DISTRIBUTED SYSTEMS

M.Sc(IT)- 1^st SEMESTER

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GULLAGONG

P.G. DEPT. OF COMPUTER SC & IT

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Distributed Systems

Models of Distributed Systems

System Models

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Distributed system models

• Model: “a simplified representation of a system or phenomenon, as in the sciences or economics, with any hypotheses required to describe the system or explain the phenomenon, often mathematically.”

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System Models

• Architectural model defines the way in which the components of the system are placed and how they interact with one another and the way in which they are mapped onto the underlying network of computers.
• Fundamental models:
• Interaction model deals with communication details among the components and their timing and performance details.
• Failure model gives specification of faults and defines reliable communication and correct processes.
• Security model specifies possible threats and defines the concept of secure channels.
• We will discuss the various models at a high level in this discussion and will elaborate on each of these as we discuss other systems.

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Architectural Model

• Concerned with placement of its parts and relationship among them.
• Example: client-server model, peer-to-peer model
• Abstracts the functions of the individual components.
• Defines patterns for distribution of data and workload.
• Defines patterns of communication among the components.
• Example: Definition of server process, client process and peer process and protocols for communication among processes; definition client/server model and its variations.

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Software and hardware service layers in distributed systems

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Middleware

• Layer of software whose purpose is to mask the heterogeneity and to provide a convenient programming model for application programmers.
• Middleware supports such abstractions as remote method invocation, group communications, event notification, replication of shared data, real-time data streaming.
• Examples: Java RMI, grid software (Globus, Open grid Services), Web services.

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�Clients invoke individual servers

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EX: browser,

web client

EX: Web server

EX: 1. File server,

2. Web crawler

A service provided by multiple servers

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EX: akamai (data duplication), now amazon aws (zones)

Web proxy server and caches

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Proxy servers + cache are used to provide increased

Availability and performance. They also play a major role

Firewall based security. http://www.interhack.net/pubs/fwfaq/

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A distributed application based on peer processes

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Ex: distributed

Whiteboard

Application;

Music sharing

Web applets

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EX: Code streaming; mobile code

Interaction Models

• Within address space (using path as addresses)
• Socket based communication: connection-oriented, connection-less
• Socket is an end-point of communication
• Lets look at some code + details

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Real-time ordering of events

Processes and channels

Omission and arbitrary failures

Timing failures

Objects and principals

�The enemy

�Secure channels

Principal

A

Secure channel

Process

p

Process

q

Principal

B

Summary of System Models

• When designing systems or analyzing systems, you want to examine at the high level the architectural model.
• Subsequent steps will explore fundamental models such as interaction model, security model, failure model, reliability model etc.

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