Database Reference
In-Depth Information
CHAPTER 9
DATABASE MANAGEMENT APPROACHES
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1. A distributed database is a single logical database that is physically divided among computers at
several sites on a network. A distributed database management system (DDBMS) is a DBMS capa-
ble of supporting and manipulating distributed databases.
3. A homogeneous DDDBS is one that has the same local DBMS at each site. A heterogeneous
DDBMS is one that does not; there are at least two sites at which the local DBMSs are different.
Heterogeneous systems are more complex.
5. Location transparency is the characteristic of a DDBMS that states that users do not need to be
aware of the location of data in a distributed database. Data should be accessible at a remote site
just as easily as it is at a local site; the only difference should be the response time.
7. Replication transparency is the characteristic that users do not need to be aware of any replica-
tion that has taken place in a distributed database. The DDBMS should handle updates to all
copies of the data without users being aware of the steps taken by the DDBMS.
9. Fragmentation transparency is the characteristic that users do not need to be aware of any data
fragmentation (splitting of data) in a distributed database. Users should feel as if they are using a
single central database, even if data is stored at different sites.
11. In a well-designed distributed database, you can often increase its capacity by increasing the
capacity at only one site. Also, you can increase capacity through the addition of new sites to the
network.
13. Increased efficiency is an advantage in a distributed database because data available locally can
be retrieved much more rapidly than data stored on a remote, centralized system.
15. Query processing is more complex in a distributed environment because of the difference
between the time it takes to send messages between sites and the time it takes to access a disk.
17. With a two-phase commit, one site (often the site initiating the update) acts as coordinator. In the
first phase, the coordinator sends messages to all other sites requesting they prepare to update
the database (or acquire all necessary locks). Once the coordinator receives positive replies from
all sites, the coordinator sends a message to each site to commit the update. At this point, each
site must proceed with the commit process, and all sites must abort if any reply is negative. Two-
phase commit guarantees consistency of the database.
19. The information-level design process is unaffected by the fact that the database is distributed.
21. The 12 rules against which you can measure DDBMSs are:
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Local autonomy: No site should depend on another site to perform its functions.
No reliance on a central site: A DDBMS should not need to rely on one site more than any
other site.
Continuous operation: Performing any function should not shut down the entire distributed
database.
Location transparency: Users should feel as if the entire database is stored at their location.
Fragmentation transparency: Users should feel as if they are using a single central database.
Replication transparency: Users should not be aware of any data replication.
Distributed query processing: A DDBMS must process queries as rapidly as possible even
though the data is distributed.
Distributed transaction management: A DDBMS must effectively manage transaction updates
at multiple sites.
Hardware independence: A DDBMS must be able to run on different types of hardware.
Operating system independence: A DDBMS must be able to run on different operating
systems.
Network independence: A DDBMS must be able to run on different types of networks.
DBMS independence: A DDBMS must be heterogeneous.
