Computer Networking

The client server building blocks

The Client Building Block

• Runs the client side of the application
• It runs on the OS that provides a GUI or an OOUI and that can access distributed services, wherever they may be.
• The client also runs a component of the Distributed System Management (DSM) element.

The Server Building Block

• Runs the server side of the application
• The server application typically runs on top of some shrink-wrapped server software package.
• The five contending server platforms for creating the next generation of client/server applications are SQL database severs, TP Monitors, groupware servers, Object servers and the Web server.
• The server side depends on the OS to interface with the middleware building block.
• The server also runs DSM component
• It may be a simple agent or a shared object database etc.

The Middleware Building Block

• Runs on both the client and server sides of an application
• This broken into three category
• • Transport Stacks
  • NOS
  • Service-specific middleware
• Middleware is the nervous system of the client/server infrastructure
• This also has the DSM component

DSM

• Runs on every node in the client/server network.
• A managing workstation collects information from all its agents on the network and displays it graphically.
• The managing workstation can also instruct its agents to perform actions on its behalf.

Server-to-server Middleware

• Server-to-server interactions are usually client/server in nature - servers are clients to other servers.
• However, some server-to-server interactions require specialized server middleware. For example, Two-Phase commit protocol may be used to coordinate a transaction that executes on multiple servers.
• Servers on mail backbone will use special server-to-server middleware for doing store-and-forward type messaging.
• But most modern software follows the client/server paradigm.

The building blocks of client/server applications are:

1. Client
2. Middleware
3. Server

These building blocks can be rearranged to use them in the following situations:

1. Client/Server for tiny shops and nomadic tribes - This is a building-block implementation that runs the client, the middleware software, and most of the business services on the same machine. It is the suggested implementation for the one-person shops, home offices, and mobile users with well-endowed laptops.

2. Client/Server for small shops and departments - This is the classic Ethernet client/single-server, building block implementation. It is used in small shops, departments, and branch offices. This is the predominant form of client/server today.

3. Client/Server for intergalactic enterprises - This is the multiserver building-block implementation of client/server. The servers present a single system image to the client. They can be spread out throughout the enterprise, but they can be made to look like they are part of the local desktop. This implementation meets the initial needs of intergalactic client/server computing.

4. Client/Server for a post-scarcity world - This model transforms every machine in the world into both a client and a server. Personal agents on every machine will handle all the negotiations with their peer agents anywhere in the universe. This dream is almost within reach.

1) Client/Server for Tiny Shops and Nomadic Tribes

• It is easy to run the client and server portion of an application on the same machine.
• Vendors can easily package single-user versions of a client/server application.
• The business critical client/server application runs on one machine and does some occasional communications with outside servers to exchange data, refresh a database and send or receive mail and faxes. Ex: Internet.

2) Client/Server for small shops and departments

• The client/server architecture is particularly well-suited for the LAN-based single server establishments.
• It consists of multiple clients talking to a local server.
• This is the model used in small businesses.
• The single-server nature of the model tends to keep the middleware simple.
• The client only needs to look into a configuration file to find its server's name.
• Security is implemented at the machine level and kept quite simple.
• The network is usually relatively easy to administer; it's a part-time job for a member of the group.
• There are no complex interactions between servers, so it is easy to identify failures- they're either on the client or on the local server.

3) Client/Server for Intergalactic Enterprises:

• The client/server enterprise model addresses the needs of establishments with a mix of heterogeneous servers.
• These models are upwardly scalable.
• When more processing power is needed for various intergalactic functions, more servers can be added, or the existing server machine can be traded up for the latest generation of superserver machine.
• The servers can be partitioned based on the function they provide, the resource they control, or the database they own.
• The servers can be replicated to provide a fault-tolerant service or to boost an application's performance.
• Multiserver capability, when properly used, can provide an awesome amount of compute power and flexibility, in many cases rivaling that of mainframes.
• To exploit the full power of multiservers, we need low-cost, high-speed bandwidth and an awesome amount of middleware features -including
• • network directory services
  • network security
  • remote procedure calls and
  • network time services.
• Middleware creates a common view of all the services on the network called a single system image.
• Good software architecture for intergalactic enterprise client/server implementations is all about creating system "ensembles" out of modular building blocks.
• Intergalactic client/server is the driving force behind middleware standards as distributed objects and the Internet.

4) Client/Server for a Post-Scarcity World

• Every machine is both a client and a full-function server.
• Because every machine is a full-function server, it will run, at a minimum, a file server, database server, workflow agent, TP Monitor, and Web server - all connected via an ORB.
• This is in addition to all the client software and middleware.
• In next few years, a hundred million machines or more may be running almost all the forms of client/server software
• In this model instead of mobile agents, personal agents will be used.

The four layers in the DoD model, from bottom to top, are:

1. The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network.
2. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol.
3. The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer's most important members.
4. The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.