• the client consists of a thin Java applet whose main purposes are to realize the JAccess user interface and to provide a suitable environment for the execution of services coming from the JAccess server; when a user visits the JAccess web page on the WWW server, the client migrates on the user’s machine;
  
  
• JAccess server interacts through the JDBC interface with one or more federated DBMSs located in a network of heterogeneous machines; the DBMS which stores the data of JAccess itself is denominated primary; JAccess server is a Java program which runs on the WWW Server machine with the following main purposes:
  
  
• 1. it translates services into Java object networks and deliver them to demanding client machines;
• 2. it provides a view of the different databases as a list of different sources, hiding to the users and the service developers both the details of the location of data and heterogeneity of DBMSs;
• 3. it addresses the client’s requests to the DBMS servers and deliver back the results.

For what concerns the second point, in the current version of the tool, a SQL statement, belonging to a service, can refer to any number of tables belonging to the same data source. Therefore, at the moment, a single service can not access two distinct data sources. But this is not a real obstacle because a service expression can have more than one service and therefore more SQL statement involving tables of different data sources.

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