Exercise 4 Local Views of Traces
Consider communications between subjects u, v in S where u sends v a request (?) or approves their
request (!). We describe the events as tuples (u,v,a) with a in {?,!}.
Consider the trace
x = ((a,b,?), (c,b,?), (a,a,!), (a,c,?), (c,d,?), (b,a,!), (c,d,?))
and determine
* the localization (purges) of x for a, b, c and d and
* the localization of property X= {x}, hat(X),
for the case that the localization function is:
a) l1(u,v,a) = u
b) l2(u,v,a) = v
c) l3(u,v,a) = {u,v}
d) l4(u,v,a) = S
Exercise 5 Trace Properties
a) For the communications in exercise 4, formally describe the following properties:
PA: No requests of a to b after any approvals have been made from b to a.
PB: No duplicated approvals (each request is only approved at most once in the trace)
PC: Every approval is answering a request (no unprompted approvals).
b) Explain which of these properties are safety and which of these properties are liveness
properties.
c) Which of these properties are authorization properties and availability properties for any choice
of a localization function l1, l2, l3 or l4 from exercise 4?
