Gateway (Or Heterogeneous Services before 11g) make a non-Oracle system appear as a remote Oracle Database server through program that are called agent.
The Oracle ODBC agent acts as an ODBC client and then the ODBC/OLEDB datasource must be accessible on the machine where the Oracle HSODBC agent is running.
The agent can be :
A transparent gateway agent is a system-specific source and behaves as if you had an Oracle database. You can then use the Oracle language (function, …) but you must purchase an agent (a special program).
Gatway enables integration with data stores such as :
When an heterogeneous services source is created, an SID is also created to refer to it.
The generic connectivity agent (DG4ODBC) don't support stored procedure and distributed transactions and must be installed on the same machine as the database while the agent gateway talk with the same language than the database and support natively the datatype conversion.
Check here for a complete list of features and restrictions by agent : Data sheet Oracle Generic Connectivity and Oracle Transparent Gateway®
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