such as views, procedures, and synonyms, can provide location transparency...

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For example, a view that joins table data from several databases provides
location transparency because the user of the view does not need to know from
where the data originates.
Site Autonomy
Site autonomy means that each database participating in a distributed database is administered separately and independently from the other databases, as
though each database were a non-networked database. Although each
database can work with others, they are distinct, separate systems that are
cared for individually.
Distributed Data Manipulation
The Oracle distributed database architecture supports all DML operations,
including queries, inserts, updates, and deletes of remote table data. To access remote data, a reference is made including the remote object’s global object
name — no coding or complex syntax is required to access remote data. For
example, to query a table named EMP in the remote database named SALES,
you reference the table’s global object name:
SELECT * FROM emp@sales;
Two-Phase Commit
Oracle provides the same assurance of data consistency in a distributed
environment as in a non-distributed environment. Oracle provides this
assurance using the transaction model and a two-phase commit mechanism.
As in nondistributed systems, transactions should be carefully planned to
include a logical set of SQL statements that should all succeed or fail as a unit.
Oracle’s two-phase commit mechanism guarantees that no matter what type
of system or network failure might occur, a distributed transaction either
commits on all involved nodes or rolls back on all involved nodes to maintain
data consistency across the global distributed database.
Introduction to the Oracle Server 1-25
Complete Transparency to Database Users
The Oracle two-phase commit mechanism is completely transparent to users
that issue distributed transactions. A simple COMMIT statement denoting the
end of a transaction automatically triggers the two-phase commit mechanism
to commit the transaction; no coding or complex statement syntax is required
to include distributed transactions within the body of a database application.
Automatic Recovery from System or Network Failures
The RECO (recoverer) background process automatically resolves the
outcome of in-doubt distributed transactions — distributed transactions in which the commit was interrupted by any type of system or network failure. After
the failure is repaired and communication is reestablished, the RECO of each
local Oracle Server automatically commits or rolls back any in-doubt
distributed transactions consistently on all involved nodes.
Optional Manual Override Capability
In the event of a long-term failure, Oracle allows each local administrator to manually commit or roll back any distributed transactions that are in doubt as a result of the failure. This option allows the local database administrator to free up any locked resources that may be held indefinitely as a result of the
long-term failure.
Facilities for Distributed Recovery
If a database must be recovered to a point in the past, Oracle’s recovery
facilities allow database administrators at other sites to return their databases to the earlier point in time also. This ensures that the global database remains consistent.
Table Replication
Note: The information in this section applies only to Oracle with the
distributed or advanced replication options.
Distributed database systems often locally replicate remote tables that are
frequently queried by local users. By having copies of heavily accessed data on several nodes, the distributed database does not need to send information
across a network repeatedly, thus helping to maximize the performance of the
database application.
Data can be replicated using snapshots or replicated master tables. Replicated master tables require the replication option. For more information about
replicating data, see Oracle8 Server Replication.
1-26 Oracle8 Server Concepts
Oracle and Net8
Net8 is Oracle’s mechanism for interfacing with the communication protocols used by the networks that facilitate distributed processing and distributed
databases. Communication protocols define the way that data is transmitted
and received on a network. In a networked environment, an Oracle database
server communicates with client workstations and other Oracle database
servers using Oracle software called Net8.
Net8 supports communications on all major network protocols, ranging from
those supported by PC LANs to those used by the largest of mainframe
computer systems.
Using Net8, the application developer does not have to be concerned with
supporting network communications in a database application. If a new
protocol is used, the database administrator makes some minor changes, while
the application requires no modifications and continues to function.
Startup and Shutdown Operations
An Oracle database is not available to users until the Oracle Server has been
started up and the database has been opened. These operations must be