Databases Reference
In-Depth Information
Figure 12-5
ASIA
• Tokyo
EUROPE
ARCTIC
OCEAN
A
B
C
U.K.
• Paris
D
E
F
GREENLAND
(DEN.)
A
B
C
D
E
F
AFRICA
CANADA
A
B
C
A
B
C
NORTH
AMERICA
PACIFIC
OCEAN
D
E
F
ATLANTIC
OCEAN
D
E
F
• New York
Los Angeles •
UNITED STATES
• Memphis
A
B
C
MEXICO
D
E
F
Caribbean
Sea
SOUTH AMERICA
Multiple database copies.
Possibly the biggest problem that data replication introduces is that of data
updates and concurrency control. If you allow updates to only one master copy
of the database that contains a consolidated data set, say the copy on New York,
you cut down significantly on the possibility of concurrency errors, but you lose
any benefits relating to telecommunication and improved access any time you
need to make a data update. If you allow updates to all of the databases, the
possibility of concurrency errors increases significantly. With either, you have the
issue of getting the updates made to all of the database copies.
12.2.2 Understanding Replicated Data
An important part of most distributed database environments is
replication.
This
is the means used by the database server to update multiple destination data-
bases with data from a central source database. The terminology used to describe
