176 Personal Content Experience: Managing Digital Life in the Mobile Age There is also always a chance to create a new, different defi nition that has, for instance, the same name but is in a different namespace. The namespace must be different for such re-defi nitions as it also implies different semantics for the property and its use. As a conse- quence, from the framework point of view, they will be totally separate defi nitions. A relation defi nition is simply the name of the relation. A relation can be described between any two metadata objects, so there is no limit to the class types of the participants in a relation and no means to defi ne such restrictions in the ontology. Events are almost as simple as relations at the ontology level. The name of an event class describes the event type. In addition, event defi nitions contain an importance level that can be used to prioritize which events are moved to a secondary storage, should storage space become an issue. Any type of an event can occur to any type of meta- data object so, as in relations, there are no restrictions concerning the type in the ontology. Context metadata is managed by a separate component, Context Engine, in the framework. Details of Context Engine and context objects are described below. Context metadata is a metadata object defi nition where its URI points to a context object stored in the Context Engine, with the possible context metadata attributes defi ned. For MdE it does not differ from any other content. In addition to the semantically meaningful properties, keywords can be attached to objects, without defi ning further meaning for the words. This construct can be used to create a full-text index, for example, of the words in an SMS message. 5.7.2.2 Making Queries Metadata Engine has three kinds of queries: 1. querying metadata objects 2. querying relationships 3. querying events. Metadata object queries are used for retrieving metadata objects from the database. The simplest metadata object query is to fi nd all proper- ties of a given metadata object, such as retrieving all properties of a photo. Metadata objects can also be searched based on their proper- ties (fi nd all metadata objects whose ‘location’ property has the value ‘London’). This query would return all objects whether they are images, music, or any other content type. You can also limit the query to