So far, we're being deliberately vague, and with good reason. Often, mapping requirements are not well known and change as more is discovered about the nuances of the messaging formats involved and their variations. It often surprises people familiar with dealing with XML-based messages that describing their validity simply in terms of schema by using XML Schema Definition (http://www.w3.org/XML/Schema) can turn out to be more complicated than it first appears. This is unfortunately sometimes to do with the different productions or instances of XML messages that can be created or produced by a single schema, often unintentionally. XSD is sometimes not precise enough, and integration is therefore often messy, requiring good tools to make things fit, while the purity of standards and specifications doesn't go far enough to avoid ambiguity in implementation. This is a theme we'll come back to many times during this book: to be successful, any integration technology must be flexible...