Database Reference
In-Depth Information
AP
1
AP
1
AP
2
W
W
X
DB
1
X
Y
Z
DBMS/OS
Computer 1
DBMS/OS
Communication
Line
Single Processing Computer
AP
2
Y
Z
DB
2
DBMS/OS
Computer 2
DB
(a)
Nonpartitioned, Nonreplicated
Alternative
(b)
Partitioned, Nonreplicated Alternative
AP
1
AP
2
W
AP
1
W
X
Y
Z
X
Y
DBMS/OS
DBMS/OS
DB
1
DB (Copy 1)
Computer 1
Communication
Line
Computer 1
Communication
Line
AP
2
AP
1
AP
2
W
Y
Z
X
Y
Z
DBMS/OS
DB
2
DB
Computer 2
DBMS/OS
DB (Copy 2)
Computer 2
(c)
Nonpartitioned, Replicated Alternative
(d)
Partitioned, Replicated Alternative
Figure 12-27
Types of Distributed
Databases
shows a database that is partitioned and replicated. Portion Y of the database is stored and
processed on Computers 1 and 2.
The portions to be partitioned or replicated can be defined in many different ways. A
database that has five tables ( for example, CUSTOMER, SALESPERSON, INVOICE, LINE_
ITEM, and PART) could be partitioned by assigning CUSTOMER to portion W, SALESPERSON
to portion X, INVOICE and LINE_ITEM to portion Y, and PART to portion Z. Alternatively,
different rows of each of these five tables could be assigned to different computers, or different
columns of each of these tables could be assigned to different computers.
Databases are distributed for two major reasons: performance and control. Having a
database on multiple computers can improve throughput, either because multiple computers
are sharing the workload or because communications delays can be reduced by placing the
computers closer to their users. Distributing the database can improve control by segregating
different portions of the database to different computers, each of which can have its own set of
authorized users and permissions.
Challenges of Distributed Databases
Significant challenges must be overcome when distributing a database, and those challenges
depend on the type of distributed database and the activity that is allowed. In the case of a fully
replicated database, if only one computer is allowed to make updates on one of the copies,
then the challenges are not too great. All update activity occurs on that single computer, and
copies of that database are periodically sent to the replication sites. The challenge is to ensure
that only a logically consistent copy of the database is distributed (no partial or uncommitted
transactions, for example) and to ensure that the sites understand that they are processing
data that might not be current because changes could have been made to the updated data-
base after the local copy was made.
Search WWH ::
Custom Search