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Semantic Web Services 193
We have discussed several approaches used to compose web services. Now, we present
a new category of problems which can not be solved using these approaches. We extend the
example from section 6.3.2 to describe the problem. In this example the goal is to move Eva
and Adam from one room the other. This situation can be described by simple propositions,
i.e. the goal is to reach a state in which certain propositions are true. Described AI planning
techniques are able to solve this problem and determine the sequence of actions which need
to be performed to achieve the goal state. We change now a little bit the problem. Consider
that in room one is a bottle of wine (bottle one). In room two, there is an empty bottle (bottle
two). The goal is to carry the wine from the bottle one into the empty bottle two. To do this
a goblet is available. We can fill the goblet with wine from bottle one, carry it to room two
and pour it into bottle two. This is being done until the whole bottle of wine is carried.
This extension of the example causes that we are not able to describe and solve the
problem using the discussed approaches. The problem is that we introduce operations with
effects that can not be described by simple propositions with true and false values. The
fill and pour operations have effects that the bottle is fillet at certain level. This can not be
described using true and false values. We need to capture the amount of wine in the bottle.
To describe the effects of the fill and pour operation arithmetic needs to be employed.
The solution of the wine carrying problem is simple. It is depicted in Figure 6-14 where
simplified process solving the problem is shown. In a loop we are carrying the wine in goblets
until the bottle one is empty. Discussed approaches to problem description and solving are
unmatured to handle this problem. First, there are no constructs to use arithmetic. Second,
the methods to find the plan – sequence of actions which lead from the initial to goal state, do
not work with constructs like conditional loops. Discussed planning methods create a plan
from which it is clear what the sequence of actions to achieve the goal is. In the wine carrying
problem, this can not be known without the information about the amount of wine in bottle
one. Concretely, the number of loops depicted in Figure 14 is dependent on the amount of
wine in bottle one, i.e. parameters decides what the sequence of actions required is. To deal
with this category of problems, new methods working with arithmetic means and complex
constructs have to be developed.
We have presented several problems related to Semantic web service composition. Discussed
problems present issues we have to deal with. Until these problems will not be solved, the
utilization of Semantic web services in practice will be limited.
6.4 Conclusions
Adding semantics to the web services description has shown to be useful in different contexts.
The additional metadata describing the meaning of the different elements of web services
makes more effective several processes. The most important are web service discovery
and composition. The exploitation of the available semantics makes these processes more
effective and the results are more relevant.
The research in the context of web service discovery is nowadays focused to solve the
problem of service matchmaking. The aim of the discovery is to find services producing
semantically suiting outputs. The suiting outputs are those which are semantically the same
as the required data or are sufficiently close to it.
The automatic semantic web service composition has shown to be a challenging problem.
A lot of research focuses on web service composition to bring solutions moving this