Xmill: an Eecient Compressor for Xml Data

We describe a tool for compressing XML data, called XMill, that usually achieves about twice the compression ratio of gzip at roughly the same speed. The intended applications are XML data exchange and archiving. XMill does not need schema information (such as a DTD or an XML-Schema), but can exploit hints about such a schema in order to further improve the compression ratio. XMill incorporates and combines existing compressors in order to compress heterogeneous XML data: it uses zlib, the library function for gzip, as well as a collection of datatype speciic compressors. XMill can be extended with new specialized compressors: this is useful in applications managing XML data with highly specialized data types, such DNA sequences, images, etc. The paper presents a theoretical justiication for the method used, XMill architecture and implementation, a new languages for expression the hints about the XML schema, and a series of experiments validating XMill on several real data sets.