Chapter 12Distributed Database Management Systems

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Let us ponder…..

““So what is a Distributed So what is a Distributed Database…sounds like Database…sounds like a Disturbing Database!”a Disturbing Database!”

““So why would anyone So why would anyone want to have a want to have a

Distributed Database, Distributed Database, anyway!”anyway!”

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

What is Distributed Database?

Wotz the difference?Centralised Database vs Distributed Database

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CDB v DDB

So wotz the difference?So then is there a difference between

“Distributed processing” and“Distributed database”?

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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 fineSo what have we got here with all this centralised, distributed processing, distributed database……?

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Levels of Data and Process Distribution Database systems can be classified

based on process distribution and data distribution

Table 10.1

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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

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Single-Site Processing, Single-Site Data

Figure 10.6

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

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

• Requires network file server• Applications accessed through LAN• Variation known as client/server architecture

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Multiple-Site Processing, Multi Data (MPMD)

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Multiple-Site Processing, Multiple-Site Data (MPMD)• Fully distributed DDBMS with support

for sites

Too easyOK, 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?

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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……

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Heterogeneous Distributed Database Scenario

Figure 10.8

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

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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

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DDBMS Disadvantages

• Two main thorns• Complex• Setup• Management

• Security & Control• Remote data• Data communications

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

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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)

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Distributed Database Components

Figure 10.5

The clever bit is….

The important part is what is invisible…..

Distributed DB Transparency

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Distributed DB Transparency

• Allows end users to feel like only database user

• Hides complexities of distributed database• Transparency features• Location• Fragmentation

• Heterogeneity • etc

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Distributed Transaction

Figure 10.12

The hard bit…

is Concurrency Controlis Query Optimisationis Integrity/Standards Controlis Security Controls

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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

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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