Distributed Systems: Concepts and Design by George Coulouris. Question 2.5

2.5) Suggest some applications for the peer process model, distinguishing between cases when the state of all peers needs to be identical and cases that demand less consistency.

Answer:

Cooperative work (groupware) applications that provide a peer process near to each user. 

Applications that need to present all users with identical state - shared whiteboard, shared view of a textual discussion 

Less consistency: where a group of users are working on a shared document, but different users access different parts or perhaps one user locks part of the document and the others are shown the new version when it is ready. 

Some services are effectively groups of peer processes to provide availability or fault tolerance. If they partition data then they don’t need to keep consistent at all. If they replicate then they do.

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 3. Question 2.4

2.4) A search engine is a web server that responds to client requests to search in its stored indexes and (concurrently) runs several web crawler tasks to build and update the indexes. What are the requirements for synchronization between these concurrent activities?

Answer:

The crawler tasks could build partial indexes to new pages incrementally, then merge them with the active index (including deleting invalid references). This merging operation could be done on an off line copy. Finally, the environment for processing client requests is changed to access the new index. The latter might need some concurrency control, but in principle it is just a change to one reference to the index which should be atomic.

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 3. Question 2.2

2.2) For the applications discussed in Exercise 2.1 state how the servers cooperate in providing a service.

Answer:

Web: Web servers cooperate with Proxy servers to minimize network traffic and latency. Responsibility for consistency is taken by the proxy servers - they check the modification dates of pages frequently with the originating web server.

Mail: SMTP servers do not necessarily hold mail delivery routing tables to all destinations. Instead, they

simply route messages addressed to unknown destinations to another server that is likely to have the relevant tables.

Netnews: All NNTP servers cooperate in the manner described above to provide the newsfeed mechanism.

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 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.

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.

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 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.

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
Chapter 1 Exercise Solutions


1.1 Give five types of hardware resource and five types of data or software resource that can usefully
be shared. Give examples of their sharing as it occurs in distributed systems.
1.1 Answer



Hardware:
CPU: compute server (executes processor-intensive applications for clients), remote object server
(executes methods on behalf of clients), worm program (shares cpu capacity of desktop machine with the
local user). Most other servers, such as file servers, do some computation for their clients, hence their cpu
is a shared resource.
memory: cache server (holds recently-accessed web pages in its RAM, for faster access by other local
computers)
disk: file server, virtual disk server (see Chapter 8), video on demand server (see Chapter 15).
screen: Network window systems, such as X-11, allow processes in remote computers to update the
content of windows.
printer: networked printers accept print jobs from many computers. managing them with a queuing
system.
network capacity: packet transmission enables many simultaneous communication channels (streams of
data) to be transmitted on the same circuits.
Data/software:
web page: web servers enable multiple clients to share read-only page content (usually stored in a file, but
sometimes generated on-the-fly).
file: file servers enable multiple clients to share read-write files. Conflicting updates may result in
inconsistent results. Most useful for files that change infrequently, such as software binaries.
object: possibilities for software objects are limitless. E.g. shared whiteboard, shared diary, room booking
system, etc.
database: databases are intended to record the definitive state of some related sets of data. They have been
shared ever since multi-user computers appeared. They include techniques to manage concurrent updates.
newsgroup content: The netnews system makes read-only copies of the recently-posted news items
available to clients throughout the Internet. A copy of newsgroup content is maintained at each netnews
server that is an approximate replica of those at other servers. Each server makes its data available to
multiple clients.
video/audio stream: Servers can store entire videos on disk and deliver them at playback speed to multiple
clients simultaneously.
exclusive lock: a system-level object provided by a lock server, enabling several clients to coordinate their
use of a resource (such as printer that does not include a queuing scheme).

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 Information Systems (DIS) Exam questions and Answers. Domain Name Service (DNS)

Clearly describe the purpose and architecture of the Domain Name Service (DNS). Your answer should:

• Discuss the structure of the name space, and how this is reflected in the organisation of the domain name database.
• Describe the functionalities of the various types of DNS server, and
• Illustrate at least one example of an iterative query and at least one example of a recursive query.

Answer:


The main purpose of the DNS is to resolve human-friendly URLs into IP addresses
(numerical, logical addresses).

Architecture of DNS
- Domain Name Service (DNS) is a hierarchical system of name servers, each authoritive for
one or more domains within the Domain Name Space.
Organisation of name servers, root servers, then those lower down the hierarchy to leaf (zonedomain)
level. Delegation of responsibility resulting in a scalable and easy-to-modify
distributed database

The DNS hierarchical structure above

The following four types of server are used in the DNS:


Primary master retrieves data from the host that it runs on and its data is held in a stored
database. The Secondary master gets its data from another master that authoritative for
the zone (i.e. a Primary Master). It contacts an authoritative name server and
pulls the zone data over greatly reduce administrative load. The Caching name server does
not have a database of mappings between IP addresses and names at start-up. It knows of
Primary and Secondary servers which can supply such information if required. The use of
caching servers are used to reduce the load on Primary and Secondary servers. The slave
name server operates in a similar way to a Caching name server however it is less
sophisticated than the other types of server and cannot not follow redirections.
Query types could be either recursive or iterative. With iterative queries the NS gives the
best answer it already knows. This might result in the name server referring the requester to a
‘closer’ name server that it knows. – There is no additional querying of other name servers.
In contrast with recursive query resolution is managed by a single NS. The NS must return
the final answer. This may involve querying other name servers and following ‘referrals’
received thus resulting in further queries being sent to other name servers.

Recursive query above

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.