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Friday, November 23, 2007

Homogeneous and Heterogeneous DBMS

Homogenous Distributed Database Systems
A homogenous distributed database system is a network of two or more Oracle databases that reside on one or more machines. illustrates a distributed system that connects three databases: hq, mfg, and sales. An application can simultaneously access or modify the data in several databases in a single distributed environment. For example, a single query from a Manufacturing client on local database mfg can retrieve joined data from the products table on the local database and the dept table on the remote hq database.

For a client application, the location and platform of the databases are transparent. You can also create synonyms for remote objects in the distributed system so that users can access them with the same syntax as local objects. For example, if you are connected to database mfg but want to access data on database hq, creating a synonym on mfg for the remote dept table enables you to issue this query:


In this way, a distributed system gives the appearance of native data access. Users on mfg do not have to know that the data they access resides on remote databases.Heterogeneous Distributed Database Systems
In a heterogeneous distributed database system, at least one of the databases is a non-Oracle system. To the application, the heterogeneous distributed database system appears as a single, local, Oracle database. The local Oracle database server hides the distribution and heterogeneity of the data.

The Oracle database server accesses the non-Oracle system using Oracle Heterogeneous Services in conjunction with an agent. If you access the non-Oracle data store using an Oracle Transparent Gateway, then the agent is a system-specific application. For example, if you include a Sybase database in an Oracle distributed system, then you need to obtain a Sybase-specific transparent gateway so that the Oracle databases in the system can communicate with it.

Alternatively, you can use generic connectivity to access non-Oracle data stores so long as the non-Oracle system supports the ODBC or OLE DB protocols.
Other than the introductory material presented in this chapter, this book does not discuss Oracle Heterogeneous Services.
Heterogeneous Services
Heterogeneous Services (HS) is an integrated component within the Oracle database server and the enabling technology for the current suite of Oracle Transparent Gateway products. HS provides the common architecture and administration mechanisms for Oracle gateway products and other heterogeneous access facilities. Also, it provides upwardly compatible functionality for users of most of the earlier Oracle Transparent Gateway releases.

Transparent Gateway Agents
For each non-Oracle system that you access, Heterogeneous Services can use a transparent gateway agent to interface with the specified non-Oracle system. The agent is specific to the non-Oracle system, so each type of system requires a different agent.

The transparent gateway agent facilitates communication between Oracle and non-Oracle databases and uses the Heterogeneous Services component in the Oracle database server. The agent executes SQL and transactional requests at the non-Oracle system on behalf of the Oracle database server.

See Also:
Your Oracle supplied gateway-specific documentation for information about transparent gateways

Generic Connectivity
Generic connectivity enables you to connect to non-Oracle data stores by using either a Heterogeneous Services ODBC agent or a Heterogeneous Services OLE DB agent--both are included with your Oracle product as a standard feature. Any data source compatible with the ODBC or OLE DB standards can be accessed using a generic connectivity agent.

The advantage to generic connectivity is that it may not be required for you to purchase and configure a separate system-specific agent. You use an ODBC or OLE DB driver that can interface with the agent. However, some data access features are only available with transparent gateway agents.

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