Chapter 2 slides

Slides for Chapter 2:
Architectural Models

From Coulouris, Dollimore, Kindberg and
Blair
Distributed Systems:
Concepts and Design
Edition 5, © Addison-Wesley 2012

Figure 2.1
Generations of distributed systems

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012
2

Figure 2.2
Communicating entities and communication paradigms

3

Figure 2.3
Clients invoke individual servers

Client invocation invocation Server

result result
Server

Client
Key:
Process: Computer:


Figure 2.4
A service provided by multiple servers

Service

Server
Client

Server

Client
Server


Figure 2.5
Web proxy server

Client Web
server
Proxy
server

Client Web
server


Figure 2.6
Web applets

a) client request results in the downloading of applet code

Client Web
Applet code server

b) client interacts with the applet

Web
Client Applet server


Figure 2.7
Software and hardware service layers in distributed systems

Applications, services

Middleware

Operating system
Platform

Computer and network hardware


Figure 2.8
Two-tier and three-tier architectures

9

AJAX

 AJAX (Asynchronous Javascript And XML) is an extension to the
standard client-server style of interaction used in the World Wide
Web.
 AJAX meets the need for fine-grained communication between a
Javascript front-end program running in a web browser and a
server-based back-end program holding data describing the state of
the application.
 Ex: In the standard web style of interaction a browser sends an
HTTP request to a server for a page, image or other resource with a
given URL. The server replies by sending an entire page that is
either read from a file on the server or generated by a program,
depending on which type of resource is identified in the URL. When
the resultant content is received at the client, the browser presents it
according to the relevant display method for its MIME type.

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The restriction of the web applications development

This standard style of interaction constrains the development of web
applications in several significant ways:
• Once the browser has issued an HTTP request for a new web page,
the user is unable to interact with the page until the new HTML
content is received and presented by the browser. This time interval
is indeterminate, because it is subject to network and server delays.
• In order to update even a small part of the current page with
additional data from the server, an entire new page must be
requested and displayed. This results in a delayed response to the
user, additional processing at both the client and the server and
redundant network traffic.
• The contents of a page displayed at a client cannot be updated in
response to changes in the application data held at the server.

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AJAX Solution

 AJAX is the second innovative step that was needed to enable
major interactive web applications to be developed and deployed. It
enables Javascript front-end programs to request new data directly
from server programs.
 Any data items can be requested and the current page updated
selectively to show the new values.
 Indeed, the front end can react to the new data in any way that is
useful for the application.

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Figure 2.9
AJAX example: soccer score updates

new Ajax.Request('scores.php?
game=Arsenal:Liverpool’,
{onSuccess: updateScore});
function updateScore(request) {
.....
(request contains the state of the Ajax request including the returned
result.
The result is parsed to obtain some text giving the score, which is
used to update the relevant portion of the current page.)
.....
}
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Figure 2.10
Figure below illustrates a thin client accessing a compute server over
the Internet.

 Advantage - simple local devices can be significantly enhanced
with a excess of networked services and capabilities.
 Drawback - in highly interactive graphical activities such as CAD
and image processing, where the delays experienced by users are
increased to unacceptable levels by the need to transfer image and
vector information between the thin client and the application
process, due to both network and operating system latencies.

Compute server
Network computer or PC

Thin network Application
Client Process

The web service architectural pattern

 The proxy pattern is a commonly repeating pattern in distributed
systems designed particularly to support location transparency in
remote procedure calls or remote method invocation.
 The use of brokerage in web services can usefully be viewed as an
architectural pattern supporting interoperability in potentially
complex distributed infrastructures.
 Reflection is a pattern that is increasingly being used in distributed
systems as a means of supporting both introspection (the dynamic
discovery of properties of the system) and intercession (the ability to
dynamically modify structure or behaviour).

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Figure 2.11
The web service architectural pattern

 Brokerage pattern consists of the trio of service provider, service
requester, and service broker (a service that matches services
provided to those requested).
 This brokerage pattern is replicated in many areas of distributed
systems.

16

Figure 2.12
Categories of middleware

17

Distributed Systems Issues

 Interaction: Computation occurs within processes; the processes
interact by passing messages, resulting in communication
(information flow) and coordination (synchronization and ordering of
activities) between processes.
 Failure: The correct operation of a distributed system is threatened
whenever a fault occurs in any of the computers on which it runs
(including software faults) or in the network that connects them.
 Security: The modular nature of distributed systems and their
openness exposes them to attack by both external and internal
agents.

18

Interaction Model

 Multiple server processes may cooperate with one another to
provide a service (ex. Domain Name System).
 A set of peer processes may cooperate with one another to achieve
a common goal: for example, a voice conferencing system that
distributes streams of audio data in a similar manner, but with strict
real-time constraints.

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Interaction Model

 Algorithm – a sequence of steps to be taken in order to perform a
desired computation.
 Distributed algorithm – a definition of the steps to be taken by
each of the processes of which the system is composed, including
the transmission of messages between them. Messages are
transmitted between processes to transfer information between
them and to coordinate their activity.

20

Performance Of Communication Channels

 The delay between the start of a message’s transmission from one
process and the beginning of its receipt by another is referred to as
latency. The latency includes:
– The time taken for the first of a string of bits transmitted through a
network to reach its destination. For example, the latency for the
transmission of a message through a satellite link is the time for a
radio signal to travel to the satellite and back.
– The delay in accessing the network, which increases significantly
when the network is heavily loaded. For example, for Ethernet
transmission the sending station waits for the network to be free of
traffic.
– The time taken by the operating system communication services at
both the sending and the receiving processes, which varies
according to the current load on the operating systems.

