1. Distributed Database System

2. Definition
A distributed Database System consists of a collection of sites, connected together via some kind of communication network, in which:
Each site is a full database system site in its own right, but
The sites has agrees to work together so that a user at any site can access data anywhere in the network exactly as if the data were all stored at the user’s own site.

3. London New York Database Server Communication network Database
Workstation
Database
Communication
network
Database Server
New York
London

4. Each site is a database system site in its own right.
Each site has it own local “real” database,
Its own local users,
Its own local DBMS and transaction Management software including it own local locking, logging recovery, and etc.),
Its own local data communication manager (DC manager)
Overall distributed system can thus be
A kind of partnership among the individual local DBMSs at the individual local site
It has a new S/W component at each site –
logical extension of the local DBMS – provides the necessary partnership functionality,
and it is the combination of these new components together with the existing DBMSs that constitutes what is usually called the “Distributed Database Management System”
Overall distributed system แต่ละ site หรือระบบจัดการฐานข้อมูลแต่ละตัวเป็นหุ้นส่วนกัน

5. Distributed Database (DDB)
DDB is a collection of multiple logically interrelated database distributed over a computer network, and a distributed database management system (DDBMS) as a software system that manages a distributed database while making the distribution transparent to the user.

6. Type of Distributed Database system
Heterogeneous DDBMS
Homogenous DDBMS

7. Advantage of Distributed Database
Why desirable?
The enterprise are usually distributed
Data is usually distributed
Each organization unit maintain data that is relevant to its own.
Total information asset of the enterprise is thus splintered into what are sometime called “islands of information”
And what a Distributed system does is provide the necessary “bridge” to connect those islands together.
It enables the structure of database to mirror the structure of the enterprise
Local data can be kept locally, where it most logical belongs
While at the same time remote data can be accessed when necessary
Relevant เกี่ยวเนื่อง
Splinter ๑. ชิ้นเศษ, เสี้ยน ๒. แตกเป็นเสี่ยง

8. Banking system SF accounts keep in SF, NY accounts keep in NY…
The advantage are surely obvious: “The distributed arrange combines efficiency of processing (the data kept close to the point where it is most frequency used” with increased accessibility (it is possible to access a LA account from SF, via the communication network)
New York
Atlanta
San Francisco
Los Angeles
Communication
Network

9. New York Atlanta Chicago San Francisco Los Angeles Communication
(headquarter)
San Francisco
Los Angeles
Communication
Network
Employees_All
Projects_All
Works_on_All
Employees_New York
Works_on_New York Employee
Employees_Atlanta
Projects_Atlanta
Works_on_Atlanta Employee
Employees_Los Angeles
Projects_Los Angeles and San Francisco
Works_on_LosAngeles Employee
Employees_San Francisco and Los Angeles
Projects_San Francisco
Works_on_San Francisco Employees

10. Function of distributed database
Keeping track of data
Distributed query processing
Distributed transaction management
Replicated data management
Distributed data management
Distributed data recovery
Security
Distributed directory (catalog) management

11. Fundamental Principle (Rule zero)
The fundamental principle of distributed database
“To the user, a distributed system should look exactly like a non-distributed system”
Local autonomy
No reliance on a central site
Continuous operation
Location independence
Fragmentation independence
Replication independence
Distributed query processing
Distributed transaction management
Hardware independence
Network independence
DBMS independence

12. Local autonomy The sites in a distributed system should be autonomous.
Local autonomous means that all operations at a given site are controlled by that site;
No site X should depend on site Y for its successful operation
Local data is locally owned and managed, with local accountability; all data “really” belongs to some local database, even if it is accessible from other site
Integrity, security and physical storage representation of local data remain under the control and jurisdiction of the local site
autonomous อิสระ

13. No reliance on a central site
All site must be treated as equals.
They must not be any reliance on a central “master” site for some central service – for example
Central query processing
Central transaction management
Centralized naming services
The entire system is dependent on that central site
Why don’t need
First, that central site might be a bottleneck;
Second, the system would be vulnerable if the central site went down, the whole system would be down (“The single point of failure” problem)
Reliance = dependence
ความไว้ใจ, ความเชื่อถือ
Bottleneck จุดติดขัด
Vulnerable = that can be wounded or physically injured

14. Continuous operation Unplanned shutdowns are undesirable
The advantage of distributed systems is provide greater reliability and greater availability
Reliability is the probability that the system is up and running any given moment.
Availability is the probability that the system is up and running continuously (available) during a time interval.
Unplanned shutdowns are undesirable
Planned shutdowns should never be required;
That is it should never necessary to shut the system down in order to perform a task such as adding a new site or upgrading the DBMS at an existing site to a new release version.
Reliability ความเชื่อถือได้
Availability สภาพพร้อมใช้งาน

