Services as a Paradigm of Computation - Formal Methods and Hybrid Real-Time Systems - page 527

to discriminate “reasonable” (e.g., terminating) services. and τ lay the ground

for a canonical, rich theory of services, covering a wealth of important notions,
questions and properties, as they are particularly important in the framework
of SOA.

⊕

3.2 The Strategies of a Service

One of the central ideas of SOA is the assumption of a provider agent, offering
a service P to the public. P is intended to be engaged by requester agents. To
engage P means to follow a strategy to interact with P in order to reach the
requester's goal. In technical terms, a strategy of P is an other service, R ,such
that the composition P

⊕ R of P and R is “reasonable”, i.e. P ⊕ R ∈ τ . So, from

the requester perspective, the most important aspect of P is the set

Strat ( P )= def { R ∈S| P ⊕ R ∈ τ }

of all strategies of P . This set may be conceived as the semantics of P .

3.3 Simulation and Equivalence

The strategies of a service P yield a canonical generalization relation

≤⊆S×S
on services: A service P generalizes P if P preserves all strategies of P :

≤ P iff Strat ( P ) ⊆ Strat ( P ) .

P

A typical scenario including this relation is the provider of P wanting to exchange
P by a service P without bothering the so-far users of P .
Consequently, two services P and P are equivalent iff they generalize each

other:

P iff Strat ( P )= Strat ( P ) .

P ∼

This equivalence is in fact the canonical counterpart of functional equivalence
in the classical setting: Two systems are equivalent iff their environment cannot
distinguish them.

3.4 Brokering of Services

The handling of services includes ways to find an adequate provider services P
for given requester services, R . This basic problem gives rise to a lot of derived
questions, including ecient decision procedures and construction algorithms.
Many of them can be posed in the general framework as developed so far. More
precisely, a given provider service P rises the quest of

- Controllability :Does P have a strategy at all, i.e. Strat ( P ) = ∅ ?

- Compatibility : For a given service R ,is R a strategy for P , i.e. R ∈ Strat ( P )?