Database Reference
In-Depth Information
DATABASE - Employee Relation
EmpNo
123
234
345
456
901
Name
City
Position
Dept
Salary
Bolton
Reno
Programmer
Info Tech
65000
Watkins
Houston
Manager
Sales
70000
Harvey
London
Analyst
Finance
69000
New York
Dillon
Dublin
Programmer
Info Tech
60000
Foster
Norfolk
Analyst
Info Tech
86000
DATABASE - Employee Relation
EmpNo
123
234
345
456
901
Name
City
Position
Dept
Salary
Bolton
Reno
Programmer
Info Tech
65000
Watkins
Houston
Manager
Sales
70000
Harvey
London
Analyst
Finance
69000
London
Dillon
Dublin
Programmer
Info Tech
60000
Foster
Norfolk
Analyst
Info Tech
86000
DATABASE - Employee Relation
EmpNo
567
678
789
Name
City
Position
Dept
Salary
Ceccatelli
Milan
Vice President Marketing
110000
Bertolini
Firenze
Manager
Finance
85000
Milan
Galileo
Turin
Vice President R & D
125000
Figure 18-12
EMPLOYEE relation: replication.
run a lot slower than queries on local data. Also, fragmentation presents a problem
with failures. When one site fails, the data at that site become inaccessible through-
out the organization.
Data Replication Suppose that, in your environment, every site needs the full
EMPLOYEE relation. What is the best method of providing for users at every site?
Keep a full copy of the relation at every site. Then every user in the organization
can have fast and easy access to employee data. The data replication method means
that we store either full copies of a relation or copies of appropriate fragments at
several sites. Figure 18-12 illustrates data replication. Observe the copies stored at
different sites.
Advantages and disadvantages Replication improves data availability. If one of
the sites containing a copy of a relation goes down, another copy may be accessed
from another site. In full replication, because each site gets a full copy, the data con-
stitute local data at every site. Therefore, you can get fast response to queries against
the relation at every site.
However, in practice, implementation of replication is complex, especially
for keeping all the copies synchronized whenever updates take place. Also, if
each site stores a full copy of a large relation with thousands of rows and sev-
eral columns, storage requirements for the distributed database can be quite
extensive.
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