In (An efficient necessary condition for compatibility, 2009) the authors discuss a method to find out, if two services are compatible or not. This obviously presupposes that the behavior of the services is modeled. One method to do this is to use BPMN 2.0 and model two or more participants offering respective public processes.

Compatibility of the services is not so obvious in the first place. The question to be answered is, if for all possible sent messages the other participant (or service) is ready to process it, and if both can complete their control flow in all cases. For example if one participants sends an order cancellation, but the other participant cannot process it, because the order is already delivered, and it has no control flow to deal with the late cancellation, then the services are not compatible.

Typically this can be checked by comparing all possible states of all participants and all messages. However this brute force method consumes much memory and computing time. Probably this is one reason, why it is not yet offered in so many or even a modeling product at all – which I do not exactly know. The proposed method in (An efficient necessary condition for compatibility, 2009) is to use a method known from petri nets: to convert them into matrices (state equation) and use the matrices to determine the compatibility in a much more efficient way. This is definitely interesting to consider.

The paper states, that this can be used for service discovery and mediator construction. In my personal opinion these use cases are very advanced research use cases, which are not yet so relevant in the practical industry application. Even the mediator construction I have severe doubts, if this is a way that will lead to somewhere at all. But I might be convinced later.

But very relevant is to support the design process of services which need to interact. If there were a repository, that would contain such compatibility determination method – that would be a great step forward. Furthermore it would provide the industry use cases that are needed for further research on the matter.

References

An efficient necessary condition for compatibility. Oanea, Olivia und Wolf, Karsten. 2009. [Hrsg.] Oliver Kopp und Niels Lohmann. Stuttgart : Universität Stuttgart, Fakultät Informatik, Elektrotechnik und Informationstechnik, 2009. Services und ihre Komposition, Erster zentraleuropäischer Workshop, ZEUS 2009. Bd. CEUR Workshop Proceedings Vol. 438, S. 81-87. http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS/Vol-438/paper13.pdf.

In A scenario is a behavioral view – Orchestrating services by scenario integration it is proposed to capture service behavior in the form of a certain petri net class, especially oclets (which is an acyclic Petri net with a local precondition) and by this to model individual scenarios. A scenario is a partial execution of the service. The behavior of a service is defined by the set of scenarios for this service.

In my personal opinion the use of preconditions is a good thing in general. However in my opinion the splitting up of the whole behavior into different scenarios has also disadvantages. For example that it is not so clear, if one scenario is changed, which side effects this has on other scenarios. Also one transition for example can occur in different scenarios. Thereby probably many scenarios need to be changed, if the behavior of one transition needs to be changed. Also a problem is, that it is not so clear how to model differen alternative behaviors. If the union set of all scenarios describes the behavior of the service, it is all one. So for example if there is one big service, but the behavior can be different – for example in one case with confirmation, in another without confirmation – this is not possible to be modeled. It is just a “confirmation” scenario in the set, but it is not modeled, that is sometimes must be used, sometimes not.

Good is that in the end of the paper it is proposed to integrate the scenarios into one view. This I agree upon. My personal opinion is, that I would rather start with the integrated view. Because all the mentioned disadvantages are not there in this case.

But I admit, that the use case proposed in the paper may still be valid in a consulting experince, where the client already has described the behavior of his services in the form of scenarios. Then, the method proposed is a good way to unify them into the desired integrated view. This may also be supported by tools, which gives an additional value add.

Also notbody should be distracted by the notion of the modeling language. This is just the underlying logic. The same method can as well be transferred to industrial languages, as is proposed in the paper and as I believe. For example I see no reason why this should not be useable in a BPMN 2.0 orchestration model environment.

A Theory of Service Behavior tries to formalize – as the name sais – the behavior of a service. This is in contrast to the syntax of a service, that merely describes the datatypes, that need to be used – which is commodity. The behavior if a service describes the order of allowed operation calls / message exchanges with a service.
This is a similar field, that BPMN 2.0 Choreography models. However the BPMN 2.0 Choreography Model is an abstraction, while there are different detailed representations, for different purposes.

So the paper proposes, that for the purposes of service validation, (automatic) service construction, service composition and service replacement there is not yet a represenations, that allows for a closed theory – i.e. a structured solution in all cases. Actually it is an opening paper that will be later completed by describing “operating guidelines”, which are intended to fulfil this purpose.

My personal opinion is, that is it a good endeavor. Such work is needed to complete the foundation for modeling tools, that allow for more comfortable modeling consistency checks and functionality.
I especially see much benefits in consistency checking. I also have one or two patents in this area. Important in my view in consistency is a clear definition of the goal of the check and a good method to explain consistency errors to the tool user. Consistency checking is in most cases useful. One needs a little bit of patience, to make a service definition fit to another, because there is always this one unwanted execution that needs to be eliminated. But it is worth the effort, because it is better to eliminate these errors in modeling time than in runtime.

Service construction is in my personal opinion difficult. In my view there are always abiguities in such an endeavor. So I prefer “human” design plus consistency check.
Service composition promises some benefit – especially if there is a skeleton composed, which can be further edited by the human designer. Also here it is important to formulate clear goals of the service composition and to have a complete – i.e. without gaps – description of the underlying services. Especially if there is a Business Process Platform that offers services, that need to be composed, and that is model driven, this is very promising. There are also other projects working on this. I may come back to this some other time.
Service Replacement is certainly an interesting application. If a service is changed, then it is certainly worthwhile to know potentially which consuming services might be affected and which not. This may be the outcome of this part of the research.

So good work – keep it going!