Unless you elect to put all your error handling inline, you will find it necessary to design one or more error handlers for your DB Interface Module applications. So far, we have deliberately kept our example error handling routines extremely simple, in order to concentrate on presenting an overview of the error model. In this section, we go into the subject in more detail.

The first thing a useful error handler needs to know is what kind of error it is dealing with. The Essential Tools Module approach is to classify errors as internal or external, depending on whether or not they can reasonably be predicted in advance by the programmer. In the Essential Tools Module, an internal error throws an exception of type RWInternalErr, and an external error throws an exception of type RWExternalErr. In the DB Interface Module, however, database errors are also possible. To simplify the situation, we have adopted the policy that all errors are external errors. This means that:

The errorCode() member function of RWDBStatus tells you the kind of error you're dealing with; for a list of error codes, see the entry for RWDBStatus in the SourcePro API Reference Guide. Of these, the codes vendorLib, serverError, and serverMessage indicate database errors or events, while the remaining error messages are generated by the DB Interface Module itself. But even though there are many more error codes generated by the DB Interface Module, our experience indicates that most errors in database applications are database errors. In order to distinguish database errors from other errors, the first part of our error handler might look like this:

Once your application receives a database error, you must first interpret it before you can handle it. The main problem here is that each database vendor has its own ways of representing and categorizing errors. The DB Interface Module solves this problem by providing class RWDBStatus with four variables reserved for database-dependent information. These are accessed using the member functions vendorError1(), vendorError2(), vendorMessage1(),and vendorMessage2(). The first two return long int, and the second two return RWCString.

Each DB Access Module defines the contents of RWDBStatus for its particular database. Each Access Module user’s guide provides details. As comparative examples, Table 9,Table 10, and Table 11 list the mappings used for three popular databases, Oracle, Sybase, and Microsoft SQL Server.

You can see in these tables that the layouts are similar but not identical. This is not surprising, since error reporting is not standardized well among database APIs. Let us now return and modify our error handler to cope with this problem:

Now let's create an error condition. The following fragment attempts to drop a database table that doesn't exist:

On our system, this is the output when the example is run against an Oracle database and a Sybase database, respectively:

(Oracle output)

(Sybase output)

Now suppose that your application needs to take some special action if a table doesn't exist. Evidently your application must be able to interpret database errors, not just report them, and to interpret them it must recognize the database. Perhaps the most straightforward way for your application to handle different databases, then, is to have separate error handlers for each database type:

When the database-dependent table does not exist error comes up, it is trapped explicitly and handled by the take special action case in the appropriate error handler. The trade-off here is between portability and specificity. The more portable your application, the less database-dependent intelligence it can use, and vice versa.

To summarize, once you make the decision to interpret information in a database-dependent way, you can do it several different ways. In addition to the examples shown here, you may consider deriving from class RWDBStatus in order to obtain a database-dependent interface to it. Or you might design a database exception class with derived database-dependent variants, and explicitly throw() instances of your class instead of using the raise() method of RWDBStatus.