and facilitates distributed computing Distributed database sys

3、tem technology is the union of what appear to be diametrically opposed approaches to data processing: Database System, Computer Network technologies A database system aims at integrating the operational data of an enterprise, and to provide a centralized and controlled access to that data,Distribute

4、d Computing System,A distributed computing system consists of a number of autonomous processing elements (not necessarily homogeneous) interconnected by a computer network cooperate in performing their assigned tasks What is distributed? Processing Logic Function Data Control,All these are necessary

5、 and important for distributed database technology,Distributed DBMS Environment,Distributed Database System,A distributed database is a collection of multiple, logically interrelated databases distributed over a computer network; stores data on multiple computers (nodes) over the network and permits

6、 access from any node to the joint data A distributed database management system (DDBMS) is a software system that permits the management of the distributed databases and makes the distribution transparent to the users.,What is not a Distributed Database System?,A DDBS is not a collection of files t

7、hat can be individually stored at each node of a computer network files are not logically related no access via common interface,Centralized DBMS on a Network,data resides only at one node the database management is no different from centralized DBMS remote processing, single servermultiple clients,

8、Multiprocessor Systems,Shared Memory (SMP),Shared Disk,Shared Nothing (network),Sequent, SGI, Sun,VMScluster, Sysplex,Tandem, Teradata, SP2,Distributed Database System Technology,The key is integration, not centralization Distributed database technology attempts to achieve integration without centra

9、lization,Database Technology,Computers Networks,Distributed Database Systems,Integration,Integration Without Centralization,Distributed Computing,Example,Multinational manufacturing company: head quarters in Macau manufacturing plants in Nanning and Kunming warehouses in Zhongshan and Dongguan R&D f

10、acilities in Beijing Data and Information: employee records (working location) projects (R&D) engineering data (manufacturing plants, R&D) inventory (manufacturing, warehouse),Promises of Distributed DBMS,transparent management of distributed, fragmented, and replicated data improved reliability and

11、 availability through distributed transactions improved performance higher system extendibility,Transparency,Transparency refers to separation of the higher-level semantics of a system from lower-level implementation details. From data independence in centralized DBMS to fragmentation transparency i

12、n DDBMS. Issues Who should provide transparency? What is the state of the art in the industry?,Improved Reliability,Distributed DBMS can use replicated components to eliminate single point failure. The users can still access part of the distributed database with “proper care” even though some of the

13、 data is unreachable. Distributed transactions facilitate maintenance of consistent database state even when failures occur.,Improved Performance,Since each site handles only a portion of a database, the contention for CPU and I/O resources is not that severe. Data localization reduces communication

14、 overheads. Inherent parallelism of distributed systems may be exploited inter-query parallelism intra-query parallelism Performance models are not sufficiently developed.,Easier System Expansion,Ability to add new sites, data, and users over time without major restructuring. Huge centralized databa

15、se systems (mainframes) are history (almost!). PC revolution (Compaq buying Digital, 1998) will make natural distributed processing environments. New applications (such as, supply chain) are naturally distributed - centralized systems will just not work.,Disadvantages of DDBSs,Lack of Experience No

16、operating true distributed database systems in existence Complexity DDBS problems are inherently more complex than centralized DBMS ones Cost More hardware, software and people costs Distribution of control Problems of synchronization and coordination to maintain data consistency Security Database s

17、ecurity + network security Difficult to convert No tools to convert centralized DBMSs to DDBSs,Complicating Factors,Data may be replicated in a distributed environment, consequently the DDBS is responsible for choosing one of the stored copies of the requested data for access in case of retrievals m

18、aking sure that the effect of an update is reflected on each and every copy of that data item If there is site/link failure while an update is being executed, the DDBS must make sure that the effects will be reflected on the data residing at the failing or unreachable sites as soon as the system rec

19、overs from the failure,Complicating Factors,Maintaining consistency of distributed/replicated data. Since each site cannot have instantaneous information on the actions currently carried out in other sites, the synchronization of transactions at multiple sites is harder than centralized system.,Dist

20、ributed DBMS Issues,Distributed Database Design Distributed Query Processing Distributed Directory Management Distributed Concurrency Control Distributed Deadlock Management Reliability of Distributed Databases Operating Systems Support Heterogeneous Databases,Distributed Database Design,The problem

21、 is how the database and the applications that run against it should be placed across the sites. The two fundamental design issues are fragmentation (the separation of the database into partitions called fragments), and allocation (distribution), the optimum distribution of fragments. The general pr

22、oblem is NPhard.,Distributed Query Processing,Query processing deals with designing algorithms that analyze queries and convert them into a series of data manipulation operations. The problem is how to decide on strategy for executing each query over the network in the most cost effective way, howev

23、er the cost is defined. The objective is to optimize where the inherent parallelism is used to improve the performance of executing the transaction,Distributed Directory Management,A directory contains information (such as descriptions and locations) about data items in the database. A directory may be global to the entire DDBS, or local to each site, distributed, multiple copies, etc.,Distributed Concur