TurboIMAGE/XL Database Management System Reference Manual MPE/iX V6.5 (30391-90011)

358 Chapter7
Logging and Recovery
DBRECOV Commands Used with Roll-Forward and Roll-Back Recovery
The initial default condition for stop time and end-of-file is that none is imposed on
recovery. When a particular date or record number has been specified by STOPTIME or
EOF, it can be changed by specifying a new date or record number.
The following provides an example of the override:
>CONTROL NOSTAMP,STAMP
Because STAMP was entered after NOSTAMP, STAMP negates NOSTAMP, so that
recovery proceeds with the time-stamp check intact.
For the options and form of the >CONTROL command, refer to the discussion of the
>CONTROL command of the DBRECOV utility in chapter 8. Note that the >CONTROL command
does not specify a database. Therefore, all >CONTROL options (except NOSTORE which
must be issued before recovery on a specified database is performed) apply to all databases
being recovered.
FILE Command
The recovery file facility is an interface between the recovery system and the application
program. With the >FILE command, you sort the log file by individual users and/or user
identifiers, and designate an MPE/iX file as the destination for each user's log records.
The recovery file facility is based on the concept of transactions within transaction blocks.
A transaction block consists of all transactions between a call to DBOPEN and a call to
DBCLOSE (see Figure 7-1. earlier in this chapter). Within each transaction block, a
transaction is defined as one of the following:
1. A single call to DBPUT, DBUPDATE,orDBDELETE if not preceded by a call to DBBEGIN (or
DBXBEGIN if logging is enabled), or
2. A sequence of calls beginning with a call to DBBEGIN or DBXBEGIN, followed by any
number of calls to DBPUT, DBUPDATE, or DBDELETE and ending with a call to DBEND or
DBXEND respectively.
For each transaction block, the >FILE command returns the initial DBOPEN log record to the
user recovery file. The DBCLOSE record is returned as well, unless one of the following
occurs:
1. All of the transactions within the block could not be recovered, or
2. There was no DBCLOSE log record for this block on the log file. This happens when the
system fails while the database is open.
Consequently, an application can determine the outcome of recovery to some extent by
examining the number of DBOPENs and DBCLOSEs or pairs of DBBEGIN and DBEND or
DBXBEGIN and DBXEND log records returned to the user recovery file. If there are as many
calls to DBCLOSE as to DBOPEN, it is likely that all transactions were successfully recovered.
However, the possibility exists that an entire transaction block was lost due to the system
failure if the block was very short. Fewer calls to DBCLOSE indicate the possibility that
some transactions were lost and need to be re-entered. More information about recovery
can be inferred from the recovery file by using the optional rmode and fmode parameters.
These parameters return transaction information to the user recovery files in addition to