Distributed Systems – Mock Exam –

Distributed Systems
– Mock Exam –
Werner Nutt
January 17, 2011
• The exam comprises 5 questions, which consist of several subquestions.
There is a total of 120 points that can be achieved in this exam. You will
have 2 hours time to answer the questions.
• Please, write down the answers to your questions in the exam booklet handed
out to you.
• For drafts use the blank paper provided by the university.
• If the space in the booklet turns out to be insufficient, please use the university
paper for additional answers and return them with the booklet.
• For students who did not submit assignments the final mark will be the same
as the exam mark. For those who submitted assignments for the labs there
is the possibility to improve the mark:
– For students who submitted all assignments, the final mark will be the
weighted average of
70% exam mark + 30% lab mark
if this is higher than the exam mark.
– For students who submitted all assignments but the last, the final mark
will be the weighted average of
80% exam mark + 20% lab mark
if this is higher than the exam mark.
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1 Protocols
(i) If you had to implement a protocol for remote method invocation, on which
transport level protocol would you base it, UDP or TCP? Explain your answer.
(5 points)
(ii) Suppose you have to implement a request-reply protocol. For each of the
three, UDP, TCP, and HTTP, discuss whether one could use it as the basis
for the new implementation. Which one would be the most suitable and
which one the least? Why?
(15 points)
2 Java RMI
Suppose a server is to be implemented for a book shop where customers can place
orders and managers can check how many copies of each book have been ordered.
There are two methods to access the server,
1. order, which takes four strings, name, cardNo, bookCode, and address
as arguments, and stores that the customer with that name and the credit
card number cardNo has ordered a copy of the book identified by bookCode
to be sent to address;
2. copiesOrdered, which takes a string bookCode as argument and returns
an integer, representing the number of copies ordered of the book with
code bookCode
(i) Write down the declarations of the interfaces and the classes (including the
method declarations) that have to be implemented on the server side.
(12 points)
(ii) Describe how you would implement a small client program that can be
started from a command shell, with the book code as argument, and returns
the number of copies that have been ordered.
(8 points)
(iii) Which compilation steps have to be made in which order before the client
can be started?
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(4 points)

3 Lamport’s Logical Clocks
Consider three distributed processes P 1 , P 2 , and P 3 . The processes are involved in
the events a, b, . . . , k listed below, which happen at spefic points in time, specified
as “wall-clock time” (WCT) and measured in ms:
At 0ms WCT:
a: P 2 sends message m 1 to P 1
b: P 3 reads a startup file
At 100ms WCT:
c: P 1 opens a file containing a user profile
d: P 3 sends message m 2 to P 2
At 200ms WCT:
e: P 1 receives message m 1
f: P 2 receives message m 2
At 300ms WCT:
g: P 1 sends message m 3 to P 2 and P 3
At 400ms WCT:
h: P 2 receives message m 3
At 500ms WCT:
i: P 2 sends message m 4 to P 1
At 600ms WCT:
j: P 1 receives message m 4
At 600ms WCT:
k: P 3 receives message m 3
(i) Which of these events are related by Lamport’s “happened before” relation
(in the lecture denoted as “E 1 → E 2 ” if event E 1 happened before
event E 2 )? Draw a directed graph where the events are vertices and where
there is an edge from E 1 to E 2 if E 1 happened before E 2 .
(10 points)
(ii) Associate to each event a logical timestamp according to the logical clock
algorithm. What is the linear order of events induced by these timestamps?
(10 points)
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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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5 Mutual Exclusion
Distributed algorithms for solving coordination problems are required to have the
properties of safeness and liveness.
(i) Briefly explain each of the two properties. What does it mean for a leader
election algorithm to have these properties?
(8 points)
In addition to leader election, organising exclusive access to shared resources is
another problem requiring distributed coordination. This problem is often specified
as the problem of Mutual Exclusion.
(ii) Briefly explain what is the problem of mutual exclusion.
(4 points)
A classical solution to mutual exclusion uses a central server and tokens, which
can be requested by clients.
(iii) Briefly explain how the basic algorithm for that approach works.
(4 points)
In addition to the two properties of safeness and liveness, an algorithm for mutual
exclusion is sometimes required to have the ordering property.
(iv) Briefly explain what it means for a mutual exclusion algorithm to guarantee
ordering.
(3 points)
(v) Does the algorithm you described above guarantee safeness, liveness, and
ordering? Give a brief explanation.
(9 points)
The basic algorithm cannot deal with failure. Suppose that clients may fail, but
the central server cannot fail. Moreover, suppose the central server uses a failure
detection service that eventually detects any failed client.
(vi) What would be a simple modification of the central server algorithm so that
it can handle the crash of a client?
(4 points)
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Suppose that the error detection algorithm is sometimes wrong in its judgement,
that is, it may assume a client has crashed although the client is still working.
(vii) How may the server detect that a client thought to have crashed is still working?
(3 points)
(viii) How can the algorithm be extended so that it is able to deal also with situations
where a client is falsely thought to have crashed?
(5 points)
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