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The ASAP Cars system, like most systems, relies on a series of UDTs that are used in many of the components. There are only two such UDTs in this case study, but a typical system contains many more. All these types are packaged together in a component called CommonTypes, as shown in Figure 13-21. By packaging all the commonly used types in a single component, the overall system s coupling is kept at a minimum. The coupling diagram in Figure 13-22 shows the components that are typecoupled to CommonTypes.

When a user logs in, it s useful to identify the type of the user and to capture certain key user attributes. You can later use this information in the security policies that are attached to the database objects. The built-in USERENV namespace is ideal for capturing these kinds of information. The USERENV namespace, of course, is just one of the application context namespaces that you can use. You ll have to create your own application context so you can define which attributes you want to use in setting your security policies. To define your own application context, you need to do the following: 1. Create a PL/SQL package that sets the context with the help of functions. 2. Create an application context that uses the package you created.

To set the application context for the hr user, you need to create a PL/SQL package. Listing 12-12 shows you how to create a simple package called HR_CONTEXT to set the application context. The package includes a single procedure that selects the value of the employee_id column into the empnum variable. Since this SELECT statement is based on a WHERE clause that determines the last_name of the employee based on the value of the SESSION_USER attribute, the employee_id will be that of the username by which the current user is authenticated by the database. Listing 12-12. Creating a Package to Set the Application Context SQL> CONNECT hr/hr Connected. SQL> CREATE OR REPLACE PACKAGE hr_context AS 2 PROCEDURE select_emp_no ; 3* END;

Figure 13-22. The type-coupling diagram of the system The star pattern shows that the system has ideal coupling. None of the outer components are coupled to each other. It is very important to note that the CommonTypes component is not coupled to any other components in the system. Since many components in the system are coupled to the component containing the shared UDTs, circular coupling may occur if a type were added to CommonTypes that type-coupled CommonTypes to other components.

SQL> / Package created. SQL> CREATE OR REPLACE PACKAGE BODY hr_context as 2 PROCEDURE select_emp_no IS 3 empnum number; 4 BEGIN 5 SELECT employee_id INTO empnum FROM employees WHERE 6 UPPER(last_name) = 7 sys_context('USERENV', 'SESSION_USER'); 8 dbms_session.set_context('employee_info', 'emp_num', empnum); 9 END select_emp_no; 10* END; SQL> / Package body created. SQL>

You can think of an application context as a named set of variable=value pairs that are specific to a session. Once you create the package (HR_CONTEXT) that helps set the application context, you can go ahead and create the application context itself as follows. Note that the hr user uses the package just created in the previous section to create the employee_info application context. SQL> CONNECT system/system_passwd; Connected. SQL> GRANT CREATE ANY CONTEXT TO hr; Grant succeeded. SQL> CONNECT hr/hr; Connected. SQL> CREATE CONTEXT employee_info USING hr.context; Context created. SQL> You can set the application context for a user in two ways. The first is to implement an application context by itself, without fine-grained access control. To do this, you just create an event trigger on a user s logon so the user will invoke the SELECT_EMP_NO procedure belonging to the HR_CONTEXT package upon logging into the database. Here s how you create the logon trigger to set the initial context for a user: SQL> CREATE OR REPLACE TRIGGER hr.security_context 2 AFTER LOGON ON DATABASE 3 BEGIN 4 hr_context.select_emp_no; 5* END; SQL> / Trigger created. SQL> The preceding logon trigger uses the SELECT_EMP_NO procedure of the HR_CONTEXT package you created to grab the user s employee_id and store it in the emp_num variable. The second way to set or reference an application context is to do so as an integral part of VPD, using a policy function implementing fine-grained access control. The following section discusses this in detail.