Distributed Systems: Concepts and Design. by George Coulouris. Exercise Solutions 3

2.1 Describe and illustrate the client-server architecture of one or more major Internet applications (for example the Web, email or netnews).

2.1 - Answer:


Web:

Browsers are clients of Domain Name Servers (DNS) and web servers (HTTP). Some intranets are
configured to interpose a Proxy server. Proxy servers fulfil several purposes – when they are located at the same site as the client, they reduce network delays and network traffic. When they are at the same site as the server, they form a security checkpoint (see pp. 107 and 271) and they can reduce load on the server. N.B. DNS servers are also involved in all of the application architectures described below, but they ore omitted from the discussion for clarity.

Email:

Sending messages: User Agent (the user’s mail composing program) is a client of a local SMTP server and passes each outgoing message to the SMTP server for delivery. The local SMTP server uses mail routing tables to determine a route for each message and then forwards the message to the next SMTP server on the chosen route. Each SMTP server similarly processes and forwards each incoming message unless the domain name in the message address matches the local domain. In the latter case, it attempts to deliver the message to local recipient by storing it in a mailbox file on a local disk or file server. Reading messages: User Agent (the user’s mail reading program) is either a client of the local file server or a client of a mail delivery server such as a POP or IMAP server. In the former case, the User Agent reads messages directly form the mailbox file in which they were placed during the message delivery. (Exampes of such user agents are the UNIX mail and pine commands.) In the latter case, the User Agent requests information about the contents of the user’s mailbox file from a POP or IMAP server and receives messages from those servers for presentation to the user. POP and IMAP are protocols specifically designed to support mail access over wide areas and slow network connections, so a user can continue to access her home mailbox while travelling.

Netnews:

Posting news articles: User Agent (the user’s news composing program) is a client of a local NNTP server and passes each outgoing article to the NNTP server for delivery. Each article is assigned a unique identifier. Each NNTP server holds a list of other NNTP servers for which it is a newsfeed – they are registered to receive articles from it. It periodically contacts each of the registered servers, delivers any new articles to them and requests any that they have which it has not (using the articles’ unique id’s to determine which they are). To ensure delivery of every article to every Netnews destination, there must be a path of newsfeed connections from that reaches every NNTP server.
Browsing/reading articles: User Agent (the user’s news reading program) is a client of a local NNTP server.
The User Agent requests updates for all of the newsgroups to which the user subscribes and presents them to the user.

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BSc (Hons) in Computing - eCommerce Assignment - EasyJet Case Study

Easy Jet Case Study

The coursework is based on the case study 8.1 available in E‐Business and E‐Commerce Management by

Dave Chaffey. 3rd Edition. Page 352.

The evolution of easy Jet’s online revenue contribution

EasyJet was founded by Stelios Haji‐Ioannou, the son of a Greek shipping tycoon who reputedly used to

'hate the Internet'. In the mid 1990s Haji‐Ioannou reportedly denounced the Internet as something 'for

nerds', and swore that it wouldn't do anything for his business. This is no longer the case since by August

1999, the site accounted for 38 per cent of ticket sales or over 135,000 seats.

This was past the company's original Internet contribution target at launch of 30 per cent of sales by

2000. In the period from launch, the site had taken more than 800,000 bookings since it was set up in

April 1998 after a shaky start of two sales in the first week and one thousand within the first month. In

March 2000 EasyJet increased its online discount to £2.50 for a single trip ‐ a higher level of permanent

discount than any other airline. By September 2000, Internet sales reached 85% of total sales. Since this

time, the growth in proportion of online sales has decreased. By 2003, over 90% of all sales were online.

The articles relate the tale of the owner's office being graced by a photo of the owner with horns on his

head and a Mexican moustache on his upper lip. The image was contributed as a complaint by an

aggrieved customer. The nature of the entrepreneur was indicated since he sent the customer two free

tickets.

The company was originally set up in 1994. As a low‐cost airline, looking to undercut traditional carriers

such as British Airways, it needed to create a lean operation. To achieve this, Haji‐Ioannou decided on a

single sales channel in order to survive. He chose the phone. At the time this was groundbreaking, but

the owner was encouraged by companies such as Direct Line insurance, and the savings, which direct

selling, would bring.