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Performance Of Communication Channels

 The bandwidth of a computer network is the total amount of
information that can be transmitted over it in a given time. When a
large number of communication channels are using the same
network, they have to share the available bandwidth.
 Jitter is the variation in the time taken to deliver a series of
messages. Jitter is relevant to multimedia data. For example, if
consecutive samples of audio data are played with differing time
intervals, the sound will be badly distorted.

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Two variants of the interaction model

 Synchronous distributed systems: The one in which the following
bounds are defined:
• The time to execute each step of a process has known lower and upper bounds.
• Each message transmitted over a channel is received within a known bounded time.
• Each process has a local clock whose drift rate from real time has a known bound.
 Asynchronous distributed systems: The one in which there are
no bounds on:
• Process execution speeds – for example, one process step may take only a
picosecond and another a century; all that can be said is that each step may take
an arbitrarily long time.
• Message transmission delays – for example, one message from process A to
process B may be delivered in negligible time and another may take several years.
In other words, a message may be received after an arbitrarily long time.
• Clock drift rates – the drift rate of a clock is arbitrary- (The term clock drift rate
refers to the rate at which a computer clock deviates from a perfect reference clock)

23

Event ordering

 We are interested in knowing whether an event (sending or
receiving a message) at one process occurred before, after or
concurrently with another event at another process. The execution
of a system can be described in terms of events and their ordering
despite the lack of accurate clocks.

 For example, consider the following set of exchanges between a
group of email users, X, Y, Z and A, on a mailing list:
1. User X sends a message with the subject Meeting.
2. Users Y and Z reply by sending a message with the subject Re:
Meeting.

 In real time, X’s message is sent first, and Y reads it and replies; Z
then reads both X’s message and Y’s reply and sends another
reply, which references both X’s and Y’s messages.
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Figure 2.13
Real-time ordering of events

 But due to the independent delays in message delivery, the messages
may be delivered as shown in Figure below, and some users may view
these two messages in the wrong order. For example, user A might see:
send receive receive
X
1 m1 4
m2
send
receive
Y 2 3 Physical
receive time

send
Z
receive receive

m3 m1 m2
A
receive receive receive
t1 t2 t3


 For example, user A might see:

 If the clocks on X’s, Y’s and Z’s computers could be synchronized, then
each message could carry the time on the local computer’s clock when it
was sent.
 For example, messages m1, m2 and m3 would carry times t1, t2 and t3
where t1<t2<t3. The messages received will be displayed to users
according to their time ordering. If the clocks are roughly synchronized,
then these timestamps will often be in the correct order.



 Since clocks cannot be synchronized perfectly across a distributed
system, a model of logical time that can be used to provide an
ordering among the events at processes running in different
computers in a distributed system.
 Logical time allows the order in which the messages are presented
to be inferred without recourse to clocks.

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Figure 2.14
Processes and channels

 Communication omission failures: Consider the communication
primitives send and receive. A process p performs a send by
inserting the message m in its outgoing message buffer. The
communication channel transports m to q’s incoming message
buffer. Process q performs a receive by taking m from its incoming
message buffer and delivering it. The outgoing and incoming
message buffers are typically provided by the operating system.

process p process q

send m receive

Communication channel
Outgoing message buffer Incoming message buffer


Figure 2.15
Omission and arbitrary failures

Class of failure Affects Description
Fail-stop Process Process halts and remains halted. Other processes may
detect this state.
Crash Process Process halts and remains halted. Other processes may
not be able to detect this state.
Omission Channel A message inserted in an outgoing message buffer never
arrives at the other end’s incoming message buffer.
Send-omission Process A process completes a send, but the message is not put
in its outgoing message buffer.
Receive-omission Process A message is put in a process’s incoming message
buffer, but that process does not receive it.
Arbitrary Process or Process/channel exhibits arbitrary behaviour: it may
(Byzantine) channel send/transmit arbitrary messages at arbitrary times,
commit omissions; a process may stop or take an
incorrect step.


Figure 2.16
Timing failures

Class of Failure Affects Description
Clock Process Process’s local clock exceeds the bounds on its
rate of drift from real time.
Performance Process Process exceeds the bounds on the interval
between two steps.
Performance Channel A message’s transmission takes longer than the
stated bound.


Figure 2.17
Security model - Objects and principals

 Figure below shows a server that manages a collection of objects on
behalf of some users. The users can run client programs that send
invocations to the server to perform operations on the objects. The server
carries out the operation specified in each invocation and sends the result
to the client. Objects are intended to be used in different ways by different
users. For example, some objects may hold a user’s private data, such
as their mailbox, and other objects may hold shared data such as web
pages. To support this, access rights specify who is allowed to perform
the operations of an object – for example, who is allowed to read or to
write its state.
Access rights Object
invocation

Client
result Server

Principal (user) Network Principal (server)

Figure 2.18
The enemy

Copy of m

The enemy
m’
Process p m Process q
Communication channel


Figure 2.19
Secure channels

 Encryption and authentication are used to build secure channels as a service layer
on top of existing communication services. A secure channel is a communication
channel connecting a pair of processes, each of which acts on behalf of a
principal.
 A secure channel has the following properties:
• Each of the processes knows reliably the identity of the principal on whose behalf the other
process is executing.
• A secure channel ensures the privacy and integrity of the data transmitted across it.
• Each message includes a physical or logical timestamp to prevent messages from being
replayed or reordered.

Principal A Principal B

Process p Secure channel Process q


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