1-2 Distributed System Models

8/12/2019 1-2 Distributed System Models

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Distributed Systems Peter Arnold [email protected]

Distributed System Models

A model is intended to provide an abstract, simplified but consistent description of a relevant aspect of a

distributed system design. There are two types of distributed system models i.e.

Architectural models Fundamental models

Architectural models

An architectural model defines the way in which the components of systems interact with one another

and the way in which they are mapped onto an underlying network of computers. The architecture of a

system is its structure in terms of separately specified components.

Model Considerations

The placement of the components across a network of computers - this defines patterns forthe distribution of data and workloads.

The interrelationships between the componentsthis defines their functional roles and the

patterns of communication between them.

There are several main paradigms commonly used to structure a distributed system. These include:

1.

Master- slave model

2.

Client/server model

3.

Peer-to-peer model

4.

Group model

Masterslave model

A master process normally initiates and controls any dialogue with the slave processes. Slave processes

exhibit very little intelligence, responding to commands from a single master process and exchange

messages only when invited by the master process. The slave merely complies with the dialogue while

rules are set by the master.

Client/ server model

Clients request a particular service. Server process is normally non-terminating (persistent) and provides

services to more than one client process. On receipt of a request the, the server executes the

appropriate operation and sends a reply back to the client. The operation of this model can be depicted

as follows.


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s1

s2 s3

s4c1

c2 c3 c4

c5

c6c7 c8

c9

c10

c11

c12

Client process

Server process

Peer-to-peer model

Every computer can be configured to be either a client or a server. Every process has the same

functionality as peer processes. Peer-to-peer (p2p) systems are usually decentralized systems where

computations may be carried out by any node in the network. The overall system is designed to take

advantage of the computational power and storage of a large number of networked computers. The

model can be depicted as follows.


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Group model (Collaborative Application model)

Processes co-operate in such a way that one process may need to send a message to all other processes

in the group and receive response from one or more members. For example, in video conferencing,

when someone writes to the blackboard facility every other participant must receive the image. In this

model, processes participate in a collaborative session as a group. Each participating process may

contribute input to part or the entire group. Processes may do so using:

Multicasting - to send data to all or part of the group

virtual sketchpads or whiteboards - allow each participant to read and write data to a shared

display

Client/server variation models

These are extensions of client server model. They include the following:

Multi-tiered architecture

3 tier architecture

Mobile code

Mobile agent

Multi-tiered architecture


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The vertical distribution or multi-tier architecture distributes the traditional server functionality over

multiple servers. The architecture can be depicted as follows.

3-Tier architecture

Three tier systems move the client intelligence to a middle tier so that stateless clients can be used. This

simplifies application deployment. This architecture is used in most web servers. It consists of:

Presentation Tier - This is the topmost level of the application. It displays information related to such

services as browsing merchandise, purchasing, and shopping cart contents. It communicates with other

tiers by outputting results to the browser/client tier and all other tiers in the network.

- The logic tier is pulled out from the presentation tier and, as

it w y, fy by fm dd .

Data Tier - This tier consists of Database Servers. Here information is stored and retrieved. The tier

keeps data neutral and independent from application servers or business logic. Giving data its own tier

helps improve scalability and performance.

The 3-Tier architecture can be depicted as follows.


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The following diagram illustrates an example of a 3-Tier architecture in use.

Mobile Code

Refers to code that is sent to a client process to carry out a specific task.m d d

m mm d.


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Mobile Agents

Refer to executing program i.e. code + data migrating amongst processes, carrying out an autonomous

task, usually on behalf of some other process. Mobile agents have the advantages of flexibility and

savings in communications cost.

Fundamental Models

They are a description of fundamental properties in processes and communication, shared among

different architectures. They refer to the aspects of the distributed system that we wish to capture in

the architectural models. They are concerned with a more formal description of the properties that are

common in all of the architectural models. They include:

1.

Interaction model

2.

Failure model

3.

Secure model

The secure model

This model addresses the possible threats to processes and communication channels. Encryption and

authentication are used to build secure channels. Each of the processes knows the identity of the

principal on whose behalf the other process is executing and can check their access rights before

performing an operation. h md b dd fw.

The interaction model

The interaction model deals with performance and with the difficulty of setting time limits in a

distributed system. It is concerned with analysis of communication and coordination (synchronization

and ordering of activities) between processes.

Factors affecting interaction process

1.

Performance of communication channels


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1-2 Distributed System Models

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