7.5 Other Ways to Validate Input

The converters and validators validate simple syntax and semantic rules, but many applications require more complex input validation. For instance, in a flight reservation application, verifying seat availability on the selected flight and validating the credit card information require access to external systems. It's common to defer this type of validation to the backend code, invoked through event handlers when all the simple tests succeed. We'll look at event handlers in the next chapter.