UNIT-1 Left over Topics

1. Basic building blocks of Client/Server

The client/server model has three basic building blocks as show in below figure

1. Client2. Server3. Middleware slash (/)

The Client Building block

It runs the client side of the application. It runs on an Operating System (OS) that provides a Graphical User Interface (GUI) or an Object Oriented User Interface (OOUI) and that can access distributed services.

Thin clients require a web browser to download Java beans and applets on demand. In all cases, the operating system most often passes the buck to the middleware building block and lets it handle the non-local services. The client also runs a component of the Distributed System Management (DSM) element.

The Server building block

It runs the server side of the application. The server application is typically runs on top of some server software. The five contending server platforms for creating the next generation of client/server applications are SQL database servers, TP Monitors, groupware servers, object servers, and the web.

The server side depends on the operating system to interface with the middleware building block that brings in the requests for service. The server also runs a DSM component.

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The middleware building block

It runs on both the client and server sides of an application. We can break this building block into three categories: transport stacks, network operating system, and service-specific middleware.

The middleware is the nervous system of the client/server infrastructure. Like the other two building blocks, the middleware also has a DSM software component.

N-Tier Middleware

In N-tier environments, the middleware must also provide a platform for running server-side components, balancing their loads, managing the integrity of transactions, maintaining high-availability and securing the environment.

Middleware in N-tier client/server environments includes pipes and platforms.

Pipes: Provides the intercomponent (and interapplication) communication services. Examples of pipes are RPCs, ORBS, etc. Pipes also include wire-level security such as SSL.

Platforms: Platforms are application servers that run the server-side components. We can typically use them across multiple operating systems to provide a unified view of the distributed environment. Examples of platforms are TP monitors, Object transaction Monitors and web application servers.

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Server-to-Server Middleware

Middleware does not include the software that provides the actual service. It does, however, include the software that is used to coordinate inter-server interactions as shown in below figure.

Server-to-server interactions are usually client/server in nature- servers are clients to other servers, and vice versa. So a server can play both client and server roles.

Some server-to-server interactions require specialized server middleware for example a two-phase commit protocol may be used to coordinate a transaction that executes on multiple servers.

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2. N-tier Architecture

The middle tier in most 3-tier applications is not implemented as a monolithic program. Instead, it is implemented as a collection of components that are used in a variety of client-initiated business transactions as shown in below figure

Each component automates a relatively small business function. Clients frequently combine several middle-tier components within a single business transaction.Component-based applications offer significant advantages over monolithic applications. With an N-tier we have the following advantages

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We can develop big applications in small steps Component based architecture allows us to develop large mission-critical applications as small projects.

Applications can reuse Components:

We can reuse components as “black-boxes”. Clients can access data and functions easily and safely:

Clients send requests to components to execute a function on their behalf. The server components encapsulate the details of the application logic and thus raise the level of abstraction.

Custom applications can incorporate off-the shelf components

Enterprises gain tremendous benefits by buying ready-made components that are packaged as applications.

Component environment don’t get older- they only get better

Component based system grows beyond a single application to become the basis for suites of applications. We can assemble applications very quickly by building new clients, adding a few new middle-tier components and reusing a number of existing components.

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