Although Haji‐Ioannou thought at the time that there was no time to worry about the Internet and that

one risk was enough, he was adaptable enough to change. When a basic trial site was launched, he kept

a close eye on how popular the dedicated information and booking phone line was (having a webspecific

phone number advertised on the site can be used to trace the volume of users on the site). A

steady rise in the number of calls occurred every week. This early success coincided with the company

running out of space at its call centre due to easy jet’s growth. Haji‐Ioannou related, 'We either had to

start selling over the Internet or build a new call centre. So our transactional site became a £10 million

decision.'

Although the success of EasyJet could be put down solely to the founder's adaptability and vision, the

company was helped by the market it operated in and its chosen business model ‐ it was already a 100

per cent direct phone sales operation. This meant it was relatively easy to integrate the web into the

central booking system. There were also no potential channel conflicts with intermediaries such as travel

agents. The web also fitted in with the low‐cost EasyJet proposition of no tickets, no travel agents, no

network tie‐ups and no in‐flight meals. Customers are given a PIN number for each order on the web

site, which they give when they get to the airport.

Sales over the Internet began in April 1998, and although easy jet’s new‐media operations were then

handled by Tableau, a few months ago easy jet took them in‐house.

The Internet is important to EasyJet since it helps it to reduce running costs, important for a company

where each passenger generates a profit of only £1.50. Savings to EasyJet made through customers

booking online enable it to offer at least £1 off to passengers who book online ‐ this is part of the online

proposition. Online buyers also benefit from paying the price of a local call, instead of the standard

national rate of easy jet’s booking line. The owner says that 'the savings on the Internet might seem

small compared to not serving a meal on a plane, which saves between £5 and £10, but when you think

how much it would cost to build a new call centre, pay every easy jet reservation agent 80 pence for

each seat sold ‐ not to mention all the middlemen ‐ you're talking much more than the £1 off we give

online buyers'. What about the risks of alienating customers who don't want to book online? This doesn't

worry the owner. He says 'I'm sure there are people who live in the middle of nowhere who say they

can't use the Internet and will fly Ryanair instead. But I'm more worried about keeping my cost base

down, and finding enough people to fill my aeroplanes. I only need six million people a year, not all 56

million.'

Promotion

The Internet marketing gurus say 'put the company URL everywhere'. EasyJet has taken this literally with

its web address alongside its Boeing 737s.

EasyJet frequently varies the mix by running Internet‐only promotions in newspapers. easy jet ran its first

Internet‐only promotion in a newspaper in The Times in February 1999, with impressive results. Some

50,000 seats were offered to readers and 20,000 of them were sold on the first day, rising to 40,000

within three days. And, according to the marketing director, Tony Anderson, most of these were seats

that otherwise would have been flying along at 600 mph ‐ empty. The scalability of the Internet helped

deal with demand since everyone was directed to the web site rather than the company needing to

employ an extra 250 telephone operators. However, risk management did occur with a micro site built

for Times readers (www.times.easyjet.com) to avoid putting a strain on easy jet’s main site.

Anderson says, 'The airline promotions are basically designed to get rid of empty seats'. He adds, 'If we

have a flight going to Nice that's leaving in 20 minutes' time, it costs us very little to put some extra

people on board, and we can get, say, £15 a head for it'. Flight promotions are intended to avoid

attracting people who'd fly with EasyJet, so advanced booking schemes are intended to achieve that.

A later five‐week promotion within The Times and The Sunday Times newspapers offered cheap flights to a choice of all EasyJet destinations when 18 tokens were collected. In total, 100,000 seats were sold during the promotion, which was worth more than £2m to the airline. Thirty per cent of the seats were

sold online, with the rest of the transactions being completed by phone; 13,000 orders were taken over

the Internet in the first day alone with over 15,000 people on the site at one point.

The web site also acts as a PR tool. Haji‐Ioannou uses its immediacy to keep newspapers informed about

new promotions and offers by phoning and e‐mailing journalists and referring them to the web site

rather than faxing.

