Requirements as Goals and Commitments Too - Intentional Perspectives on Information Systems Engineering - page 141

Ta b l e 1 A purchase protocol expressed in terms of commitments

Domain-specific message Commitment-oriented message

Offer ( E,A, $12, BNW ) Create ( E, A, $12, BNW )

Accept ( A,E,BNW, $12) Create ( A, E, BNW , $12)

Reject ( E,A , $12, BNW )

Release(E, A, $12, BNW )

Deliver ( E,A,BNW )

Declare ( E, A, BNW )

Pay ( A,E, $12) Declare ( A, E, $12)

Table 1 shows an alternative purchase protocol specified in terms of com-
mitments. The semantics of domain-specific messages are explained in terms of
commitment operations. For example, an Offer message is interpreted as a Create
operation, whereas a Reject message releases the debtor from the commitment.
Table 2 introduces the commitments used in Figs. 2 and 3.
Let us walk through the interaction of Fig. 2, which shows a possible enactment
of the protocol described in Table 1. Upon sending Create ( c B ), EBook infers c B ;
upon receiving the message Alice infers c B . Upon sending Declare ($12), Alice
infers that $12 holds. Consequently, she infers that c B is detached, yielding c UB .
When EBook receives Declare ($12), it infers c UB . EBook finally sends Declare
(BNW), thus concluding that its commitment is discharged. When Alice receives
Declare (BNW), she draws the same inference.
Notice that Table 1 does not specify any ordering constraints on messages. In
effect, each party can send messages in any order . Figure 3 shows some additional
enactments of the purchase protocol of Table 1. Neither the enactments of Fig. 3( b)
and (c) nor the one in Fig. 2 are legal according to the FSM in Fig. 1.
So when is an agent compliant with a protocol? The answer is simple: an agent
complies if its commitments are discharged, no matter if delegated or otherwise

Ta b l e 2 Commitments used
as running examples in this
chapter

Name Commitment

c A C (Alice, EBook, BNW, $12)

c B C (EBook, Alice, $12, BNW)

c UA C (Alice, EBook, T , $12)

c UB C (EBook, Alice, T ,BNW)

Fig. 2 An enactment of the
protocol of Table 1 in terms
of ( a ) domain-specific
messages and ( b )
commitments. We show only
the strongest commitments at
each point. For example,
because c UB is stronger than
c B , c UB is sufficient