XML Usage
Nil Values in Schema
For any particular data element, there is an important distinction between the following cases:
...
The following table shows an example sequence of messages, and the expected actions taken by the data recipient. It should be noted, however, that the action is up to the recipient. The message from the sender is a notification, and not a command for the recipient to take action.
Message contents | Expected action by recipient |
|---|---|
aircraftRegistration element missing | Does not set or change aircraft registration value |
<aircraftRegistration>ABC123</aircraftRegistration> | Sets the value to ABC123 |
aircraftRegistration element missing | Does not change the value |
<aircraftRegistration xs:nil=’true’> | Clears the aircraftRegistration value |
Note that the sender should not use blank data elements such as <aircraftRegistration/> or <aircraftRegistration></aircraftRegistration> as these may cause validation errors. For example, if the schema specifies a format or a minimum length for an element, then a zero-length element will be invalid. This problem does not occur if using the nil attribute.
...