The web site is also used as an aggressive tool in what is a very competitive marketplace. Haji‐Ioannou

says 'Once we had all these people coming to our site, I asked myself: "Why pay a PR company to

publicise what we think when we have a captive audience on the site?'" For example, easy jet ran a

competition in which people had to guess what BA's losses would be on 'Go', its budget rival to EasyJet

(the figure turned out to be £20m). Within minutes of the BA results being announced on 7 September,

the EasyJet site had the 50 flight‐ticket winners from an incredible 65,000 people who had entered. In a

similar vein a section of the site was entitled 'Battle with Swissair', giving easy jet's view that Swissair's

head had persuaded the Swiss government to stop EasyJet being granted a commercial scheduled

licence on the Geneva‐Barcelona route. EasyJet also called itself 'The web's favourite airline', in 1999, a

direct counterpoint to British Airways slogan of 'The world's favourite airline' for which it enjoyed a court

battle.

easyEverything

Following the brand extension success of Virgin, EasyJet has used the 'easy' prefix to offer additional

services as part of the easyGroup:

• easyEverything, a chain of 400‐seat capacity Internet cafes originally offering access at £1 an

hour. This is run as an independent company and will charge EasyJet for banner ads, but clearly

the synergy will help with click through between two to three per cent. The only concession

easyEverything makes towards EasyJet is that cafe customers can spend time on the easyjet site

for free.

• easyRentacar, a low‐cost car rental business offering car rental at £9 a day. These costs are

possible through offering a single car type and being an

Internet‐only business.

Implementation

The articles report that Russell Sheffield, head of new‐media agency Tableau, who initially worked with

EasyJet had an initial problem of colour! 'He says there was a battle to stop him putting his favourite

colour all over the site.' The site was intended to be highly functional, simply designed and without any

excess baggage. He says 'the home page (orange) only had four options ‐ buy online, news, info, and a

topic of the moment such as BA "go" losses ‐ and the site's booking system is simpler to use than some

of its competitors'. He adds: 'great effort was put into making the navigation intuitive ‐ for example,

users can move directly from the timetables to the booking area, without having to go via the home

page'. The site was designed to be well integrated into easy jet’s existing business processes and systems. For example, press releases are fed through an electronic feed into the site, and new destinations appear

automatically once they are fed into the company's information system.

Measurement of the effectiveness of the site occurred through the dedicated phone number on the site,

which showed exactly how many calls the site generated, and the six‐month target within six weeks.

Web site log file analysis showed that people were spending an average of eight minutes a time on the

site, and better still, almost everyone who called bought a ticket, whereas with the normal phone line,

only about one in six callers buys. Instead of having to answer questions, phone operators were doing

nothing but sell tickets.

Once the web site generated two‐fifths of EasyJet business, it was taken in‐house and Tableau now acts

solely as a strategic advisor.

Coursework Specification

You are invited, as an experienced consultant, to put together a strategic plan based on the information

supplied in the Easy Jet Case Study. The strategic plan should include the following deliverables which

should be presented as sections in your technical report:

Introduction

Background – briefly describe the background context of the organisation. This can include, for

example, historic information on the company’s formation, the financial performance, and the

key products and services.

Situation Analysis

Present Position and Industry Analysis: You should consider analysing the organisation's current

position using both of the following tools: SWOT and PEST

eMarketing Strategy

eMarketing strategy

What eMarketing strategy has the organisation adopted or is planning to adopt?

What is the value proposition and differential advantage of this strategy?

In this section you should build upon your Situation Analysis (You should also evaluate the

organisations’ ‘marketing mix’).

Implementation

How is the management creating new core and extended value for customers?

How is the management balancing their online and offline promotion methods?

What impact is the implementation of strategy having upon the performance of the business?

Legal and ethical issues

What legal and ethical issues does the organisation need to consider with regard to the

gathering, processing, distribution and use of information on the Internet?

Technical Report

In addition to the strategic plan, your report entitled, “An eMarketing Consultation Document for

Easy Jet" should include the following:

Management Summary

Summarises the approach taken and the key findings.

Background

Describes the background context of the organisation. This can include, for example, historical

information on the company’s formation, the financial performance, and the key products and

services.

Conclusion

Presents the conclusions that you have reached based upon your findings and the

recommendations to the management of Easy Jet.

The report should be between 4,000 and 5,000 words in length and all references to other sources of

information must be properly acknowledged using the Harvard Referencing System.

