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Basic VSP Programming

VS-B-6 Error Recovery

Error handling in VSPs

Preliminaries

• When processing some form data in VSPs, there may be errors in the PL code (for example due to deadlock condition).
• The most popular way of catching the SQL errors is to declare an exception handler.

  declare error varchar;
  error := null;
  declare exit handler for sqlstate '*', not found result { error := __SQL_MESSAGE; };
   {
      -- some operation(s) that may cause a SQL error
   }
  if (error is not null)
    -- print the error message

• To simplify VSP production, the error handler can be written in a common file and included in each page.
• The external file can be included using the following markup:

  <?include path_to_the_external_file.extension ?>

Deadlock Example

The first example demonstrates the creation of a universal deadlock handler. If this file is included in any VSP the deadlock condition will cause a retry up to 3 times on an operation that follows the handler. The VSP simulates the deadlock situation by signalling the error. The error handler is therefore called.

Calculator with error handler

The second example is the vs_b_3 Calculator example. In this example there is an added handler to trap errors such as division by zero, and entering characters instead of numbers.