HP 3000 Manuals HP 3000 Manuals
Defining Transactions
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.