Client/Server Architecture The client/server architecture significantly decreased network traffic by providing a query response rather than total file transfer. It allows multi-user updating through a GUI front end to a shared database. Remote Procedure Calls (RPCs) or standard query language (SQL) statements are typically used to communicate between the client and server.
The following are the examples of client/server architectures.
1) Two tier architectures A two-tier architecture is where a client talks directly to a server, with no intervening server. It is typically used in small environments(less than 50 users).
In two tier client/server architectures, the user interface is placed at user's desktop environment and the database management system services are usually in a server that is a more powerful machine that provides services to the many clients. Information processing is split between the user system interface environment and the database management server environment.
2) Three tier architectures The three tier architecture is introduced to overcome the drawbacks of the two tier architecture. In the three tier architecture, a middleware is used between the user system interface client environment and the database management server environment.
These middleware are implemented in a variety of ways such as transaction processing monitors, message servers or application servers. The middleware perform the function of queuing, application execution and database staging. In addition the middleware adds scheduling and prioritization for work in progress.
The three tier client/server architecture is used to improve performance for large number of users and also improves flexibility when compared to the two tier approach.
The drawback of three tier architectures is that the development environment is more difficult to use than the development of two tier applications.
The widespread use of the term 3-tier architecture also denotes the following architectures:
• Application sharing between a client, middleware and enterprise server
• Application sharing between a client, application server and enterprise database server.
i) Three tier with message server. In this architecture, messages are processed and prioritized asynchronously. Messages have headers that include priority information, address and identification number. The message server links to the relational DBMS and other data sources. Messaging systems are alternative for wireless infrastructures.
ii) Three tier with an application server This architecture allows the main body of an application to run on a shared host rather than in the user system interface client environment. The application server shares business logic, computations and a data retrieval engine. In this architecture applications are more scalable and installation costs are less on a single server than maintaining each on a desktop client.
3-tier architecture provides
• A greater degree of flexibility
• Increased security, as security can be defined for each service, and at each level
• Increased performance, as tasks are shared between servers
The basic characteristics of client/server architectures are:
1) Combination of a client or front-end portion that interacts with the user, and a server or back-end portion that interacts with the shared resource. The client process contains solution-specific logic and provides the interface between the user and the rest of the application system. The server process acts as a software engine that manages shared resources such as databases, printers, modems, or high powered processors.
2) The front-end task and back-end task have fundamentally different requirements for computing resources such as processor speeds, memory, disk speeds and capacities, and input/output devices.
3) The environment is typically heterogeneous and multivendor. The hardware platform and operating system of client and server are not usually the same.Client and server processes communicate through a well-defined set of standard application program interfaces (API's) and RPC's.
4) An important characteristic of client-server systems is scalability. They can be scaled horizontally or vertically. Horizontal scaling means adding or removing client workstations with only a slight performance impact. Vertical scaling means migrating to a larger and faster server machine or multiservers.
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