With Version 12.1 ORACLE provided a new way to determine that a program unit may only be called by other program units which are part of a whilelist. This new feature was implemented using the ACCESSIBLE BY clause. ACCESSIBLE BY was declared on top level (e.g. package specification).

ORACLE 12.2 provides a higher granularity for the whitelist. It is now possible to define the access on a packaged procedure/function too.

Code Example:

CREATE OR REPLACE PACKAGE emp_tapi IS
   TYPE emp_tapi_rect IS RECORD (ename    emp.ename%TYPE
                               , comm     emp.comm%TYPE
                               , hiredate emp.hiredate%TYPE
                               , empno    emp.empno%TYPE
                               , mgr      emp.mgr%TYPE
                               , job      emp.job%TYPE
                               , deptno   emp.deptno%TYPE
                               , sal      emp.sal%TYPE);

   TYPE emp_tapi_tabt IS TABLE OF emp_tapi_rect;

-- ========================================================================
-- insert
-- ========================================================================
   PROCEDURE ins (emp_tapi_rec IN OUT NOCOPY emp_tapi_rect)
      ACCESSIBLE BY (PACKAGE emp_maint_up);

-- ========================================================================
-- update
-- ========================================================================
   PROCEDURE upd (emp_tapi_rec IN emp_tapi_rect) 
      ACCESSIBLE BY (PACKAGE emp_maint_up);

-- ========================================================================
-- delete
-- ========================================================================
   PROCEDURE del (p_empno IN emp.empno%type)
      ACCESSIBLE BY (TRIGGER emp_io_d_t);
END emp_tapi;
/

CREATE OR REPLACE PACKAGE BODY emp_tapi IS
-- ========================================================================
-- insert
-- ========================================================================
   PROCEDURE ins (emp_tapi_rec IN OUT NOCOPY emp_tapi_rect)
      ACCESSIBLE BY (PACKAGE emp_maint_up)
   IS
   BEGIN
      ...

As you can see, ACCESSIBLE BY clause has to be specified in specification and body, if we want to define the whitelist on subprograms. 

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Conclusion

The ACCESSIBLE BY clause is very useful (e.g. refactoring) and the higher granularity will help to make it even better.