15. Location independence/transparency
Basic idea
Users should not have to know where data is physically stored, but rather should be able to perform
– at least from a logical standpoint
– as if data were all stored at their own local site.
Desirable because
It simplifies application programs and user activities
It allows data to migrate from site to site without invalidating any of those program and activities
(migratability is desirable because it allows data to be move around the network in respond to changing performance requirement)

16. Fragmentation independence / transparency
Support data fragmentation
Base table can be divided into pieces or fragments for physical storage purposes, and distinct fragments can be stored at different sites.
Fragmentation is desirable for performance reasons:
Data can be stored at the location where it is most frequently used, so that most operations are local and network traffic is reduces

17. User perception
Emp
Emp# Dept# Salary
E1 D
E2 D
E3 D
E4 D
E5 D
Site A
Site B
S1_Emp
S2_Emp
Emp# Dept# Salary
E1 D
E2 D
E5 D
Emp# Dept# Salary
E3 D
E4 D
FRAGMENT EMP AS
S1_EMP AT SITE ‘SITE_A’ WHERE DEPT# = DEPT#(‘D1’) OR DEPT# = DEPT#(‘D3’)
S1_EMP AT SITE ‘SITE_B’ WHERE DEPT# = DEPT#(‘D2’)

18. Fragmentation Fragmentation type
Horizontal Fragmentation
Vertical Fragmentation
Reconstructing the original base relvar from the fragments in done via suitable join (for vertical) and union operations (for horizontal)
Fragmentation independence implies that users will be presented with a view of the data in which the fragments are logically recombines by means of suitable joins and unions. (no fragmentation)
The optimizer responds to determine which fragments need to be physically accessed in order to satisfy any given user request.
Emp where salary > and dept# = dept#(‘D1’)
Optimizer will know from the fragment definitions (in catalog) that the entire result can be obtained from site_A

19. Replication independence
Support Data replication
Desirable because
First, it can mean better performance.
Application can operate on local copies instead of having to communicate with remote sites.
Second, it can also mean better availability.
A given replicated object remains available for processing – at least for retrieval as long as at least one copy reminds available
Disadvantage
A given replicated object is updates, all copies of that object must be updated (the update propagation)

20. Replication transparency
Transparency to the user
User should be able to behave, at least from a logical standpoint, as if the data were in fact not replicated at all.
Desirable
It simplifies application programs and end-user activities;
It allows replicas to be created and destroy anytime in response to changing requirements, without invalidating any of those programs or activities.
Replication independence implies that it is the responsibility of the optimizer to determine which replicas physically need to be access in order to satisfy any given user request.

21. Site_A
Site_B
S2_Emp
S1_Emp
Emp# Dept# Salary
E3 D
E4 D
Emp# Dept# Salary
E1 D
E2 D
E5 D
Emp# Dept# Salary
E1 D
E2 D
E5 D
Emp# Dept# Salary
E3 D
E4 D
S12_Emp (S2_EMP replica)
S21_Emp (S1_EMP replica)

22. Distributed query processing
In distributed system data store in many sites and may replicate
Optimization is even more important in a distributed system that it is in a centralized one.
Query that involve several sites A B CN

23. Database S{S#,CITY} 10,000 stored at Site A
P{P#,Color} 100,000 stored at site B
SP{S#,P#} 1,000,000 stored at A
Assume every stored tuple is 25 bytes (200bits)
Query (Get supplier numbers for LD suppliers of red parts”
((S JOIN SP JOIN P) where CITY = “LD” and
COLOR = (‘Red’)) {S#}
Estimated cardinalities of certain intermediate results:
Number of read parts = 10
Number of shipments by LD suppliers = 100,000
Communication assumptions:
Data Rate = 50,000 bits per second
Access delay = 0.1 second

24. 6 strategies for processing this query and for each i calculate the total communication time Ti from the formula
(total access delay) + (total data volume/data rate)
Become in second
No of message/10 + No of bits/50000
Move parts to Site A and process the query at A
T1 = ( * 200) /50000
Move supplier and shipments to site B and process the query at B
T2 = (( ) * 200)/5000
Join suppliers and shipments at site A, restrict the result to LD suppliers and then, for each of those supplier in turn, check site B to see whether corresponding part is red. Each of these checks will involved 2 messages – a query and a respond. The transmission time for these messages will be small compared with the access delay
T3 = seconds approx.

25. Restrict parts at site B of those the red, and then, for each of those parts in turns, check site A to see whether there exists a shipment relating the part to a LD supplier. Each of these checks will involve 2 messages; transmission time for these message will be small compared with the access delay
T4 = 2 seconds approx.
Join supplier and shipments at site A, restrict the result to LD suppliers, project the result over S# and P#, and move the result to site B. Complete the processing at site B
T5 = (10000 * 200)/50000
Restrict parts at site B to those that are red and move the result to site A. complete the processing at site A
T6 = (10 * 200) / 50000

26. Distributed transaction management
Related to
Transaction management
Recovery and concurrency
2 phase commit
Prepare phase
Commit phase
(see the previous slide)
behalf of = in the interest of

27. Hardware / Network / DBMS independence