Client/Server Architectural Style

The client/server architectural style describes distributed systems that involve a separate client and server system, and a connecting network. The simplest form of client/server system involves a server application that is accessed directly by multiple clients, referred to as a 2-Tier architectural style.

Historically, client/server architecture indicated a graphical desktop UI application that communicated with a database server containing much of the business logic in the form of stored procedures, or with a dedicated file server. More generally, however, the client/server architectural style describes the relationship between a client and one or more servers, where the client initiates one or more requests (perhaps using a graphical UI), waits for replies, and processes the replies on receipt. The server typically authorizes the user and then carries out the processing required to generate the result. The server may send responses using a range of protocols and data formats to communicate information to the client.

Today, some examples of the client/server architectural style include Web browser—based programs running on the Internet or an intranet; Microsoft Windows® operating system—based applications that access networked data services; applications that access remote data stores (such as e-mail readers, FTP clients, and database query tools); and tools and utilities that manipulate remote systems (such as system management tools and network monitoring tools).

Other variations on the client/server style include:

• Client-Queue-Client systems. This approach allows clients to communicate with other clients through a server-based queue. Clients can read data from and send data to a server that acts simply as a queue to store the data. This allows clients to distribute and synchronize files and information. This is sometimes known as a passive queue architecture.
• Peer-to-Peer (P2P) applications. Developed from the Client-Queue-Client style, the P2P style allows the client and server to swap their roles in order to distribute and synchronize files and information across multiple clients. It extends the client/server style through multiple responses to requests, shared data, resource discovery, and resilience to removal of peers.
• Application servers. A specialized architectural style where the server hosts and executes applications and services that a thin client accesses through a browser or specialized client installed software. An example is a client executing an application that runs on the server through a framework such as Terminal Services.

The main benefits of the client/server architectural style are:

• Higher security. All data is stored on the server, which generally offers a greater control of security than client machines.
• Centralized data access. Because data is stored only on the server, access and updates to the data are far easier to administer than in other architectural styles.
• Ease of maintenance. Roles and responsibilities of a computing system are distributed among several servers that are known to each other through a network. This ensures that a client remains unaware and unaffected by a server repair, upgrade, or relocation.

Consider the client/server architectural style if your application is server based and will support many clients, you are creating Web-based applications exposed through a Web browser, you are implementing business processes that will be used by people throughout the organization, or you are creating services for other applications to consume. The client/server architectural style is also suitable, like many networked styles, when you want to centralize data storage, backup, and management functions, or when your application must support different client types and different devices.

However, the traditional 2-Tier client/server architectural style has numerous disadvantages, including the tendency for application data and business logic to be closely combined on the server, which can negatively impact system extensibility and scalability, and its dependence on a central server, which can negatively impact system reliability. To address these issues, the client-server architectural style has evolved into the more general 3-Tier (or N-Tier) architectural style, described below, which overcomes some of the disadvantages inherent in the 2-Tier client-server architecture and provides additional benefits.

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