Minutes of the 2nd GXL/GTXL-Meeting on March 1-2, 2001 at Universität Bremen

Aachen: Boris Böhlen
Berlin: Gabi Taentzer
Bremen: Giorgio Busatto, Berthold Hoffmann, Peter Knirsch, Hans-Jörg Kreowski,
Sabine Kuske
Budapest: Dániel Varró.
Erlangen: Mark Minas
Kent: Peter Rodgers
Koblenz: Andreas Winter
München: Andy Schürr
Paderborn: Gregor Engels, Szilvia Gyapay (on leave from Budapest)


0. Welcome and Introduction
1. Report on "Homework" given at the last meeting in Paderborn
(Sept. 5/6, 2000).
2. Report from other meetings concerned with graph exchange formats,
3. Further work on our proposal for GXL
4. Discussion of further aims
5. Distribution of work
6. Miscellaneous

0. The agenda sent with the invitations was approved.
Point 1 is scheduled for the first day, and the rest is left for the secon day.

The overall aim of this meeting is to appove a first version of GXL as a
standard, and to make considerable progress towards a standard for GTXL.

5 Pieces of homework were distibuted at the last meeting in Paderborn.

The following pieces have been done:
A Using XML-Schema to define GXL (Dániel Varró)
B Update of GXL according to the Paderborn Meeting
(Andy Schürr, Andreas Winter)
C GTXL draft  inside of UNIGRA-paper on GXL and GTXL(Gabi Taentzer und Michael Matz)

The following pieces have not been done:
D Proposal for control structures for GTXL (Albert Zündorf, Uni Paderborn)
E Translation of XML-Schema to DTDs (Reiko Heckel, Ralf Depke, Uni Paderborn)

1. Report on (Done) Homework

1A. Dániel Varró presents his report (with András Pataricza) on
"An XML-Schema Description of Graph Transformation Systems".
He points out advantages of XML-Schema wrt. typing of references /links
and inheritance.

The group favors the idea of defining GXL in XML-Schema, as it seems to be
the technology of the future.

1B. Andreas Winter reprots on joint work with Andy Schürr and Ric Holt
on the GXL-draft designed at the last meeting.

There has been some discussion via email, in particular on how hierarchical
graphs can be represented. The changes in response to that discussion are

Andreas explains the difference between object level (0), meta level (1) and
meta-meta level (2), and points out that no more than two meta levels are
needed. Conformance of graphs on level i with graph schemata on (meta) level
i+1 has to be checked outside the XML schema (i>=0).

Andy is not yet content with the definition of attribute types, as they
mostly re-invent list structures etc. that are well-known from programming
languages, and make up a considerable part of the standard.

GXL should provide for hooks were extensions can be built in, e.g. for
layout information.

There are still (too) many ways to represent hierarchical graphs; case studies
shall explore which of them are really needed, and guidelines to GXL users
shall define which ones should be used.

1C. GTXL draft (Gabbi Taentzer und Michael Matz)

Gabi reports on the modifications in the GTXL class diagram, which is
presented in her paper at UniGra'01.

The control structure is still unclear, as Albert failed to make a proposal.
A module structure is still missing.

It also is not quite clear how GTSes can be exchanged between different tools
so that their semantics is preserved.

2. Report on related standardizing efforts

Andreas reports that GXL has recently been accepted as a standard by the
re-engineering community.

The XMI-standard for exchanging UML diagrams is supported by several
available tools.

Andreas reports that the graph drawing community is starting to work on a
standard too, but is still in a very early stage of discussion.

Further communities with ongoing activities on standardization include:
- Petri nets (PNML)
- RuleML (related to AI)

Our group should approach the graph drawing community, and try to convince
them to base their standard on GXL, or cooperate in an adaptation of GXL so
that they can base their standard on GXL.

People from those comunities shall be invited to a special event at the 1st
int. GraGra conference.

3. Further work on our proposal for GXL

GXL - with some modifications discussed on this meeting - is accepted as
Version 1 of an exhange format. Requests for changing the format should be
sent to Andreas Winter.

Open points in the present version include:
- How should hierarchical graphs be represented?
- How should attributes be incorporated in the format?
- How can a package structure be incorporated?
- Should special kinds of graphs, like trees and the like, have a special
representation, like hypergraphs and binary graphs?

4. Discussion of further aims

GXL shall be consolidated by using it "by hand", and in tool implementations.
GTXL shall be elaborated.

5. Distribution of work

GXL homework

- Send links to XML-activities on graph-like exchange formats to Gabi,
for being added to the GXL homepage (Petri net, Graph drawing, rules,
conceptual graphs, CASE tool developers ...).
- Experiment with GXL, especially with representing hierarchical graphs.
- Start developing tools using GXL.

- Ask Jürgen Ebert for references to notions of hierarchical graphs.
(and tell Giorgio!)

Andreas , Dániel
- Define XML-Schema for GXL.

- Explore layout fascilities of Argo (?) and Rational Rose
(in order to learn for GXL).

- Send links to GXL-producing/parsing tools at Aachen to Andreas Winter
(and Andy Schürr).

Gabi, Peter
- Make a proposal to add layout information to GXL
(along the lines of an older AGG proposal).

- Investigate different notions of hierarchical graphs and how they can be
encoded in GXL.

- Contact key mebers of communities developping graph-like exchange formats
with the perspective of inviting them to the 1st Int. GraGra Conference
(2002 near Barcelona).

- Study representations of hierarchical graphs by extending the Statecharts
editor to produce GXL.

GTXL homework

- Proposal for control structures in GTXL
- A FUJABA example in GTXL

Andy (and Manfred Münch)
- A ProGres example in GTXL

- A term graph rewriting example in GTXL
- A DIAPLAN example in GTXL (with Mark)

- Update the class diagramm for GTXL
- An AGG example in GTXL

Hans-Jörg and Sabine
- A Hyperedge replacement example in GTXL

- A DIAGEN example in GTXL

- A Grrrr example in GTXL

Homework is due until 1. August. (2001!)

6. Miscellaneous

The next meeting shall be on 20/21. September 2001 at Erlangen.
Its topics shall include:

1. Representing hierarchical graphs in GXL
2. Representing graph transformation programs in GTXL

Berthold Hoffmann, 19. März 2001