We present an actor language which is an extension of a simple
functional language, and
provide an operational semantics for this extension. Actor configurations
represent open
distributed systems, by which we mean that the specification of an actor
system
explicitly
takes into account the interface with external components. We study the
composability of
such systems. We define and study various notions of testing equivalence
on
actor expressions
and configurations. The model we develop provides fairness. An important
result
is that the
three forms of equivalence, namely, convex, must, and may equivalences,
collapse
to two in
the presence of fairness. We further develop methods for proving laws of
equivalence and
provide example proofs to illustrate our methodology.