Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

1.6 Distributed Database Features

One of the strongest features of the Oracle database is its ability to
scale up to handle extremely large volumes of data and users. Oracle
scales not only by running on more and more powerful platforms, but
also by running in a distributed configuration. Oracle databases on
separate platforms can be combined to act as a single logical
distributed database.

This section describes some of the basic ways that Oracle handles
database interactions in a distributed database system.

1.6.1 Distributed Queries and Transactions

Data within an organization is often spread among multiple databases
for reasons of both capacity and organizational responsibility. Users
may want to query this distributed data or update it as if it existed
within a single database.

Oracle first introduced distributed databases in response to the
requirements for accessing data on multiple platforms in the early
1980s. Distributed queries
can retrieve data from multiple databases.
Distributed
transactions can insert, update, or delete data on
distributed databases. Oracle's two-phase commit
mechanism, which is described in detail in Chapter 12, guarantees
that all the database servers that are part of a transaction will
either commit or roll back the transaction. Background recovery
processes can ensure database consistency in the event of system
interruption during distributed transactions. Once the failed system
comes back online, the same process will complete the distributed
transactions.

Distributed transactions can also be implemented using popular
transaction monitors (TPs) that interact with Oracle via XA, an
industry standard (X/Open) interface. Oracle8i
added native transaction coordination with the
Microsoft Transaction Server
(MTS), so you can implement a distributed transaction initiated under
the control of MTS through an Oracle database.

1.6.2 Heterogeneous Services

Heterogeneous Services allow non-Oracle
data and services to be accessed from an Oracle database through
generic connectivity via ODBC and OLE-DB included with the database.

Optional Transparent Gateways use agents specifically tailored for a
variety of target systems. Transparent Gateways allow users to
submit Oracle SQL statements to a non-Oracle distributed database
source and have them automatically translated into the SQL dialect of
the non-Oracle source system, which remains transparent to the user.
In addition to providing underlying SQL services, Heterogeneous
Services provide transaction services utilizing
Oracle's two-phase commit with non-Oracle databases
and procedural services that call third-generation language routines
on non-Oracle systems. Users interact with the Oracle database as if
all objects are stored in the Oracle database, and Heterogeneous
Services handle the transparent interaction with the foreign database
on the user's behalf.

1.6.3 Availability

All the technologies discussed in this section are included in both
editions of the database, although Transparent Gateways are an add-on
available at additional cost.