==========================================

Strategic Plan for Easy Jet

Table of Contents

Introduction............................................................2

           Background.................................................2

Situation Analysis....................................................4

           SWOT Analysis...........................................4

           PEST Analysis.............................................5

eMarketing Strategy.................................................6

           eMarketing Strategy......................................6

           Implementation.............................................7

Legal and Ethical Issues............................................8

Technical Report......................................................8

          Management Summary...................................8

          Background...................................................8

          Conclusion....................................................8

References..............................................................9

 

Introduction

This strategic plan is developed with an assistance of Easy Jet. It provides with a roadmap for a company development, customer-oriented support and services. Easy Jet provided all the information we needed to complete this strategic plan.

This strategic plan addresses some of the general fundamental problems like finding customers to fill the airline’s planes, how to stay in the market while other budget airlines are also growing fast and how to be ahead of everyone in the innovation curve. 

This strategic plan also addresses some of the problems that occur while dealing with day-to-day business processes and some of the problems that need to be kept under control and some of the problems might have been ignored by the current airline management.

During a development of this strategic plan some of the alternative or possible market growth areas also observed. We tried to find answers to questions like: “Is current Easy Jet management giving enough attention to keeping the website technologically up-to-date?” and “Is current Easy Jet management benefiting from one of the fastest growing and very effective marketing platforms like social media, blogging and twitting?”

SWOT Analysis and PEST Analysis tools are used to analyse the Easy Jet’s current market position. With an assistance of this strategic plan Easy Jet will be able to assess their strong sides, problems they encounter during a daily business processes, opportunities they have and possible future threats and coordinate themselves according to the choices which are revealed in this strategic plan.

Background

Budget airline EasyJet was founded by Greek-Cypriot businessman Stelios Haji-Ioannou in 1994. In the beginning Haji-Ioannou was ignorant towards the internet. He thought the internet would never make money for his airline.

The airline started selling travel tickets online in April 1998. It could only sell two tickets online in the first week and only one thousand in whole month. But after the slow start ticket selling online has grown very fast reaching 38% of all ticket sales or 135,000 seats total by August 1999. EasyJet introduced the biggest permanent discount of £2.50 for a single trip in March 2000. 

Online ticket sales didn’t stop there. If it was 85% of total sales in September, it climbed to 90% of total sales by year 2003. 

The airline founder saw the opportunity of using phone lines as a sales channel. It was groundbreaking at the time, because it helped Easy Jet to cut the operational costs and it would enable the airline to compete with existing airlines.

Haji-Ioannou had to be convinced to sell tickets online. Trial site was launched and a web specific phone line was allocated to track the volume of online users. Number of calls kept rising every week. This early success led the budget airline to focus on selling tickets online. New website easily integrated to the airline’s already existing central booking system. Prior to this the company’s business model was selling tickets through direct phone lines. Introducing a web based sales system enabled the airline keep costs low. With the introduction of a new system customers who book their flights online are given a PIN number per order. The only thing the customers need to do was to give the PIN number at the airport. Customers didn’t have to pay the price of a local call when they book their flights online. 

Easy Jet was saving through selling tickets online and not serving food on planes. 

Easy Jet was running Internet only promotions in The Times and The Sunday Times. They even introduced a macro site Times readers to avoid the scalability problem on the site. First promotion which was offered in the Times newspaper in February 1999 was 50,000 seats and 20,000 of them sold in a first day. This helped the airline to fill the empty seats. 

Another promotion which was offered in The Times and The Sunday Times sold 100,000 seats and made £2million for the airline. 

Easy Jet started using the website as powerful PR tool as well. Newspapers and journalists were informed about the promotions and updates and were referred to the website. They started running competitions on the website. Guessing British Airways losses questionnaire was a great successful PR action attracting 65,000 people answering the question and giving away 50 tickets. 

Easy Jet later added easyEverything, a chain of internet cafes and easyRentacar, a low cost car rental business to its existing travel flights business. 

Easy Jet’s website was designed and operated by a new-media agency Tableau, but when the online ticket sales reached the two-fifths of the total sales, they decided to take it in-house and Tableau would become a strategic advisor.

 

Situation Analysis

SWOT Analysis is used to analyse the current market position of the Easy Jet. Whole diagram is shown below.

Strengths of the airline help them to take an advantage on the opportunities. On the other hand weaknesses make them weak when they face threats. 

