Chapter 12 Distributed Database Management Systems

2 Let us ponder….. “So what is a Distributed Database…sounds like a Disturbing Database!” “So why would anyone want to have a Distributed Database, anyway!” So, how does it work – this fancy DDB?”

What is Distributed Database?

Wotz the difference? Centralised Database vs Distributed Database

5 CDB v DDB

So wotz the difference? So then is there a difference between “Distributed processing” and “Distributed database”?

7 Distributed Processing vs DDB Shares database’s logical processing among physically, networked independent sites Stores logically related database over physically independent sites

Ja well no fine So what have we got here with all this centralised, distributed processing, distributed database……?

9 Levels of Data and Process Distribution Database systems can be classified based on process distribution and data distribution Table 10.1

10 Single-Site Processing, Single-Site Data (SPSD) All processing on single CPU or host computer All data are stored on host computer disk DBMS located on the host computer DBMS accessed by dumb terminals Typical of mainframe and minicomputer DBMSs Typical of 1st generation of single-user microcomputer database

11 Single-Site Processing, Single-Site Data Figure 10.6

12 Multiple-Site Processing, Single-Site Data (MPSD)

13 Multiple-Site Processing, Single-Site Data (MPSD) Requires network file server Applications accessed through LAN Variation known as client/server architecture

14 Multiple-Site Processing, Multi Data (MPMD)

15 Multiple-Site Processing, Multiple-Site Data (MPMD) Fully distributed DDBMS with support for sites

Too easy OK, so then the Database is at all the sites, the processing takes place at all the sites….Ummm… Do all the sites have the same type of database? Huh?

17 Homogeneous vs Heterogeneous DDB Homogeneous Integrate one type of centralized DBMS over the network Heterogeneous Integrate different types of centralized DBMSs over a network No kidding……

18 Heterogeneous Distributed Database Scenario Figure 10.8

So why? “So why would anyone want to have a Distributed Database, anyway!”

20 DDBMS Advantages Two main reasons Speed Data located near site with greatest demand Multiple/Parallel processing Reliability Improved communications Redundant copies – no single point failure Yeah…but surely it’s not all roses

21 DDBMS Disadvantages Two main thorns Complex Setup Management Security & Control Remote data Data communications

So How? So, how does it work – this fancy DDB?”

23 DDBMS Components Computer workstations Network hardware and software components Communications media Transaction processor (TP) Also called application manager (AP) or transaction manager (TM) Data processor (DP) Also called data manager (DM)

24 Distributed Database Components Figure 10.5

The clever bit is…. The important part is what is invisible….. Distributed DB Transparency

26 Distributed DB Transparency Allows end users to feel like only database user Hides complexities of distributed database Transparency features Location Fragmentation Heterogeneity etc

27 Distributed Transaction Figure 10.12

The hard bit… is Concurrency Control is Query Optimisation is Integrity/Standards Control is Security Controls

29 Date’s 12 Commandments for Distributed Databases 1. Local Site Independence 2. Central Site Independence 3. Failure Independence 4. Location Transparency 5. Fragmentation Transparency 6. Replication Transparency

30 Date’s 12 Commandments for Distributed Databases 7. Distributed Query Processing 8. Distributed Transaction Processing 9. Hardware Independence 10. Operating System Independence 11. Network Independence 12. Database Independence