1. 10 1 Chapter 10 Distributed Database Management Systems Database Systems: Design, Implementation, and Management, Sixth Edition, Rob and Coronel

2. 10 2 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel In this chapter, you will learn: What a distributed database management system (DDBMS) is and what its components are How database implementation is affected by different levels of data and process distribution How transactions are managed in a distributed database environment How database design is affected by the distributed database environment

3. 10 3 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel The Evolution of Distributed Database Management Systems Distributed database management system (DDBMS) –Governs storage and processing of logically related data over interconnected computer systems in which both data and processing functions are distributed among several sites

4. 10 4 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel The Evolution of Distributed Database Management Systems ( continued ) Centralized database required that corporate data be stored in a single central site Dynamic business environment and centralized database’s shortcomings spawned a demand for applications based on data access from different sources at multiple locations

5. 10 5 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Centralized Database Management System

6. 10 6 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel DDBMS Advantages Data are located near “greatest demand” site Faster data access Faster data processing Growth facilitation Improved communications Reduced operating costs User-friendly interface Less danger of a single-point failure Processor independence

7. 10 7 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel DDBMS Disadvantages Complexity of management and control Security Lack of standards Increased storage requirements Greater difficulty in managing the data environment Increased training cost

8. 10 8 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Processing Environment

9. 10 9 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Database Environment

10. 10 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Characteristics of Distributed Management Systems Application interface Validation Transformation Query optimization Mapping I/O interface Formatting Security Backup and recovery DB administration Concurrency control Transaction management

11. 10 11 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Characteristics of Distributed Management Systems ( continued ) Must perform all the functions of a centralized DBMS Must handle all necessary functions imposed by the distribution of data and processing Must perform these additional functions transparently to the end user

12. 10 12 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Fully Distributed Database Management System

13. 10 13 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel DDBMS Components Must include (at least) the following components: –Computer workstations –Network hardware and software –Communications media –Transaction processor (or, application processor, or transaction manager) Software component found in each computer that requests data –Data processor or data manager Software component residing on each computer that stores and retrieves data located at the site May be a centralized DBMS

14. 10 14 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Database System Components

15. 10 15 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Database Systems: Levels of Data and Process Distribution

16. 10 16 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Single-Site Processing, Single-Site Data (SPSD) All processing is done on single CPU or host computer (mainframe, midrange, or PC) All data are stored on host computer’s local disk Processing cannot be done on end user’s side of the system Typical of most mainframe and midrange computer DBMSs DBMS is located on the host computer, which is accessed by dumb terminals connected to it Also typical of the first generation of single-user microcomputer databases

17. 10 17 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Single-Site Processing, Single-Site Data (Centralized)

18. 10 18 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Multiple-Site Processing, Single-Site Data (MPSD) Multiple processes run on different computers sharing a single data repository MPSD scenario requires a network file server running conventional applications that are accessed through a LAN Many multi-user accounting applications, running under a personal computer network, fit such a description

19. 10 19 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Multiple-Site Processing, Single-Site Data

20. 10 20 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Multiple-Site Processing, Multiple-Site Data (MPMD) Fully distributed database management system with support for multiple data processors and transaction processors at multiple sites Classified as either homogeneous or heterogeneous Homogeneous DDBMSs –Integrate only one type of centralized DBMS over a network

21. 10 21 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Multiple-Site Processing, Multiple-Site Data (MPMD) ( continued ) Heterogeneous DDBMSs –Integrate different types of centralized DBMSs over a network Fully heterogeneous DDBMS –Support different DBMSs that may even support different data models (relational, hierarchical, or network) running under different computer systems, such as mainframes and microcomputers

22. 10 22 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Heterogeneous Distributed Database Scenario

23. 10 23 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Database Transparency Features Allow end user to feel like database’s only user Features include: –Distribution transparency –Transaction transparency –Failure transparency –Performance transparency –Heterogeneity transparency

24. 10 24 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distribution Transparency Allows management of a physically dispersed database as though it were a centralized database Three levels of distribution transparency are recognized: –Fragmentation transparency –Location transparency –Local mapping transparency

25. 10 25 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Summary of Transparency Features

26. 10 26 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Fragment Locations

27. 10 27 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Transaction Transparency Ensures database transactions will maintain distributed database’s integrity and consistency

28. 10 28 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Requests and Distributed Transactions Distributed transaction –Can update or request data from several different remote sites on a network Remote request –Lets a single SQL statement access data to be processed by a single remote database processor Remote transaction –Accesses data at a single remote site

29. 10 29 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Requests and Distributed Transactions ( continued ) Distributed transaction –Allows a transaction to reference several different (local or remote) DP sites Distributed request –Lets a single SQL statement reference data located at several different local or remote DP sites

30. 10 30 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Remote Request

31. 10 31 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Remote Transaction

32. 10 32 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Distributed Transaction

33. 10 33 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Distributed Request

34. 10 34 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Another Distributed Request

35. 10 35 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Concurrency Control Multisite, multiple-process operations are much more likely to create data inconsistencies and deadlocked transactions than are single-site systems

36. 10 36 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel The Effect of a Premature COMMIT

37. 10 37 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Two-Phase Commit Protocol Distributed databases make it possible for a transaction to access data at several sites Final COMMIT must not be issued until all sites have committed their parts of the transaction Two-phase commit protocol requires each individual DP’s transaction log entry be written before the database fragment is actually updated

38. 10 38 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Performance Transparency and Query Optimization Objective of query optimization routine is to minimize total cost associated with the execution of a request Costs associated with a request are a function of the: –Access time (I/O) cost –Communication cost –CPU time cost

39. 10 39 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Performance Transparency and Query Optimization ( continued ) Must provide distribution transparency as well as replica transparency Replica transparency: –DDBMS’s ability to hide the existence of multiple copies of data from the user Query optimization techniques: –Manual or automatic –Static or dynamic –Statistically based or rule-based algorithms

40. 10 40 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Distributed Database Design Data fragmentation: –How to partition the database into fragments Data replication: –Which fragments to replicate Data allocation: –Where to locate those fragments and replicas

41. 10 41 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Data Fragmentation Breaks single object into two or more segments or fragments Each fragment can be stored at any site over a computer network Information about data fragmentation is stored in the distributed data catalog (DDC), from which it is accessed by the TP to process user requests

42. 10 42 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Data Fragmentation Strategies Horizontal fragmentation: –Division of a relation into subsets (fragments) of tuples (rows) Vertical fragmentation: –Division of a relation into attribute (column) subsets Mixed fragmentation: –Combination of horizontal and vertical strategies

43. 10 43 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel A Sample CUSTOMER Table

44. 10 44 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Horizontal Fragmentation of the CUSTOMER Table by State

45. 10 45 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Table Fragments in Three Locations

46. 10 46 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Vertically Fragmented Table Contents

47. 10 47 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Mixed Fragmentation of the CUSTOMER Table

48. 10 48 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Data Replication Storage of data copies at multiple sites served by a computer network Fragment copies can be stored at several sites to serve specific information requirements –Can enhance data availability and response time –Can help to reduce communication and total query costs

49. 10 49 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Table Contents After the Mixed Fragmentation Process

50. 10 50 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Data Replication

51. 10 51 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Replication Scenarios Fully replicated database: –Stores multiple copies of each database fragment at multiple sites –Can be impractical due to amount of overhead Partially replicated database: –Stores multiple copies of some database fragments at multiple sites –Most DDBMSs are able to handle the partially replicated database well Unreplicated database: –Stores each database fragment at a single site –No duplicate database fragments

52. 10 52 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Data Allocation Deciding where to locate data Allocation strategies: –Centralized data allocation Entire database is stored at one site –Partitioned data allocation Database is divided into several disjointed parts (fragments) and stored at several sites –Replicated data allocation Copies of one or more database fragments are stored at several sites Data distribution over a computer network is achieved through data partition, data replication, or a combination of both

53. 10 53 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Client/Server vs. DDBMS Way in which computers interact to form a system Features a user of resources, or a client, and a provider of resources, or a server Can be used to implement a DBMS in which the client is the TP and the server is the DP

54. 10 54 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Client/Server Advantages Less expensive than alternate minicomputer or mainframe solutions Allow end user to use microcomputer’s GUI, thereby improving functionality and simplicity More people with PC skills than with mainframe skills in the job market PC is well established in the workplace Numerous data analysis and query tools exist to facilitate interaction with DBMSs available in the PC market Considerable cost advantage to offloading applications development from the mainframe to powerful PCs

55. 10 55 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Client/Server Disadvantages Creates a more complex environment, in which different platforms (LANs, operating systems, and so on) are often difficult to manage An increase in the number of users and processing sites often paves the way for security problems Possible to spread data access to a much wider circle of users  increases demand for people with broad knowledge of computers and software  increases burden of training and cost of maintaining the environment

56. 10 56 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel C. J. Date’s Twelve Commandments for Distributed Databases 1.Local site independence 2.Central site independence 3.Failure independence 4.Location transparency 5.Fragmentation transparency 6.Replication transparency 7.Distributed query processing 8.Distributed transaction processing 9.Hardware independence 10.Operating system independence 11.Network independence 12.Database independence

57. 10 57 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Summary Distributed database stores logically related data in two or more physically independent sites connected via a computer network Database is divided into fragments Distributed databases require distributed processing Main components of a DDBMS are the transaction processor and the data processor

58. 10 58 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Summary ( continued ) Current database systems can be classified by extent to which they support processing and data distribution DDBMS characteristics are best described as a set of transparencies A transaction is formed by one or more database requests A database can be replicated over several different sites on a computer network Client/server architecture refers to the way in which two computers interact over a computer network to form a system