Computing Models

Microsoft Exchange uses a client/server computing model to implement its messaging system. To better understand the client/server model, two other models are discussed briefly to provide a context for the client/server model. In all, the three models discussed in this section are:

• Mainframe computing

• Shared-file computing

• Client/server computing

Mainframe Computing

Mainframe computing consists of a powerful host computer, such as a mainframe computer or minicomputer, and numerous input-output devices attached to the host, such as terminals, printers, and personal computers running terminal emulation software. The advantages of this architecture are its powerful, centralized processing, administration, and backup. These features permit a large number of users on systems built according to this model. The disadvantages are that these features incur high costs, that personal computing power and applications are not leveraged, and that most of these systems have a proprietary architecture. Examples of messaging systems that use this type of model are IBM PROFS (Professional Office System) and OfficeVision.

Figure 1.3 illustrates the host-based computing model.

Figure 1.3: Host-based computing model

Shared-file Computing

This network computing model works in a local area network (LAN) context. At least one computer is used as a server computer to store files. Users, working on their own networked personal computers, access and share the files on the server computer. Microsoft Mail is a messaging system that uses this type of architecture.

Using this model, a shared-file messaging system has active clients and passive servers. Each mail user is assigned a mailbox. A mailbox is actually a directory on the server where mail messages are placed. The server software is passive in that its main task is to store mail messages. The client software is said to be active because it performs almost all mail activities. Along with the normal mail activities of creating and reading mail, the client software is also responsible for depositing mail in the correct recipient mailboxes and checking its own mailbox for new mail (this is referred to as polling ) .

This model could be compared to a postal system where people must take their outgoing mail to the post office and place it in the respective recipients ‚ mail slots and also visit the post office to check their mail slots for any new mail. The primary duty of the post office is to store the mail. This is analogous to the shared-file messaging system in that the people (clients) are active and the post office (server) is passive.

The advantages of shared-file messaging systems include the following:

Minimal server requirements Because the server has a passive role, it does not need to run on a high-end hardware platform.

Minimal server configuration in a single-server environment Because the server is mainly a storage location, it does not need a lot of configuration.

The disadvantages of shared-file messaging systems include the following:

Limited security Because the client software is responsible for sending mail to a recipient ‚ s mailbox, each client must have write permissions on each mail directory. Each client must also have read permissions on the entire mail directory structure in order to read forwarded or copied messages. From a security standpoint, this is considered an excessive level of permissions.

Increased network traffic The periodic client polling of mailboxes for new mail increases network traffic.

Increased client load The active clients do almost all of the processing work.

Limited scalability These systems cannot accommodate large numbers of users because of the shared-file model. Users must access common files that can be opened by only one process at a time.

Single point of failure A single mail server in this type of arrangement becomes a single point of failure for clients seeking to access its services.

Figure 1.4 illustrates a shared-file messaging system.

Figure 1.4: Shared-file messaging system

Client/Server Computing

In client/server messaging , a task is divided between the client processes and server processes. Each side works to accomplish specific parts of the task. The two processes are usually running on separate computers and are communicating over a network. The communication is in the form of requests and replies passed back and forth through messages.

The client side includes a user ‚ s personal computer or workstation and client software. The client software provides the interface for the user to employ when manipulating data and making requests to and receiving replies from the server. The processing power to carry out those tasks is provided by the client ‚ s computer.

The server side includes the server computer and server software. The server software receives and processes client requests, provides storage capabilities, implements security, provides for administrative functions, and performs many more duties . The server ‚ s processor, or processors, powers these functions.

When this model is applied to a mail system, both the client side and the server side are active participants . Mail activities are divided between the two sides in a way that takes advantage of both parties. The client software enables users to initiate mail activities such as creating, sending, reading, storing, and forwarding mail and attachments.

The server software also has an active role. Some of its tasks are implementing security, placing messages in mailboxes (as opposed to the client software doing it), notifying clients of new mail (which eliminates the need for clients to poll their mailboxes), and performing specified actions on mail, such as applying rules, rerouting messages, and many other tasks. Many of the mail activities that are initiated by the client software are actually implemented on the server. For example, when a client initiates the reading of a message, the client software sends a read request to the server where the message physically resides. The server software receives this request, processes it (for example, checks security to see if this user is permitted to read this message), and then sends the message to the client. The user can then use the client software and processor to manipulate the message (edit the message, for example). This illustrates how both sides are active.

In this model, the software running on the client machine is frequently referred to as the front-end program, while the software running on the server is referred to as the back-end program.

Note ‚  

Exchange Server 2003 now supports front-end and back-end servers, a designation that allows an Exchange administrator to balance the various loads placed on Exchange servers among multiple computers. This concept is discussed in detail in Chapter 2, ‚“Microsoft Exchange Architecture, ‚½ and Chapter 7. Do not confuse front- and back-end programs with front- and back-end servers.

The advantages of the client/server model include the following:

Distributed computer processing The computer processing power of both the client and server machines is utilized. The client processor handles the end-user mail activities, such as creating, reading, and manipulating mail, while the server processor (or processors) handles the security, routing, and special handling of mail. This spreads the processing load over a multitude of client processors, while still utilizing the powerful processing of the server machine.

Tight security The server software is responsible for the security of the mail system. The server software is the entity that actually places messages in mailboxes. The clients therefore do not need permissions to all mailboxes. This creates a much more secure mail system.

Reduced network traffic Because the server software informs clients of new mail, the client software does not have to poll the server, thus reducing network traffic.

Scalable The term scalable relates to the ability to grow easily. A client/server mail system can scale to any size organization.

The primary disadvantage of the client/server model is the following:

Increased server hardware requirements Because the server has an active role in the messaging environment, there are greater requirements for the server hardware platform. This should not be seen as much of a disadvantage in light of scalability, central administration, backup, and other advantages.

Figure 1.5 illustrates the client/server mail system.

Figure 1.5: Client/server mail system

Exchange is a client/server messaging system. The Exchange Server 2003 software runs as a series of services on a Windows Server 2003 or Windows 2000 Server computer. It provides server-side messaging functions for the client applications.

So far, you have learned what features make up Exchange Server 2003 and how the system is implemented, namely the client/server model. Now we need to turn our attention to the context or scale in which Exchange can be implemented.