Chapter 5: Realizing a Metadata Framework 159 science, and in this book, we defi ne it as “data models that describe all objects that can exist in our framework, including their properties, and the relationships between them.” Our whole metadata framework is basically an ontology framework. What is especially important is that ontology describes and defi nes accurately all things that can exist in our framework, what kind of properties they may have, how those properties can be used, and what 19 kind of relationships there are between the objects. For example, our ontology declares that a property named bitrate exists, and that it can be used as a metadata attribute to describe certain video and music fi les. It also states that if a bitrate value is used, it must be an integer number with the quantity of bits per second. Another example is that our ontology declares that there can be only one creation date for any given object, and therefore trying to add another creation date results in an error. Thus, the ontology is used both to ensure interoperability of metadata between applications, and to detect some possible error situations. Obviously, no ontology can detect all possible error cases. For example, our ontology declares that all songs have a metadata attribute “artist” that can contain any text. If someone enters the name of the composer instead, there is little our framework can do to detect the mistake. Better defi nition for “artist” and other attributes having a person as a value, would be URIs identifying people. Unfor- tunately, there are no reasonable solutions available yet, but there are 20 some interesting proposals such as OpenID . The main thing is that semantics and possible values of any given metadata property is docu- mented as accurately as possible and still give application developers and users enough fl exibility to cover many possible different use scenarios. To conclude, without a clearly defi ned ontology, the system would quickly deteriorate to the situation where each application manages the metadata without any regard to interoperability. We emphasize that all metadata management systems must have clearly defi ned ontology in order to provide a useful service to more than one applica- tion. We also understand the need for fl exibility and for creating our own taxonomies (so-called folksonomies) and tags. Our framework 19 Since the ontology describes metadata, it is actually data about metadata, or meta- metadata. Remember we earlier emphasized the importance of meta-metadata? This is because our whole framework is basically about meta-metadata! 20 http://openid.net