Chapter 5: Realizing a Metadata Framework 175 5.7.2.1 Defi ning Schemas The MdE ontology follows principles discussed in previous sections. It consists of metadata object, relation, and event defi nitions. Now we will take a closer look at these defi nitions and how they are used in MdE. A metadata object defi nition is a class with a name and a set of property defi nitions. The metadata attributes of a metadata object are described with property instances. A property instance always relates to a metadata object, i.e., there are no stand-alone properties without metadata object defi nition. A property defi nition has a name, a data type, and a value range. The data type can be any of the following: text, time, Boolean, double, or a 8-, 16-, 32-, or 64-bit signed or unsigned integer. The value range can be used to restrict the possible values of the property instances to a given range. For example, property defi nition for rating can limit its possible values to a range between 1 and 5. In MdE, one property defi nition can be associated with more than one metadata object defi nition. However, it is recommended to use inheritance instead of giving same property defi nitions to more than one metadata object class. There are some exceptions, such as when different cardinalities are needed in inherited metadata defi nitions. In addition to the type and value, property instances have source and confi dence. Source is a URI describing who has created or set the value of the property. Confi dence describes how confi dent the source was about the value when it was set. This is especially required when the property is the result of data mining or some probabilistic algorithm such as trying to detect who is in a photo. The cardinality of properties is set separately for each class they belong to. The properties can be defi ned obligatory; also the number of values of a property can be defi ned. For example, our ontology defi nes creation date property to be obligatory and unique for all content objects, i.e., its cardinality is one, and only one meaning, therefore every object must have a creation date and no more than one creation date. The cardinality can also be defi ned optional and infi nite so that the object may have several different values for the same property, or even no value. This is useful in instances of MP3 objects, where some may have several artists and others none. Note that when schema objects inherit properties from other objects, also property defi nitions are inherited as they stand. There is no way of changing defi nitions or even the cardinality. However, since the cardinality of the property is set separately to each metadata object defi nition, it means that it is possi- ble to defi ne one property defi nition that is optional for some metadata object defi nitions but compulsory in some other defi nition.