What is DB Adaptor and how it is using in SOA Suite

DB ADAPTER:-

1). The Oracle Database Adapter enables a BPEL process to communicate with Oracle databases or third party databases through JDBC. The Oracle Database Adapter service is defined within a BPEL process partner link by using the Adapter Configuration Wizard of Oracle BPEL Process Manager (Oracle BPEL PM).

2). The Oracle Database Adapter enables Oracle SOA Suite and Oracle Fusion Middleware to communicate with database end points. These include Oracle database servers and any relational databases that comply with ANSI SQL and provide JDBC drivers.

3). The Oracle Database Adapter is a JCA 1.5 connector, which runs on the Oracle Application Server. It relies on an underlying JDBC connector/driver to enact the database communication.

How the Oracle Database Adapter Works:--

NOTE:-   Any adapter can not talk database directly 
            Adapter talk to JCA Connection Factory  like above picture 
In 10G:-
            If we create DB Adapter,it will generate Concrete WSDL with binding,JCA Binding instead of     SOAP Binding. 
In 11G:-  In run time The JCA file and Abstract WSDL file much together and the Concrete WSDL file Generated on top of it. 
TOP LINK LAYER :- 
            Top link layer file generated only for tables Operations Like SELECT, DELETE,INSERT AND    UPDATE...........etc
If we want execute a PURE SQL Query, The TOP LINK LAYER is not generated.
IF you need more about TOP LINK, Please see this site:-
http://docs.oracle.com/cd/E17904_01/web.1111/b32441/undtl.htm#CHDGCDDB