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.
