System models in distributed system

System Models

• Threats to consider for modeling- Widely varying modes of use: component parts are

subject to wide variations in workload.- Wide range of system environments: Able to

accommodate heterogenous h/w, s/w, OS etc.- Internal problems: Failures, inconsistency of data etc- External threats: Attack on data integrity.• Types- Physical- Architectural - Fundamental

Isha Padhy, Department of CSE, CBIT, Hyderabad 1

Physical Model

• Model that capture the hardware composition of a system in terms of computer and their interconnecting networks.

• Three generations of distributed systems:–Early distributed systems– Internet-scale distributed systems: CORBA–Contemporary distributed systems


Architectural Model

• Architecture: The structure of a system specifying its components and their interrelationships.

• Why architecture?– To ensure that the structure will meet the current and

future demands on it– To provide a reliable, manageable, adaptable and cost-

effective system(MARC)• Ways of organization of DS- Software Architecture- System Architecture


Architectural model

• Architectural elements: How to understand the fundamental building

blocks of a distributed system?• What are the entities that are communicating in the

distributed system (communicating entities)?• How do they communicate, or, what communication

paradigm is used (communication paradigms)?• What roles and responsibilities do they have in the

overall architecture (roles and responsibilities)?• How do they map on to the physical distributed

architecture (what is their placement)?


Architectural elements

• Communicating entities: Entities that take part

- Processes- Objects- Components- Web services


Architectural ElementsCommunication paradigms: How they communicate

Inter-process communication – low-level process support. E.g. message passing primitives, direct access to API, multicast communication

Remote invocation – common communication paradigm in distributed systems. Two-way exchange between communicating entities, common techniques; RPC, RMI, request-reply protocols

Indirect communication– E.g. group communication, publish-subscribe systems, message queues, tuple spaces, distributed shared memory


Indirect communication

• Group communication• Publish- subscribe systems(Event Based

systems)• Message queues• Tuple spaces• Distributed shared memory


Centralized Architecture:Client- Server Model

- Components are server, client , communication

Server

Client

Client

invocation

result

Serverinvocation

result

Process:Key:

Computer:


Layered Architecture

- A complex system partitioned into layers with each layer using service offered by layer below.

Application , Services

Middleware

Operating System

Computer and network hardwarePlatform


Example : Internet search engine


Tiered Architecture

• Technique to organize functionality of a given layer and place this functionality into appropriate servers.

• The functional decomposition is as- Presentation Logic- Application logic- Data Logic


2 tier Architecture• Server provides processing and data management; client provides simple graphical

display (thin-client)• At the other extreme, all application processing and some data resides at the client

(fat-client approach)


3 Tier Architecture

An example of a server acting as client.

-In some applications servers may also need to be clients, leading to a three level architecture. Ex-Distributed transaction processing -Web servers that interact with database servers


Education

System models in distributed system