GraphML 1.0

GraphML 1.0 is the first specified release of the Graph Markup Language, an XML-based file format for exchanging graph structure data. It grew out of the graph drawing community: a file-format workshop held on the eve of the Graph Drawing 2000 symposium in Williamsburg formed the task force, and a proposal for the structural layer was presented at GD 2001 in Vienna. The 1.0 schema was published under the namespace http://graphml.graphdrawing.org/xmlns/1.0/graphml-structure.xsd and carries a 2001/2002 GraphML Team copyright. It was conceived as a successor to the older ASCII-based format:gml (itself a Graph Drawing product from Passau) and as a general interchange target alongside peers such as format:gxl-1.0, format:xgmml-1.0, format:gexf-1.3, and the DOT language of format:dot.

The data model is deliberately minimal at its structural core. A root <graphml> element contains one or more <graph> elements, each holding <node> and <edge> children. Every node carries a unique id; each edge references endpoints via source and target. Directedness is controlled per graph through the edgedefault attribute (directed or undirected), and edges may override it individually, so directed, undirected, and mixed graphs are all expressible. Beyond simple graphs, GraphML supports nested (hierarchical/compound) graphs, in which a node may itself contain a <graph>; <hyperedge> elements with multiple <endpoint> children for edges spanning more than two nodes; and <port> elements on nodes, which may be nested, to give edges fine-grained attachment points.

Application data is carried through a separate attribute mechanism rather than baked into the topology. <key> elements, declared as children of <graphml>, define named data functions with a for scope (node, edge, graph, port, and so on), an attr.name, and an attr.type drawn from string, boolean, int, long, float, and double; a <default> supplies fallback values. <data> elements then attach concrete values to the corresponding structural elements. This clean separation of structure from semantics, plus permissive extension points, lets vendors layer their own namespaces (yFiles for tool:yed, for example) without breaking generic parsers.

GraphML 1.0 is widely readable: tool:networkx, tool:igraph, tool:gephi, tool:graphml2gv, and the Boost Graph Library import or export it, and the tool:graphinout converter treats it as a hub format. Its main limitations are practical: XML verbosity makes large graphs bulky, and full support for hyperedges, ports, and deep nested graphs is uneven across tools, so those features are often lost on round-trips. The later 1.1 revision refined the schema without changing the core model.