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Semantic Web Services 185
space. These use different heuristics or approaches utilizing evolution algorithms such as
hill climbing. Different approaches tailored to the web service composition problem dealing
with the space searching are discussed later. The correspondence between AI planning and
web service elements is depicted in Table 6-1.
Table 6-1. Correspondence between web services and space–base planning elements.
AI planning element
s 0
G
A
Γ
Web service element
initial state
desired goal state
available web services
web service state change function
OWL-S is the language containing elements directly corresponding with state-space based
AI planning. The state change produced by the execution of the web service is described by
the preconditions and effects of the OWL-S ServiceProfile.
Graph Based Planning
In this approach graph structures are used when solving the planning problem. This structure
is called planning graph [21]. It is a directed leveled graph consisting of two types of nodes:
action nodes and proposition nodes. These nodes are organized into altering levels (one level
includes nodes only of one type). The first level of the planning graph contains proposition
nodes describing the initial situation. It is then followed by a level of action nodes. This level
contains nodes for each action whose preconditions are satisfied by the proposition nodes of
the first level. Next level is again proposition level. It contains each node from the precedent
proposition level and nodes representing the effects of the actions from the precedent action
level. The construction of the planning graph stops when the successive proposition levels
are identical. If the goal is not reached either in the last proposition layer or in previous
layers, the plan can not be constructed and the given problem is unsolvable.
In Figure 6-13 a planning graph example is depicted. It presents a situation, when Eva
and Adam are in room number one (R1) and they want to move into the room number two
(R2). To do this, the room R1 must be opened and only a man can open a room. We have the
following predicates:
(at ?who ?r) - true if who is in a room r (opened ?r) - true if r
is opened (man ?m) - true if m is a man
At the beginning, the following propositions hold:
– Eva is in the room R1
– Adam is in the room R1
– Adam is a man
– The room R1 is closed