Distributed Systems – Mock Exam –

4 Invocation Semantics
Three different semantics have been defined for remote method invocation, “maybe”,
“at-least-once”, and “at-most-once” semantics.
(i) Briefly explain what each of the three means.
(6 points)
(ii) How can one implement at-most-once semantics? Which are the problems
that need to be addressed?
(5 points)
In the context of invocation semantics, on distinguishes between idempotent and
non-idempotent invocations.
(iii) When is a method invocation idempotent? Give an example of an invocation
that is idempotent and of one that is not!
(4 points)
(iv) How can the implementation of at-most-once semantics be simplified if the
methods invoked are idempotent?
(3 points)
As part of the implementation of at-most-once semantics, a site executing remote
method invocations will store results of previous invocations.
(v) When is the earliest point in time that such a stored result can be safely
removed? Explain your answer.
(3 points)
On some networks it may happen that an earlier message be a sender to a recipient
is overtaken by a later message of the same sender to the same recipient. For this
reason it may be the case that a message with a method invocation is overtaken by
another message with the same method invocation.
(vi) How can one extend the basic implementation of at-most-once semantics so
that no reply needs to be sent out for an invocation message that has been
overtaken by another invocation message?
(5 points)
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