An xfMap document contains mapping rules that specify the construction of certain objects, with the most important one being an FME feature. Other objects that may be constructed are group and reference objects, but these are just “helper” objects that the XML Reader uses to construct its FME features.
A feature is a generic container capable of holding attribute and geometric information. FME contains a variety of feature processing facilities such as functions and factories. These functions and factories are available internally to the XML Reader through xfMap group objects. The XML Reader may further process the FME features with these group objects before outputting them to the FME.
The advantage of a streaming reader is the ability to read very large XML documents. A streaming reader does not hold an entire XML document in memory, so problems may arise if the current element being read requires information from previously read elements. This inherent streaming limitation may be overcome by using xfMap reference objects. Reference objects can hold information needed by the construction of features and groups when these require data from elements that were seen much earlier in the input XML document stream.
The XML Reader can currently construct three types of objects: FME features, xfMap groups, and xfMap references. Each type of object is specified through its corresponding xfMap mapping rule type.