GXL provides a standardized notation for exchanging graphs, together with their structure definition, using Extensible Markup Language (XML) documents. Section 2 describes a variety of reengineering tools using graphs as an internal data representation, and some converters between different formats. From this, the main concepts of a universal tool are derived. Nodes and (hyper-)edges are identifiable elements and may be attributed. An attribute may again be a graph, thus allowing hierarchical graphs.

Section 3 describes the syntax of GXL. The abstract syntax graph of a small C program is explained in detail. Hypergraphs and hierarchical graphs are briefly presented by single elements. The features supported are specified by a unified modeling language (UML) class diagram and are formally defined by an XML document type definition (DTD). Additional constraints, such as edges connecting only nodes within the same graph, must be checked outside the DTD.

In section 4, the authors consider graph schemas representing the constraints typical of a domain of interest. These constraints are illustrated by unified modeling language (UML) class diagrams and are coded in GXL so that they can be exchanged together with the graphs. The final section summarizes some of the uses of GXL. There are many filters for converting GXL documents into local formats, and vice versa, to facilitate the exchange of data between different graph transformation tools.

A list of 116 references wraps up this clearly written introduction to GXL. It is recommended to people interested in graphs as research objects, as well as those interested in graph applications.