Oracle distributed Features
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There has been a debate about a standard
definition for distributed Oracle databases. To Oracle, a
distributed database is a geographically distributed system
composed entirely of Oracle products. To the GUI/tools vendors,
a distributed database is a system that is distributed
architecturally, having systems with different architecture's
and access methods.
To the hardware vendors, a distributed
database is a system composed of different database all running
on the same hardware platforms. In fact, each of these
descriptions fits the overall model for distributed, but there
are some distinguishing characteristics between a real
distributed database and a loosely-coupled system. The standard
definition of distributed database has been developed by C. J.
Date, where he stated twelve specifications for an ideal
distributed database:
1. Local Autonomy
2. No reliance on a central site
3. Continuous operation
4. Location independence
5. Fragmentation independence
6. Replication independence
7. Distributed query processing
8. Distributed transaction management
(update processing)
9. Hardware independence
10. Operating system independence
11. Network independence
12. Database independence
1. Local Autonomy
Local autonomy means that all of the
data in the distributed network is owned and managed locally.
For example, a site in New York would have a remote database
that participates in a national distributed system. While
functioning as a part of the distributed network, the New York
database continues to process local operations independently
from the overall distributed system, and the New York database
does not rely on the distributed system to function. Oracle
allows this feature in Net8 by allowing each database to
function independently from other "linked" databases.
2. No reliance on a central site.
Ideally, all sites are equally "remote",
and no one site has governing authority over another node. Each
site retains it's own Oracle data dictionary and table security.
3. Continuous operation
Each site is capable of processing
independently from the other remote sites, and the addition of a
new site will not effect the overall system. While each site
maintains its own unique identity and control it functions as a
part of a unified federation such that other remote site may
access information from the site in a seamless fashion.
Continuous operations also refers to the ability of each node to
be available to the overall system 24 hours per day, seven days
per week. To accomplish this goal, remote sites may have a "hot
backup" tool such as the Oracle Enterprise Backup Utility (EBU)
or the Oracle8 Recovery Manager (RMAN) to back-up the database
while it remains available for update. Other Oracle continuous
operations tools include Oracle Parallel Server and Oracle
snapshots.
4. Location independence
End-user do not necessarily know, or
care, about the physical location of the databases that comprise
the system. Data is retrieved without any specific reference to
the physical sites. Oracle Net8 accomplishes this by the use of
transparent service names that hide the IP address, protocol and
remote database name.
5. Fragmentation independence
Fragmentation independence refers to the
ability of the end-users to store logically related information
at different physical locations. There are two types of
fragmentation independence, vertical partitioning and horizontal
partitioning. Horizontal partitioning allows for different rows
of the same table to be stored at different remote sites. This
is commonly used by organizations that maintain several branch
offices, each with an identical set of table structures.
Vertical partitioning refers to the ability of a distributed
system to fragment information such that the data columns from
the same logical tables are maintained across the network.
Oracle accomplishes this with Oracle "views" that hide specific
columns and rows in a table.
6. Replication independence
Replication is the ability of a database
to create copies of a master database at remote sites. These
copies are sometimes called snapshots and may contain the whole
database or any sub-component of the database. In relational
databases, a CUSTOMER table may be snapped to many remote sites
for read-only query. Subsets of the customer table may be
specified, requesting only specific row and columns, and these
replication are refreshed on a periodic basis. Oracle
accomplishes this feature with Oracle snapshots and replication.
7. Distributed query processing
Distributed query processing is more
than the ability to execute a query against more than one
database. In Oracle, the query is executed at the node that the
user is signed-on to, while other database's partition a
distributed query into sub-queries, executing each subquery on
it's host processor. In Oracle, a distributed query might query
data items from widely distributed databases in a single query.
Oracle implements distributed queries with database links.
8. Distributed transaction management
(update processing)
Distributed transaction management
refers to an Oracle database that can manage an update, insert,
or delete to multiple databases from a single query. Most
database vendors use the two-phase commit to implement this
process, and Oracle implements this feature with the RECO
process. The two-phase commit insures that all of the remote
database have successfully completed their sub-updates before
the entire transaction is COMMITTED to the database.
9. Hardware independence
This refers to the ability of a query to
query and update information regardless of the hardware platform
on which the data resides. A single query from a PC might
retrieve information from an IBM 3090, a local database on the
PC, and an HP-9000 in a single transaction. Of course a Net8
implementation does not care what type of processor is included
in the network.
10. Operating system independence
Again, a query should not be dependent
upon an operating system and Oracle multi-protocol facility
manages this aspect if distribution.
11. Network independence
Network protocols should not be an issue
for distributed databases. Protocol conversion routines such as
Net8 allow synchronous channels (such as those on IBM
mainframes) to communicate with the asynchronous Unix world.
They also accommodate differences in topology so that LU6.2 can
communicate with TCP/IP, and so on.
12. Database independence
Database independence refers to the
ability to retrieve and update information from many different
database and database architectures. Oracle has not fully
implemented this feature, although there are gateway products
that allow Oracle to communicate with other databases such as
IBM's DB2 database product.
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