Jones is running a collection of processes p1 p2 pN . Each process pi contains a variable vi . She wishes to determine whether all the variables v1 v2 vN course of the execution.

Question

(i) Jones’ processes run in a synchronous system. She uses a monitor process to determine whether the variables were ever equal. When should the application processes communicate with the monitor process, and what should their messages contain?
(ii) Explain the statement possibly ( v1= v2 = =vN ). How can Jones determine
whether this statement is true of her execution?

Answer 1

(i) communicate new value when local variable vi changes;
with this value send: current time of day C(e) and vector timestamp V(e) of the event of the change, e.
(ii) possibly (...): there is a consistent, potentially simultaneous global state in which the given predicate is true. Monitor process takes potentially simultaneous events which correspond to a consistent state, and checks
predicate v1 =

v2 = =

vN .
Simultaneous: estimate simultaneity using bound on clock synchronization and upper limit on message propagation time, comparing values of C.
Consistent state: check vector timestamps of all pairs of potentially simultaneous events ei , ej : check Vei i Vej i .