Exchanging Graphs with GXL

GXL (Graph eXchange Language) is designed to be a standard data exchange format for graph-based tools. GXL is defined as an XML sublanguage, which offers support for exchanging instance graphs together with their appropriate schema information in a uniform format. Formally, GXL is based on typed, attributed, directed, ordered graphs which are extended by concepts to represent hypergraphs and hierarchical graphs. Using this general graph model, GXL offers a versatile support for exchanging nearly all kinds of graphs. 1 Motivation and Background A great variety of software tools relies on graphs as internal data representation. A standardized language for exchanging those graphs offers a first step in improving interoperability between these tools. In software reengineering, for instance, various graphbased tools are used. These include extractors (e. g. scanner, parser), abstractors (e. g. query tools, structure recognition tools, slicing tools etc.), and visualizer (e. g. graph and diagram visualizer, code browser). Currently, these tool components are used more or less independently. [29] gives an overview on existing combinations of tools used in various reengineering projects. Using a common graph interchange format, these tools can be composed to build a genral and powerful reengineering workbench. The development of GXL (Graph eXchange Language) originally started to support data interoperability between reengineering tools. But since GXL was developed as a general format for describing graph structures, it is applicable in further areas of tool interoperability. Especially, GXL is used to support interoperability between graph transformation systems [45] or graph visualization systems. Now, the work on GXL aims at offering a general exchange format for graph-based tools. Exchanging graphs with GXL deals with both, instance graphs and their corresponding graph schemas. Firstly, GXL offers a versatile support for exchanging all kinds of data based on typed, attributed, directed, ordered graphs including hypergraphs and hierarchical graphs. Secondly, GXL offers means for exchanging graph schemas representing the graph structure i. e. the definition of node and edge classes, their attribute schemas and their incidence structure. Both, instance graphs and graph schemas, are exchanged by XML documents (Extended Markup Language) [47]. After a short survey of the genealogy of GXL in section 2, the major concepts of GXL to exchange instance graphs are introduced in section 3. The language definition of GXL is given by its XML document type definition (DTD) in section 3.4. Section 4 describes the exchange of graph schemas. The current and intended usage of GXL is summarized in section 5. More information on GXL can be found in [29] and [27]. Up-to-date information including tutorials and further GXL documents are collected at http://www.gupro. de/GXL.