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Semantic Web Services 187
output. Here we look for services producing the required output data. The goal is to achieve
a situation that each service has defined a source for its input data. If it is possible to compose
a service from the available ones, backward chaining is a method which always finds it.
Despite this, pure backward chaining is unusable effectively because of the computational
complexity of this approach.
Planning Based on Logical Programming
There exist approaches which uses logical programming to solve the planning problem. In
these, the planning problem is encoded as logic program composed by a set of Horn clauses.
The plan is created using reasoning. In [75] it is shown that the planning problem can be
converted to a logic program in linear time.
Hierarchical Task Network Planning
HTN (Hierarchical Task Network) provides a hierarchical abstraction to deal with the complexity
of the planning problem. HTN like other planning techniques assumes a set of
operators applicable when preconditions are satisfied to achieve defined effects. In addition
it assumes also methods prescribing how tasks can be decomposed into subtasks. These
prescriptions are a part of the domain knowledge. The decomposition is applied recursively
until the resulting set of tasks consists only from executable ones. After each step of
decomposition it is tested whether given conditions are violated. The goal is to achieve decomposition
consisting of executable tasks without violating any given condition. A variant
of HTN planning is an ordered task decomposition planning. Here the tasks are planned
in the same order that they will be executed. This reduces the complexity of the planning
problem.
Temporal Planning
Temporal planning is not a planning technique, but it rather refers to the ability of planners
to deal with temporal aspects of the planning problem. It takes into the consideration the real
duration of the actions and allows their parallel execution. Concerning also these aspects
makes the problem more difficult but better reflects the real conditions of the problem that
the planner solves.
6.3.3 Web Services Execution
The result of the web service compositors is a plan describing a sequence of actions which
need to be performed to achieve a goal. Each sequence is bound with a corresponding
executable web service. To execute a composed web service an engine interpreting the
composition is required [43, 44]. The execution can be performed in distributed manner [59].
In the case of services with high computational requirements, the execution is distributed
in Grid environment. There exist approaches for description, construction and execution of
Semantic web services in Grid [6, 36, 38]. Project K-Wf Grid 5 has brought results in this area.
5 http://www.gridworkflow.org/snips/gridworkflow/space/K-Wf+Grid/