This paper discusses detection of global predicates in a distributed
program. Earlier algorithms for detection of global predicates
proposed by Chandy and Lamport work only for stable predicates.
A predicate is stable if it does
not turn false once it becomes true.
Our algorithms detect even unstable predicates without excessive overhead.
In the past, such predicates have
been regarded as too difficult to detect.
The predicates are specified using a logic described formally in this paper.
We discuss detection of weak conjunctive predicates which are formed
by conjunction of predicates local to processes in the
system.  Our detection methods will detect if such a predicate is true for
any interleaving of events in the system, whether the predicate is
stable or not.  Also, any predicate which can be
reduced to a set of weak conjunctive predicates is detectable.
This class of predicates captures many global
predicates that are of interest to a programmer. The message complexity
of our algorithm is bounded by the number of messages used by the program.
The main application of our results are in debugging and testing of
distributed programs. Our algorithms have been incorporated in
a distributed debugger which runs on a network of Sun Workstations under 
SunOS.