Databases Reference
In-Depth Information
Which brings up an interesting question: where should the directory itself be
stored? As with distributing the database tables themselves, there are a number of
possibilities, some relatively simple and others more complex, with many of the
same kinds of advantages and disadvantages that we've already discussed. The
entire directory could be stored at only one site, copies of the directory could be
stored at several of the sites, or a copy of the directory could be stored at every site.
Actually, since the directory must be referenced for every query issued at every site
and since the directory data will change only when new database tables are added
to the database, database tables are moved, or new replicated copies or partitions
are set up (all of which are fairly rare occurrences), the best solution generally is to
have a copy of the directory at every site.
Distributed DBMSs: Advantages and Disadvantages
At this point it will be helpful to pause, review, and summarize the advantages
and disadvantages of the distributed database concept and its various options.
Figure
12.11
provides
this
summary,
which
includes
the
advantages
and
disadvantages of a centralized database for comparison.
Centralized Database—Like Figure 12.5
Advantages:
Single site provides high degree of security, concurrency, and backup and
recovery control.
No need for a distributed directory since all of the data is in one place.
No need for distributed joins since all of the data is in one place.
Disadvantages:
All data accesses from other than the site with the database incur commu-
nications costs.
The site with the database can become a bottleneck.
Possible availability problem: if the site with the database goes down, there
can be no data access.
Dispersing Tables on the Network (without replication or partitioning)—Like Figure 12.6
Advantages:
Local autonomy.
Reduced communications costs because each table can be located at the site
that most heavily uses it.
Improved availability because portions of the database are available even if
one or some of the sites are down.
Disadvantages:
Several sites have to be concerned with security, concurrency, and backup
and recovery.
Requires a distributed directory and the software to support location trans-
parency.
Requires distributed joins.
F I G U R E 12.11
Advantages and disadvantages of
centralized and distributed database
approaches
(Continues)
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