STRENGTH

• Having a visionary company founder

• Relatively cheap ticket prices

• Responding to customer complaints quickly

• Ready to change to adapt into the new business environment

• Already having a central booking system

• Having no middle man in ticket selling

• Using a website a strong PR tool

• Saving on advertisement expenses mainly sticking to the website centred advertising

• Having a strong brand image/colour WEAKNESS

• Being overcautious about using new technological opportunities for a company growth

• Alienating some customers who may not want to book their tickets online

• Being content with the existing number of customers

OPPORTUNITIES

• Could observe more new business areas using “easy” branding like easyHotels, easyClubs and etc.,

• Could do more promotions using all the media tools available including newspapers

• Could engage with the existing and potential new customers using social media

• Could create smart phone and all other handheld device apps and create opportunities for people to buy tickets using their smart phones and handheld devices THREATS

• Business model could be copied by other budget airlines

• Existing budget airlines like Ryanair could steal customers from Easy Jet

 

PEST Analysis

PEST analysis stands for Political, Economical, Social and Technological Analysis. PEST Analysis is a very useful tool to identify external environment factors like opportunities and threats to the organisation. PEST Analysis for Easy Jet is shown below.

Political Environment

• Specific governments might provide obstacles for Easy Jet to protect their national airlines

• Governments increase ecology related taxes raising the issue of air pollution or non-ecology friendly plain engines

• Home country or foreign governments might get more protectionist as it happens quite often these days in many industries because of an effect of a recent financial crisis Economical Environment

• Current economic situation might put people off from travelling

• Travellers might consider buying cheaper options and keen on hunting promotional offers

• Easy Jet’s major destination countries like Greece, Portugal, Spain and Ireland are in deep recession

• Holidays being seasonal brings many difficulties like filling the plains when it is not a holiday season and adding more flights and hiring more staff on holiday season time

• Rising foul prices

• Interest rate changes on major European currencies like Euro and Pound

Social Environment

• Protecting brand image is a very important factor for Easy Jet, actually for any organisation. It is also fragile. If anything goes wrong with the plains or bad customer service or because of the similar sort of issues brand image could be destroyed and it would hurt the company in a big time.

• Having a good relationship with the media is an important aspect of staying in business

• Changes in the demographics of some countries in a long term might affect the Easy Jet’s business Technological Environment

• Benefitting from the social media is a very important. Social Media (Facebook, Twitter and etc.) provides an incredible platform for advertising and selling tickets online directly using the services they offer. If Easy Jet can’t stay ahead of that curve, it might affect the Easy Jet’s business

• Acquiring the environmentally friendly plains and other facilitating technologies are very important

• Being up-to-date with the latest technology and browsers on the website to sell online is very important

 

eMarketing Strategy

eMarketing Strategy

Easy Jet has adopted many eMarketing strategies. They are:

• Selling through phone calls

• Selling tickets online

• Using their official website as a PR tool

• Going green

Phone Lines: From the first day of it is foundation Easy Jet had its unique way selling tickets. It started with selling tickets through phone calls.  They stick to their selling technique for a while. 

Selling tickets online: When they tried the internet for selling tickets they saw the great potential of internet as another sales channel. Sales increased rapidly over the years and internet became the main selling channel.

Website as a PR tool: Later Easy Jet founder saw the potential of the website as a PR tool. They started organising questionnaires and giving away some free tickets for the lucky contestants. They organised promotions in the newspapers. This method helped them to sell the empty seats. Official website of the airline kept up-to-date with all the information. Journalists and newspapers were informed about any changes and referred to the website.

Going green: Lately Easy Jet set a target for itself on going green. According the Easy Jet’s official website they have an environmental code based on 3 promises:

“

1. To be environmentally efficient in the air

2. To be environmentally efficient on the ground

3. To lead in shaping a greener future for aviation, for example:

o carbon offsetting

o shaping future aircraft design - for example, the ecoJet

“

(http://www.easyjet.com/EN/Environment/index.shtml, [Accessed: 25 November 2010])

Using Social Media: According to the article “Easy Jet wants to sell flights on Facebook” published on netsparsh.com on 1 April 2010: 

“European budget airline easyJet is poised to become the first airline company to enable users to plan and book flights entirely on Facebook.

easyJet currently offers a “Holiday Planner” Facebook app that helps fans plan trips with their friends through Facebook and email. Users can coordinate destinations and dates, propose a range of budget options and then invite friends along for the trip.

