Nil Values in Schema
For any particular data element, there is an important distinction between the following cases:
(a) the element is missing
(b) the element is present but has a 'nil' attribute assigned to it
In the first case, this means that the sender is supplying no information about the element. This would typically be because no information is available, or because there has been no change in the value of the element. A recipient would not be expected to take any action as a result of this. In particular, it would not be expected to clear any existing value.
In the second case, this means that the sender has explicitly cleared the element. As a result of this, a recipient would typically clear any value that it had previously stored for this element.
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> | 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.
Note - Where a field is defined as mandatory in the schema, it must not contain a nil value.