Scheme
A scheme is an identifier for a Controlled Vocabulary which is the MDDL way of identifying a list of permitted values for a property. MDDL provides many controlled vocabularies for its various elements. Some of the schemes are ubiquitous (like the ISO currency codes) while others may be specific to a provide (i.e. a provider can provide its own list of values for a particular element).

Example of a scheme in use:
<currency scheme="http://www.mddl.org/ext/scheme/iso4217-alpha-3.xml">GBP</currency>

In this case, the scheme identified is the default value provided in the MDDL W3C schema so it can be removed and the default is assumed:
<currency>GBP</currency>

Note that schemes where the number of items in the list is small and finite can be contained within a specific file thus the scheme ends in the ".xml" extension (as is done with the ISO 4217 Currency Codes). In the event a list is too large - or changes too frequently - to be kept in a disk file that is published, the ".xml" extension is dropped and the URL identifies a Web Service that can be used to validate the value.

Example of a Web Service scheme in use:
<code scheme="http://www.example.com/mddl/scheme/iso6166">AQ01234567890</currency>

In this example, the scheme identifies a web service (hosted by example.com) where the value ('AQ01234567890') can be passed to validate the data. For more information, contact us regarding mddlQuery and mddlService. Validation will generally return one of three responses: