HP 3000 Manuals HP 3000 Manuals
Defining Transactions
You define transactions in a program unit to control what changes get
committed to a DBEnvironment and when they get committed.
A transaction consists of all the SQL commands that are executed between
a BEGIN WORK command and either a COMMIT WORK command or a ROLLBACK WORK
command. When a COMMIT WORK command is successfully executed, all
operations performed by the transaction it ends have a permanent effect
on the DBEnvironment. The opposite is true for a ROLLBACK WORK command;
no operations performed by the transaction it ends have a permanent
effect on the DBEnvironment.
The number and duration of transactions in an application program depend
on such factors as:
* Concurrency: Concurrent DBE sessions may compete for data and
index locks and buffers.
* Update activities: Applications that are update-intensive should
issue COMMIT WORK commands more frequently to avoid data re-entry
in the event of a failure.
* Data consistency: Program changes to a table that are meaningful
only if changes are made to another table should be committed or
undone at the same time to ensure the data remains consistent.
The commands at 4 and 5 in subroutines BeginTransaction and
EndTransaction in Figure 3-1 start and end a transaction that consists of
a single execution of the SELECT command at 8 in subroutine QueryTable.
The BEGIN WORK command in subprogram unit BeginTransaction is optional
but recommended. If you omit a BEGIN WORK command, ALLBASE/SQL
automatically issues a BEGIN WORK on your behalf before executing the
first SQL command that requires that a transaction be in progress.
The COMMIT WORK command in subprogram unit EndTransaction terminates the
transaction after each execution of the SELECT command. Because the
program does no DBEnvironment updates, this command is used to terminate
the transaction even if an error is encountered. In programs that update
data in a DBEnvironment, a ROLLBACK WORK command could be used to undo
the effects of any database changes that occurred during a transaction
before the error occurred.