TurboIMAGE/XL Database Management System Reference Manual (30391-90012)

Chapter 2 53
Database Structure and Protection
Protecting the Database
Specify no access for a user class to an entire data set by excluding the user class from the
read and write class lists of the data set. To allow no access to a specific data item, include
the user class in the read class list of the data set and exclude the user class from the read
and write class lists of the data item.
Note that the read or write portions of the user class list can be left empty; this is known
as a null list. In the example below, only the database creator has write access:
(11,14/) The write class list is null.
Creator-Only Access If you specify an empty data set user class list, as shown below,
only the database creator has access:
(/) This is called a null list.
Sample Read and Write Class Lists Table 2-2. contains sample lists for the
CUSTOMER data set and CREDIT-RATING data item in the ORDERS database.
Because a write class list of 14 implies that user class 14 is in the read class list, the
CREDIT-RATING read class list is redundant. However, it could be included as a reminder
in the schema of the total capability granted to user class 14.
Table 2-4. later in this chapter contains examples of the effects of read and write class lists.
Note that the examples take into account how the database access mode affects the data
set write list.
Null and Absent Lists
A distinction is made between the absence of both read and write class lists (which by
default allows read access) and a null list. When you specify the lists in the schema, they
are enclosed in parentheses and separated by a slash, for example, (11,14/15). A null list
can be one of the following:
(/) Both read and write class lists are null.
(11,14/) The write class list is null.
Because the existence of a write class list implies a read class list, even if no user classes
are listed in the read list and at least one user class is specified in the write list, the read
class list is not considered null.
An absent list and the following null write list, in which the read portion contains all user
classes and the write portion is null, yield the same result:
(0,1,2,3,...63/)
The effect of null and absent lists is illustrated in Figure 2-8. later in this chapter.
Table 2-2. Sample Read/Write Class Lists
Read Class List Write Class List
CUSTOMER (data set) 11,14 11,18
CREDIT-RATING (data item)14 14