Introduction to distributed systems Dr. S. Indran 23 January 2004

Organisation Architecture and programming paradigms for distributed system development The concepts of clients, servers, and middleware Distributed paradigms

A simple e-commerce system

Distributed system A distributed system in which the computer power in the system is distributed geographically around a number of computers which share the processing load of system

Network concepts Network topologies Layered models The Internet Internet addresses Domain names and naming Clients and servers Ports and sockets

Network topologies Bus network Ring network Hub network

Bus network

Ring network

Hub network

Domain names and naming Top-level domain nameOrganisation comCommercial company edu or acEducational govGovernmental milMilitary bizShow business orgOrganisations not catered for above aeroAeronautics

Internet domain name system

Architecture and programming paradigms Message passing Distributed objects technology Event-based bus architectures Remote Procedure call Tuple architecture

Message passing Protocols Fixed and adaptive protocols Synchronous and asynchronous message passing

Message passing: Requirements Communication needs very simple High performance is required Rapid implementation is required The system uses a technology that no existing protocols will communicate with

A message passing architecture

Protocols A language which embodies the functions required by an entity in a distributed system (usually a client) which another entity provides (usually a server)

Synchronous and asynchronous message passing Synchronous message passing involves (usually a client) in the message passing process and sending a message and a second entity (usually a server) receiving it, carrying out some processing and then sending back some responses while the first entity pauses waiting In Asynchronous message passing, each entity does not have to wait for the next part of the dialogue they are engaged in and can carry out some other task

Distributed objects technology A distributed object is an object which is resident on one computer and for which methods can be invoked associated with code resident on other computers This technology hides the network from the designer and the programmer. Examples: CORBA, and RMI.

Distributed object architecutre

n-Tiered Architecture A tier is a logically separated and encapsulated set of processes. There are three basic parts: The user interface tier (representation layer). The data tier (server-side component), which is the database data processed by the Web application. The middle tier (middle-tier component), which processes complex business processing (usually place on a separate server called the application server).

Three-tier architecture

Middle-Tier Technologies The middle-tier application development is a separated and encapsulated set of processes usually related to business applications. The applications in the middle-tier are build as components which are accessed from the server-side components (data tier). Popular technologies for building middle-tier components are: the Component Object Model (COM) with C++ and VB, and the JavaBeans with Java The middle tier components are usually place on a separate server called the application server.

Clients and servers A server is a computer which carries out a service such as printing out a file or responding to a web page A client is some computer which require service

Clients and servers Clients Servers Middleware Protocols Internal and external services Enterprise frameworks Why client-server?

Types of server A file server provides files for the clients that ask for its service A database servers are computers which store large collections of data which are structured A groupware is software which organises the work of a number of staff in some enterprise A web server is a typical type of file server that contain files which store the various components of a web site A mail server is a computer which has the task of receiving, storing, and sending Object servers Print servers Application servers

Tiered architectures Two-tier architecture (a presentation and logic layer, and a data layer) Three-tier architecture (a presentation layer, a processing (application server) layer, and a data layer

Middleware The intermediate software between clients and severs is known as middleware General middleware is software which is associated with services required by all clients and servers Service middleware is associated with a specific service such as a printing a file on a remote computer

Enterprise frameworks Features The J2EE framework The.Net framework

Enterprise frameworks features Multi-language support Support for legacy code Support for high-volume transactions Messaging support Web server programming

The J2EE framework components (Continued) Java Database Connectivity (JDBC) Remote object access Enterprise JavaBeans The Java Messaging Service Java Naming and Directory Interface (JNDI) Legacy interfacing

The J2EE framework components XML processing Web server programming Security in J2EE Access to standard protocols

The.Net framework Provides similar features to J2EE framework Involved configuring many of microsoft products such as Excel, Word, MS Access together with supporting language such as Visual Basic, C++, and CSharp.

J2EE vs..Net J2EE is platform independent.Net is a multi-language approach XML is placed more centrally in.Net than J2EE