However, users must go to a destination website to complete the transaction — a step easyJet wants to remove altogether by adding booking functionality to their Facebook app.” 

(http://netsparsh.com/wp/blog/2010/04/01/easyjet-wants-to-sell-flights-on-facebook/#axzz16GWcw4A8, [Accessed: 25 November 2010])

When we checked Easy Jet’s Facebook site, we see that airline already started selling tickets on Facebook. http://www.facebook.com/easyJet?v=app_10442206389 Clicking on this link travellers can book their tickets on Easy Jet’s Facebook page.

Future Plans: Easy Jet should definitely create iPhone and iPad application. Easy Jet should definitely create Android applications. These applications should be used as an information tool for customers. Easy Jet could certainly be able to sell tickets through those apps mentioned. Smart phone and hand-held device sales are rocketing for the last year especially. Millions of users in UK as well as in all the major destinations of the Easy Jet are already using those devices. This could create a major selling channel for Easy Jet. 

What are the advantages of these strategies?

 There are many advantages of the strategies Easy Jet adopted. Some of them are listed below:

• Being the first airline company that introduced the online sales made them to capture the big share in the travel flights market

• Using Social Media tools and attracting lots of potential customers through those tools and using them as strong Marketing tool and selling channels

• Introducing cheap tickets and direct flights

• Having a catchy and attractive company slogan

eMarketing Strategy Implementation

According to the Easy Jet’s official website (http://www.easyjet.com/EN/Environment/index.shtml, [Accessed: 25 November 2010]) 

“

• Easy Jet uses only the main airports with good public transport in holiday destination countries

• No flight changes. Direct flights only.

• Easy Jet offers cheap tickets, quick and simple online booking system.

• Customers saves on phone call charges

• Customers can make a comment, access to the other’s comments on Facebook and Twitter

• Through Facebook and Twitter Easy Jet establishes a good communication with their existing customers and gain access to the potential new customers. These tools also provide a valuable feedback for the airline company

• Easy Jet organises questionnaires online and online promotions. They award many lucky customers

Legal and Ethical Issues

  

 

References: 

1. http://www.easyjet.com/asp/en/book/index.asp?lang=en

                Easy Jet’s official website where you can book flights, hotels and holidays

2. http://twitter.com/#search?q=easyJet

               “easyJet” search results on Twitter

3. http://www.facebook.com/easyJet

               Easy Jet’s Facebook site

4. http://twitter.com/easyjet

               Easy Jet’s Twitter site

5. http://www.youtube.com/easyjetbroadcast

               Easy Jet’s YouTube site

6. http://www.advanced-web-metrics.com/blog/2010/11/17/how-much-money-does-easyjet-loose/

               This is a website where you can find information about Easy Jet’s Google Chrome problem

7. http://corporate.easyjet.com/about-easyjet/industry-awards/2009.aspx

              Easy Jet’s Industry Awards. List of awards are very impressive.

8. http://en.wikipedia.org/wiki/EasyJet

               Easy Jet’s on Wikipedia

9. http://netsparsh.com/wp/blog/2010/04/20/virgin-america-announces-expansion-via-twitter/#axzz16GX4UIWp

               “Virgin Atlantic expands via Twitter” article

10. http://netsparsh.com/wp/blog/2010/04/01/easyjet-wants-to-sell-flights-on-facebook/#axzz16GWcw4A8

               Easy Jet introduces ticket selling on Facebook

If you found the answer useful, please visit my other website. Exercise or Fitness Training has huge psychological and physical benefits. Train and dress up for the occasion. 

Distributed Systems: Concepts and Design By George Coulouris - Exercise Solutions

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


1.9 Suppose that the operations of the BLOB object are separated into two categories – public
operations that are available to all users and protected operations that are available only to certain
named users. State all of the problems involved in ensuring that only the named users can use a
protected operation. Supposing that access to a protected operation provides information that
should not be revealed to all users, what further problems arise?

