Serialization: One missing feature for UML
The story starts in Montreal, at the OMG technical meeting of June 1997. Once again this was at the time OMG knew where to organize nice meeting and Montreal in June, just before the Jazz festival, is obviously quite a different experience than this awful Hyatt hotel in Santa Clara! So this OMG meeting supported by the Ericsson company in Montreal was the place where UML got stabilized. More precisely, there was a final meeting at this nice hotel in the old Montreal area, in a big room completely crowded (probably more than 100 people attending). In front of the room, all the major contributors of UML were there, including the three amigos (Jim, Ivar and Grady).
All the issues having been settled before, the meeting turned out to be a self-congratulation meeting. Most talks were about how nice this UML language was and how important it was going to become for the whole profession.
At some point of time, there was a small question from someone in the assembly to the panel: “Knowing how successful UML was going to become, it is likely that several tools vendors were going to provide UML support in addition of the Rational company. What do you suggest for exchanging UML models between tools provided by several vendors?”
It was funny to realize that nobody on the panel was prepared to answer this simple (and natural) question. After a long silence, finally somebody (in my memory it was somebody from Rational), started to answer and made this funny reply: “There is no problem and we anticipated this problem. This is why we have provided a standard way to plug any UML tool on the CORBA bus. Thus if another tool vendor wishes to exchange UML models with Rational Rose, the simple solution is to use a CORBA connection“.
This answer closed the discussion on the subject of model interchange. But then many people started to ask themselves additional questions. What if the tools could not be connected in permanent mode through CORBA? What if people wished to exchange UML models on Internet? (the answer was then IIOP in theory). What if people wished to exchange UML models with floppy disks? The final discussion of this meeting was about collecting ideas on how to improve the UML practice. In the list of idea, there was a mention of needed serialization.
The Montreal meeting finished thus without any satisfactory answer to this problem of UML model exchange from and to the dominant Rational Rose tool. But the problem was stated and immediately after the meeting in June 1997, some people started immediately to deal with this missing feature and as soon as December 1997, a new RFP was ready (see document OMG 97-12-03.pdf) launching the SMIF initiative (Stream-based Model Interchange Format):
The SMIF Problem Statement stated that:
“The OMG has adopted specifications for an Object Analysis and Design (OAD)Facility and a Meta-Object Facility (MOF). The OAD Facility uses the Unified Modeling Language (UML) as the meta-model and graphical notation for OADmodels. The MOF provides a set of IDL interfaces for distributed meta-object management. However, in addition to a graphical modeling language anddynamic interchange of model information using OMG IDL, these and other planned OMG facilities (e.g. Workflow Management Facility) also need a stream-based model interchange format.”
More precisely, the objective of this RFP was stated as:
“This RFP asks for a stream-based model interchange format (SMIF).The specific objectives of this RFP are the following:
· Establish an industry standard specification for a stream-based model interchange format,
· Provide a generic format that can be used to transfer a wide variety ofmodels,
· Demonstrate that it can be used to exchange OMG Object Analysis andDesign Facility (OADF) compliant models (UML based) and models compliant to other MOF-compliant metamodels and extensions (e.g.Workflow Management Facility and Business Object Facility metamodels),and
· Leverage existing vendor-neutral transfer formats as much as possible.
This RFP solicits proposals for the following:
· A transfer format specification for file export/import of models. The scope for the type of models that can be exported / imported using thisspecification are those that are MOF-compliant representations of industry standard metamodels or metamodel extensions that are compliant with theMOF’s metamodel extension specification. This includes OMG approved metamodels, future OMG approved meta-models, and other current and future industry standard metamodels such as the committee draft (CD)International Standards Organization (ISO) / International ElectrotechnicalCommission (IEC) Joint Technical Committee 1 (JTC1) Subcommittee 7(SC7) Working Group 11 (WG11) CASE Data Interchange Format (CDIF)meta-model standards (CD 15476) and the metamodels that are parts of theISO International Standards (IS) for the Exchange of Product Data (STEP,ISO 10303).
· A transfer format specification for unique identification of the version of the MOF meta-metamodel and any metamodels referenced but not included in an SMIF-compliant transfer. An example use of the transfer format specification for one or more models complaint with the OMG approved Unified Modeling Language (UML) meta-model specification.”
The funny thing is that if you search for the string “XML” in this document, you’ll find no occurrence. That means that initially the OMG was looking for a native solution to serialization of UML models in december 1997.
The rest of the story is more well known. Instead of directly defining a serialization file format for UML graphs, the study revealed that the XML community had already a natural serialization format of trees. The decision was thus taken to go from graphs to trees, and then from trees to text files by using the intermediary XML encodings. Furthermore it was possible to exploit the properties of XML to make a correspondence from <model,metamodel> to <document,XMLschema> (or at least <document,DTD> at the beginning).
From that point in time, nobody heard about SMIF anymore and all efforts were spent on XMI. Due to the efforts of a group around Sridhar Iyengar and Steve Brodsky, this recommendation was stabilized in november 1998 (98-10-17.pdf):
The implementation of XMI went on quite rapidly and in december 1998, it was possible to illustrate the progresses by the so-called Burlingame demonstration where four different UML tools were able to exchange UML models: the interoperability problem was solved, thanks to XMI.
We know today that this was an impression and that the real model interchange is still a problem, but probably more a semantic problem than a syntactic problem.
This story teaches us many things. One of them is that the OMG is able to learn rapidly from its errors or omissions. Due to the short meeting cycle (one meeting every 3 months), it was possible to find rapidly a solution to a problem that had been overlooked.
Was the solution to use an XML vocabulary a good solution? At that time, and in view of the urgency of the problem probably yes. But today XMI is not probably seen as the best OMG proposal. XML is verbose and probably the cholesterol of Internet. It the SMIF problem had to be solved today, it is probable that solutions like JSON would probably be much better (see JSON, the fat-free alternative to XML).