iX V6.5 (30391-90011)

Chapter 2 59

Database Structure and Protection

Protecting the Database

User Classes and Locking

TurboIMAGE/XL does not consider user classes when locking a database entity. Any data

set or any data item can be referenced in a lock request by any user of a database

regardless of his or her user class.

Protection in Relation to Library Procedures

All access to a database is achieved through database control blocks that reside in

privileged MPE/iX ﬁles which are not directly accessible to database users. Because no

user process can read or modify these control blocks, TurboIMAGE/XL guarantees

protection of the database from unauthorized programmatic access. Refer to the detailed

description of these control blocks in chapter 10. For more information about MPE/iX ﬁles

and privileged mode, refer to the MPE/iX Intrinsics Reference Manual.

All TurboIMAGE/XL library procedures that structurally modify the database execute in

critical mode. This defers any requested process termination while modiﬁcations are in

progress. If any ﬁle system failures occur during such database modiﬁcation,

TurboIMAGE/XL causes process termination because the database integrity is

questionable.

The Database Buffer Area Control Block (DBB) contains pointers to the data set blocks

that are used to transfer data (see chapter 10 for additional information). All data set

blocks whose contents are changed, reﬂecting a modiﬁcation of the database, are always

logged by an internal MPE/iX service called Transaction Management (XM) before the

library procedure returns to the calling program. This guarantees database integrity

despite any program termination that might occur between successive procedure calls.

However, deferred output (AUTODEFER) allows the user to override this scheme. When

AUTODEFER is enabled, the database does not use MPE/iX Transaction Management.

Instead, AUTODEFER uses the MPE/iX ﬁle system default recovery mode. This mode keeps

data pages in memory for as long as possible until ﬁle close time. In this mode, a system

failure can cause the loss of database integrity. For more information about AUTODEFER,

refer to the >>ENABLE command of DBUTIL in chapter 8.

Protection Provided by the TurboIMAGE/XL Utilities

The TurboIMAGE/XL utilities perform various checks to ensure database integrity:

• They acquire exclusive or semi-exclusive access to the database being processed.

(Chapter 4 contains more information about types of access in the discussion of opening

a database.)

• Only the database creator or a user supplying the correct maintenance word can

execute the utilities. The database creator deﬁnes the maintenance word when the

database is created with the DBUTIL utility (refer to chapter 8). In addition, anyone

without system manager (SM) capability intending to use the DBUTIL >>SHOW

command or anyone running the utilities other than DBRECOV must be logged on to the

group in which the database resides (refer to chapter 8).

If no maintenance word is deﬁned, only the database creator can execute the utilities.

The exception to this rule is that a user with system manager (SM) capability can use

the DBUTIL >>SHOW command on any database without having to supply the