1.9 - Answer

Each request to access a protected operation must include the identity of the user making the request. The
problems are:
• defining the identities of the users. Using these identities in the list of users who are allowed to access
the protected operations at the implementation of the BLOB object. And in the request messages.
• ensuring that the identity supplied comes from the user it purports to be and not some other user
pretending to be that user.
• preventing other users from replaying or tampering with the request messages of legitimate users.
Further problems.
• the information returned as the result of a protected operation must be hidden from unauthorised users.
This means that the messages containing the information must be encrypted in case they are intercepted
by unauthorised users.

1.10 - The INFO service manages a potentially very large set of resources, each of which can be accessed
by users throughout the Internet by means of a key (a string name). Discuss an approach to the
design of the names of the resources that achieves the minimum loss of performance as the number
of resources in the service increases. Suggest how the INFO service can be implemented so as to
avoid performance bottlenecks when the number of users becomes very large.

1.10 - Answer

Algorithms that use hierarchic structures scale better than those that use linear structures. Therefore the
solution should suggest a hierarchic naming scheme. e.g. that each resource has an name of the form ’A.B.C’
etc. where the time taken is O(log n) where there are n resources in the system.
To allow for large numbers of users, the resources are partitioned amongst several servers, e.g. names
starting with A at server 1, with B at server 2 and so forth. There could be more than one level of partitioning
as in DNS. To avoid performance bottlenecks the algorithm for looking up a name must be decentralised. That
is, the same server must not be involved in looking up every name. (A centralised solution would use a single
root server that holds a location database that maps parts of the information onto particular servers). Some
replication is required to avoid such centralisation. For example: i) the location database might be replicated at multiple root servers or ii) the location database might be replicated in every server. In both cases, different
clients must access different servers (e.g. local ones or randomly).

1.11 - List the three main software components that may fail when a client process invokes a method in
a server object, giving an example of a failure in each case. To what extent are these failures
independent of one another? Suggest how the components can be made to tolerate one another’s
failures.
1.11 - Answer

The three main software components that may fail are:
• the client process e.g. it may crash
• the server process e.g. the process may crash
• the communication software e.g. a message may fail to arrive
The failures are generally caused independently of one another. Examples of dependent failures:
• if the loss of a message causes the client or server process to crash. (The crashing of a server would cause
a client to perceive that a reply message is missing and might indirectly cause it to fail).
• if clients crashing cause servers problems.
• if the crash of a process causes a failures in the communication software.
Both processes should be able to tolerate missing messages. The client must tolerate a missing reply message
after it has sent an invocation request message. Instead of making the user wait forever for the reply, a client
process could use a timeout and then tell the user it has not been able to contact the server.
A simple server just waits for request messages, executes invocations and sends replies. It should be
absolutely immune to lost messages. But if a server stores information about its clients it might eventually fail
if clients crash without informing the server (so that it can remove redundant information). (See stateless
servers in chapter 4/5/8).
The communication software should be designed to tolerate crashes in the communicating processes.
For example, the failure of one process should not cause problems in the communication between the surviving
processes.
1.12 - A server process maintains a shared information object such as the BLOB object of Exercise 1.7.
Give arguments for and against allowing the client requests to be executed concurrently by the
server. In the case that they are executed concurrently, give an example of possible ‘interference’
that can occur between the operations of different clients. Suggest how such interference may be
prevented.
1.12 - Answer
 For concurrent executions - more throughput in the server (particularly if the server has to access a disk or
another service)
Against - problems of interference between concurrent operations
Example:
Client A’s thread reads value of variable X
Client B’s thread reads value of variable X
Client A’s thread adds 1 to its value and stores the result in X
Client B’s thread subtracts 1 from its value and stores the result in X
Result: X := X-1; imagine that X is the balance of a bank account, and clients A and B are implementing credit
and debit transactions, and you can see immediately that the result is incorrect.
To overcome interference use some form of concurrency control. For example, for a Java server use
synchronized operations such as credit and debit.

1.13 - A service is implemented by several servers. Explain why resources might be transferred between
them. Would it be satisfactory for clients to multicast all requests to the group of servers as a way
of achieving mobility transparency for clients?
1.13 - Answer

Migration of resources (information objects) is performed: to reduce communication delays (place objects in
a server that is on the same local network as their most frequent users); to balance the load of processing and
or storage utilisation between different servers.
If all servers receive all requests, the communication load on the network is much increased and servers must
do unnecessary work filtering out requests for objects that they do not hold.

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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.