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

126 Chapter4
Using the Database
TurboIMAGE/XL Logging Services
TurboIMAGE/XL Logging Services
TurboIMAGE/XL provides logging through the following two MPE/iX services:
• Transaction Management (XM) logging
• User logging
XM logging is used to ensure the physical integrity of the database. It is transparent to the
user because it is automatically enabled unless AUTODEFER is enabled. Dynamic
roll-back recovery (explained in chapter 7) works in conjunction with XM and ensures the
logical integrity of the database. Dynamic roll-back uses XM to roll back dynamic
transactions online while other database activity is occurring. Dynamic transactions are
defined later in this chapter. User logging, which is described below, is not required for
dynamic roll-back recovery, but is recommended to guard against a hard disk crash.
User logging is used to ensure the logical integrity of the database. It is initiated by the
user and can be used for several purposes. For example, user logging is required to perform
roll-forward recovery. It can also be used to keep a record of individual database
transactions (see the description of the >FILE command of DBRECOV in chapter 8). In
addition, it is sometimes used for audit purposes to keep a record of all database
transactions. Because some user logging considerations are important to applications, they
are discussed below. For a more complete discussion of TurboIMAGE/XL logging services,
refer to chapter 7.
What User Logging Does
The user logging and recovery facility enables all database modifications to be logged
automatically to a tape or disk log file. In the event of a system failure the log file is read to
re-execute transactions or identify incomplete transactions, depending on what type of
recovery process is being used. In addition, the transaction logging system can be a useful
tool for auditing. The log file is actually a record of all modifications to the database. The
intrinsic DBMEMO, capable of logging user text, facilitates interpretation of the log files for
future reference.
The database administrator is responsible for enabling or disabling the logging and
recovery processes and generating backup database copies, thus making logging a global
function controlled at the database level rather than at the individual user level. A process
is said to be logging if all of the following are true:
• The database has been enabled for logging by the database administrator.
• A logging process has been initiated from the system console.
• The user is accessing the database in one of modes 1 through 4.
How User Logging Works
The following TurboIMAGE/XL intrinsics are automatically logged when the database is
enabled for logging and a user opens the database in a mode which permits modifications:
DBOPEN, DBCLOSE, DBPUT, DBUPDATE, DBDELETE, DBBEGIN, DBEND, DBMEMO,