Saturday, 30 April 2011

Distributed Systems: Concepts and Design. by George Coulouris. Exercise Solutions 2

Edition 3
By George Coulouris, Jean Dollimore and Tim Kindberg
Addison-Wesley, ©Pearson Education 2001
Chapter 1 Exercise Solutions

1.2 How might the clocks in two computers that are linked by a local network be synchronized without
reference to an external time source? What factors limit the accuracy of the procedure you have
described? How could the clocks in a large number of computers connected by the Internet be
synchronized? Discuss the accuracy of that procedure.

1.2 Answer

Several time synchronization protocols are described in Section 10.3. One of these is Cristian’s protocol.
Briefly, the round trip time t to send a message and a reply between computer A and computer B is measured
by repeated tests; then computer A sends its clock setting T to computer B. B sets its clock to T+t/2. The setting
can be refined by repetition. The procedure is subject to inaccuracy because of contention for the use of the
local network from other computers and delays in the processing the messages in the operating systems of A
and B. For a local network, the accuracy is probably within 1 ms.
For a large number of computers, one computer should be nominated to act as the time server and it
should carry out Cristian’s protocol with all of them. The protocol can be initiated by each in turn. Additional
inaccuracies arise in the Internet because messages are delayed as they pass through switches in wider area
networks. For a wide area network the accuracy is probably within 5-10 ms. These answers do not take into
account the need for fault-tolerance. See Chapter 10 for further details.

1.3 A user arrives at a railway station that she has never visited before, carrying a PDA that is capable
of wireless networking. Suggest how the user could be provided with information about the local
services and amenities at that station, without entering the station’s name or attributes. What
technical challenges must be overcome?

1.3 Answer

The user must be able to acquire the address of locally relevant information as automatically as possible. One
method is for the local wireless network to provide the URL of web pages about the locality over a local
wireless network.
For this to work: (1) the user must run a program on her device that listens for these URLs, and which gives
the user sufficient control that she is not swamped by unwanted URLs of the places she passes through; and
(2) the means of propagating the URL (e.g. infrared or an 802.11 wireless LAN) should have a reach that
corresponds to the physical spread of the place itself.

1.4 What are the advantages and disadvantages of HTML, URLs and HTTP as core technologies for
information browsing? Are any of these technologies suitable as a basis for client-server
computing in general?
1.4 Answer
HTML is a relatively straightforward language to parse and render but it confuses presentation with the
underlying data that is being presented.
URLs are efficient resource locators but they are not sufficiently rich as resource links. For example, they may
point at a resource that has been relocated or destroyed; their granularity (a whole resource) is too coarsegrained
for many purposes.
HTTP is a simple protocol that can be implemented with a small footprint, and which can be put to use in many
types of content transfer and other types of service. Its verbosity (HTML messages tend to contain many
strings) makes it inefficient for passing small amounts of data.
HTTP and URLs are acceptable as a basis for client-server computing except that (a) there is no strong typechecking
(web services operate by-value type checking without compiler support), (b) there is the inefficiency
that we have mentioned.

1.5 Use the World Wide Web as an example to illustrate the concept of resource sharing, client and
server.
Resources in the World Wide Web and other services are named by URLs. What do the initials URL denote? Give examples of three different sorts of web resources that can be named by URLs.
1.5 Answer
Web Pages are examples of resources that are shared. These resources are managed by Web servers.
Client-server architecture. The Web Browser is a client program (e.g. Netscape) that runs on the user's
computer. The Web server accesses local files containing the Web pages and then supplies them to client
browser processes.
URL - Uniform Resource Locator
(3 of the following a file or a image, movies, sound, anything that can be rendered, a query to a database or to
a search engine.

1.6 Give an example of a URL.
List the three main components of a URL, stating how their boundaries are denoted and illustrating
each one from your example.
To what extent is a URL location transparent?
1.6 Answer
• The protocol to use. the part before the colon, in the example the protocol to use is http ("HyperText
Transport Protocol").
• The part between // and / is the Domain name of the Web server host www.dcs.qmw.ac.uk.
• The remainder refers to information on that host - named within the top level directory used by that Web
server research/distrib/book.html.
The hostname www is location independent so we have location transparency in that the address
of a particular computer is not included. Therefore the organisation may move the Web service to
another computer.
But if the responsibility for providing a WWW-based information service moves to another
organisation, the URL would need to be changed.

1.7 A server program written in one language (for example C++) provides the implementation of a
BLOB object that is intended to be accessed by clients that may be written in a different language
(for example Java). The client and server computers may have different hardware, but all of them
are attached to an internet. Describe the problems due to each of the five aspects of heterogeneity
that need to be solved to make it possible for a client object to invoke a method on the server
object.
1.7 Answer
As the computers are attached to an internet, we can assume that Internet protocols deal with differences in
networks.
But the computers may have different hardware - therefore we have to deal with differences of
representation of data items in request and reply messages from clients to objects. A common standard will be
defined for each type of data item that must be transmitted between the object and its clients.
The computers may run different operating systems, therefore we need to deal with different operations
to send and receive messages or to express invocations. Thus at the Java/C++ level a common operation would
be used which will be translated to the particular operation according to the operating system it runs on.
We have two different programming languages C++ and Java, they use different representations for data
structures such as strings, arrays, records. A common standard will be defined for each type of data structure
that must be transmitted between the object and its clients and a way of translating between that data structure
and each of the languages.
We may have different implementors, e.g. one for C++ and the other for Java. They will need to agree
on the common standards mentioned above and to document them.

1.8 An open distributed system allows new resource sharing services such as the BLOB object in
Exercise 1.7 to be added and accessed by a variety of client programs. Discuss in the context of
this example, to what extent the needs of openness differ from those of heterogeneity.
1.8 Answer
To add the BLOB object to an existing open distributed system, the standards mentioned in the answer to
Exercise 1.7 must already have been agreed for the distributed system To list them again:
• the distributed system uses a common set of communication protocols (probably Internet protocols).
• it uses an defined standard for representing data items (to deal with heterogeneity of hardware).
• It uses a common standard for message passing operations (or for invocations).
• It uses a language independent standard for representing data structures.
But for the open distributed system the standards must have been agreed and documented before the BLOB
object was implemented. The implementors must conform to those standards. In addition, the interface to the
BLOB object must be published so that when it is added to the system, both existing and new clients will be
able to access it. The publication of the standards allows parts of the system to be implemented by different
vendors and to work together.

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