DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 576

Site 4 Site 3 Site 2 Site 1

synchronous
asynchronous

E

E

Update all
copies

E

E

E

E

Update
most
copies

E

E

E

UPDATE EMPLOYEE
SET Salary

E

Update at
primary
site

Salary * 1.05;

=

E

E

E

E

Update at
multiple
primary
sites

E

E

Figure 18-18 Update propagation techniques.

Update Propagation to Replicas In an earlier subsection on replication,
we briefly mentioned two major methods for keeping replicated copies of data-
base objects synchronized when updates take place. We covered synchronous
and asynchronous replication techniques with an example of updates to an
EMPLOYEE relation replicated at different sites. Now let us explore the topic
a little further and study a few techniques. Figure 18-18 illustrates four common
techniques.
Note the following brief descriptions of the update propagation techniques.

1. Update all copies
This is a synchronous replication method. Locking is managed centrally. The
central lock manager coordinates all local lock managers. All copies of the
database object are updated before the transaction commits. This method
imposes excessive overhead on the central site. This site has to handle all the
traffic to coordinate the locking and unlocking tasks.
2. Update most copies
This method is a slight variation of the synchronous replication method. As
the central or coordinator site prepares to update its copy synchronously, it
issues update requests to all sites where replicated copies are held. If suffi-
cient number of sites respond positively, the coordinator completes the update
process. This is similar to majority voting to authorize a process. A number of
schemes exist to determine how many sites constitute a majority in a given
situation.

3. Update at primary site

One site is designated as the primary site where updates take place. The repli-
cated copy at the primary site is called the primary or master copy. All other