The $ZQGBLMOD function enables an application to determine whether it can safely apply a lost transaction to the database. A lost transaction is a transaction that must be rolled off a database to maintain logical multisite consistency. $ZQGBLMOD() always applies to data-level (level-0) nodes.
The format for the $ZQGBLMOD function is:
$ZQGBLMOD(gvn)
The subscripted or non-subscripted global variable name (gvn) specifies the target node.
A return value of zero (0) means the value of the global variable has not changed since the last synchronization of the originating and replicating instances.
A return value of one (1) means the value of the global variable may have changed since the last synchronization of the originating and replicating instance.
$ZQGBLMOD function produces an error if you submit an argument that is not a global variable name.
Internally, $ZQGBLMOD (gvn) compares the GT.M transaction number in the database block in which the global variable name is (or would be) stored with the value in the Zqgblmod_Trans field stored in the database file header.
For example, if x is the transaction number of the level-0 database block in which gvn resides, and y is the value of Zqgblmod_Trans of region reg containing gvn, then the following is true:
If x <= y, no transaction modified the level-0 database block z in which gvn resides since the originating and replicating instances synchronized with each other. $ZQGBLMOD() returns a zero (0).
If x > y, some transaction modified z, but not necessarily gvn, after the originating and replicating instances synchronized with each other. $ZQGBLMOD() returns a one (1).
If a transaction is a lost transaction that has been rolled back and it is determined that for all the M globals set and killed in the transaction $ZQGBLMOD() is zero (0), it is probably safe to apply the updates automatically. However, this determination of safety can only be made by the application designer and not by GT.M. If the $ZQGBLMOD() is one (1) for any set or kill in the transaction, it is not safe to apply the update.
![[Note]](images/note.jpg) |
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The test of $ZQGBLMOD() and applying the updates must be encapsulated inside a GT.M transaction. |
Another approach to handling lost transactions would be to store in the database the initial message sent by a client, as well as the outcome and the response, and to reprocess the message with normal business logic. If the outcome is the same, the transaction can be safely applied.
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If restartable batch operations are implemented, lost batch transactions can be ignored since a subsequent batch restart will process them correctly. |