Architectual Models Distributed System Presentation

From Coulouris, Dollimore, Kindberg and
Blair
Distributed Systems:
Concepts and Design
Edition 5, © Addison-Wesley 2012
Slides for Chapter 2:
Architectural Models

2
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012
Figure 2.1
Models
1. Fundamental Models: based on some fundamental properties. Examples-
• Interaction Models
• Failure Model
• Security Model
2. Architectural Models: based on architectural style. Examples-
• Client Server
• Peer-to-peer

3
Figure 2.1
Generations of distributed systems

4
Figure 2.2
Communicating entities and communication paradigms

5
Figure 2
Architectural Model
Architectural Models deals with organization of components across the network of
computers and their relationships.
Client Server
Peer to Peer

6
Figure 2.3
Client Server Model
• Client Server Model is the most popular and most widely used
distributed system architecture.
• Client Server architecture is also known as request-response
architecture.
• In this architectural model the client makes a request to the server
and the server will fulfill the response.
• Client and server roles are assigned and changeable.
Advantages of client server model:
• Centralized system where all the data can be stored in a single
place.
• Requires less maintenance cost and entire system is maintained by
the server.
• Increases the speed of the resource sharing

Figure 2.3
Clients invoke individual servers

8
Figure 2.4
Peer to Peer Model
• Unlike client-server model, Peer to Peer Model (P2P) does not
distinguish between client and server instead each node can either be
a client or server depending on whether the node is requesting or
providing the services.
Advantages of peer to peer models:
•Setup and maintenance of network is easy.
•Cost efficient as each node in this model acts as a server.
•Each node is independent of each other so if one node stops
working it won't affect other node.

Figure 2.4a
Peer-to-peer architecture

Figure 2.4b
A service provided by multiple servers

Figure 2.5
Web proxy server

Figure 2.6
Web applets

Figure 2.7
Software and hardware service layers in distributed systems

14
Figure 2.8
Two-tier and three-tier architectures

15
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.)
.....
} 15

Figure 2.10
Thin clients and compute servers
Thin
Client
Application
Process
Network computer or PC
Compute server
network

17
Figure 2.11
The web service architectural pattern

18
Figure 2.12
Categories of middleware

Figure 2.13
Real-time ordering of events

Figure 2.14
Processes and channels

21
Figure 2.4
Fundamental Models
• Interaction Models
• Failure Models
• Security Models

22
Figure 2.5
Interaction Models Models
• Computation occurs within the process
•The processes interact within the process interact by passing
messages in
•Communication
•Coordination
•Interaction model reflects the facts that communication takes place
with delay
•Performance:
•Latency
•Bandwidth
•Jitter
•Two variants of the interaction model
•Synchronous
•Asynchronous

23
Figure 2.5
Interaction Models Models
• Computation occurs within the process
•The processes interact within the process interact by passing
messages in
•Communication
•Coordination
•Interaction model reflects the facts that communication takes place
with delay
•Performance:
•Latency
•Bandwidth
•Jitter
•Two variants of the interaction model
•Synchronous
•Asynchronous

24
Figure 2.5
Interaction Models contd…
•Synchronous Distributed System has bounds on:
• Process is executing in a known lower/upper bound time
• Message is received within a known bound time
• known local clock drifts rates
•Asynchronous Distributed System has no bounds on :
• Process execution speed
• Message transmission delay
• Clock drift rate

25
Figure 2.5
Faliure Models
• Failure model defines & classifies the faults
• It is important to understand the kinds of failure that may occur in a
system Distributed System has bounds on:
•Types:

Figure 2.15
Types of Failure Models: 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
(Byzantine)
Process or
channel
Process/channel exhibits arbitrary behaviour: it may
send/transmit arbitrary messages at arbitrary times,
commit omissions; a process may stop or take an
incorrect step.

Figure 2.11
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.

28
Figure 2.5
Security Models
• There are several potential threats a system designer need to be
aware of
•Threats to process: messages sent with fake identity
•Threats to communication channels: channel is spoofed
• Denial of service: channel or process is made unavailable

Figure 2.17
Objects and principals

Figure 2.18
The enemy
Communication channel
Copy of m
Process p Process q
m
The enemy
m’

Figure 2.19
Secure channels
Principal A
Secure channel
Process p Process q
Principal B

Architectual Models Distributed System Presentation

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