Topic Map Merging

Merging is a cornerstone of the topic map paradigm. The merge process enables distributed and modular creation of topic map "knowledge-bases". There are three ways in which two topic maps can be merged.

• Under explicit application control. A topic map application may provide the user with the ability to selectively merge topic maps. There is no restriction in the XTM specification to prevent an application from merging topic maps at runtime as needed or as directed by the application user.
• Processing a <mergeMap> element. The <mergeMap> element allows an author to explicitly request the merge of another topic map with the map that he or she creates. The mark-up for the <mergeMap> element also allows the author to define a set of topics to be added to the scope of every characteristic in the external map. This additional, externally defined, scope can be useful for preventing unwanted topic merges from occurring, or else to indicate the source of a characteristic in the final merged map.
• Processing a <topicRef> element. A <topicRef> element is not limited to referring only to topics contained within the same topic map document. A reference can be made to a topic in another topic map document. If a reference is made to a topic in an external topic map, then a topic map processor is required to retrieve the entire topic map containing that topic and to merge it with the topic map containing the reference.