You define transactions in an executable section to control what changes
are committed to a DBEnvironment and when they are 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
within the transaction are permanent in the
DBEnvironment. When a ROLLBACK WORK command is executed, none of
the changes remain in the DBEnvironment.
The number and duration of transactions in an application program
depend on the following factors:
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.
For detailed information regarding transaction management, see
the document, "Performance Guidelines".
The commands at 4 and 5 in Figure 3-1 start and
end a transaction that consists of a single execution of the
SELECT command in procedure SelectData.
The BEGIN WORK command in procedure 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.
However, such an implicit BEGIN WORK always has the default isolation level
of Repeatable Read (RR). If you need to increase concurrency with a different
isolation level, use an explicit BEGIN WORK command.
The COMMIT WORK command in procedure
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.
