An XML processing model enables applications that use an XML stream to perform metadata-based or other processing of data during a data validation operation while preserving a streaming processing model. For example, while an XML node is being validated, requests can be received regarding the status of the validation and any processing that may be required with the node in order to conform it to requirements of an external application. A validator exposes public APIs that allow such validation-time requests from an event handler that is associated with an external application and that is registered with the XML stream. Messages that identify schema annotation definitions are provided to an external application to direct the type of processing to be performed on nodes at application runtime. Thus, applications can process a node according to the annotation definition concurrently with validation of the given node by the validator.