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Purposes of Status Checking
Status checking is performed primarily for the following reasons:
* To gracefully handle runtime error and warning conditions.
* To maintain data consistency.
* To return information about the most recently executed command.
Handling Runtime Errors and Warnings
A program is said to be robust if it anticipates common runtime errors
and handles them gracefully. In online applications, robust programs may
allow the user to decide what to do when an error occurs rather than just
terminating. This approach is useful, for example, when a deadlock
occurs.
If a deadlock occurs, sqlcode is set to -14024 and an SQLEXPLAIN call
retrieves the following message:
Deadlock detected. (DBERR 14024)
ALLBASE/SQL rolls back the transaction containing the SQL command that
caused the deadlock. You may want to either give the user the option of
restarting the transaction, automatically re-execute the transaction a
finite number of times before notifying the user of the deadlock, or
re-execute the transaction until the deadlock is resolved.
Maintaining Data Consistency
Two or more data values, rows, or tables are said to be consistent if
they agree in some way. Changes to such interdependent values are either
committed or rolled back at the same time in order to retain data
consistency. In other words, the set of operations that form a
transaction are considered as an atomic operation; either all or none of
the operations are performed on the database. Status checking in this
case determines whether to commit or roll back work.
For example, in the sample database (SampleDBE), each order is defined by
rows in two tables: one row in the PurchDB.Orders table and one or more
rows in the PurchDB.OrderItems table. A transaction that deletes orders
from the database has to delete all the rows for a specific order from
both tables to maintain data consistency. A program containing such a
transaction should commit work to the database only if it is able to
delete the row from the PurchDB.Orders table and delete all the rows for
the same order from the PurchDB.OrderItems table:
EXEC SQL BEGIN WORK;
EXEC SQL DELETE FROM PurchDB.OrderItems
WHERE OrderNumber = :OrderNumber;
If this command succeeds, the program
submits the following command.
EXEC SQL DELETE FROM PurchDB.Orders
WHERE OrderNumber = :OrderNumber;
If this command succeeds, the program
submits a COMMIT WORK command. If this
command does not succeed, the
program submits a ROLLBACK WORK command.
This ensures that the previous delete won't
remove part of the information about this
order when the rest of the information could not
be deleted.
Checking the Most Recently Executed Command
Depending on which ALLBASE/SQL command was most recently executed, you
can make checks to insure that the command executed in a manner
appropriate to the program's context. The following section, "Using the
sqlca," gives explanations based on each sqlca element. Later in this
chapter, the section "Explicit Status Checking Techniques" provides
examples based on specific programming tasks.