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Distributed Systems Architectures

Architectural design for software that

executes on more than one processor

Distributed Systems

 Virtually all large computer-based systems are now distributed systems

 Information processing is distributed over several computers rather than confined to a single machine

 Distributed software engineering is now very important

Distributed System Characteristics

 Resource sharing

 Openness

 Concurrency

 Scalability

 Fault tolerance

 Transparency

Distributed system

disadvantages :

 Complexity

 Security

 Manageability

 Unpredictability

Distributed Systems Architectures

 Client-server architectures

 Distributed services which are called on by clients. Servers that provide services are treated differently from clients that use services

 Distributed object architectures

 No distinction between clients and servers. Any object on the system may provide and use services from other objects

1. Multiprocessor Architectures

 Simplest distributed system model

 System composed of multiple processes which may (but need not) execute on different processors

 Architectural model of many large real-time systems

 Distribution of process to processor may be pre-ordered or may be under the control of a dispatcher

A Multiprocessor Traffic Control System

Traffic lights

controlLight process

Traffic flowprocessor Traffic light controlprocessor

Traffic flow sensorsand cameras Operator consoles

processorSensor

controlSensor process

Displayprocess

A Client-Server System

Client process

 - s - s2 s 

• c1 s
  • c2 c3 c
    • c
      • c6 c7 c - c - c - c - c

Computers in a C/S Network

Network SC2^ SC

CC1 CC2 CC

CC4 CC5 CC

computerServer

computerClient

s1, s2 s3, s

c5, c6, c

c1 c2 (^) c3, c

c8, c9 c10, c11, c

Application Layers

Presentation layer

Application processing layer

Data management layer

Thin and Fat Clients

 Thin-client model

 In a thin-client model, all of the application processing and data management is carried out on the server. The client is simply responsible for running the presentation software.

 Fat-client model

 In this model, the server is only responsible for data management. The software on the client implements the application logic and the interactions with the system user.

Thin Client Model

 Used when legacy systems are migrated to client server architectures.  The legacy system acts as a server in its own right with a graphical interface implemented on a client

 A major disadvantage is that it places a heavy processing load on both the server and the network

Fat Client Model

 More processing is delegated to the client as the application processing is locally executed

 Most suitable for new client-server systems where the capabilities of the client system are known in advance

 More complex than a thin client model especially for management. New versions of the application have to be installed on all clients

Three-Tier Architectures

 In a three-tier architecture, each of the application architecture layers may execute on a separate processor

 Allows for better performance than a thin-client approach and is simpler to manage than a fat-client approach

 A more scalable architecture - as demands increase, extra servers can be added to the data management or application processing layers.

A 3-Tier Client-Server Architecture

Client

Server Data management

Presentation Server Application processing