When people refer to a piece of a hardware as "a server," they typically mean that it is running one or more pieces of server software. You must go under the hood to really understand what the server is doing.
When people refer to a piece of a hardware as "a server," they typically mean that it is running one or more pieces of server software, may or may not be dedicated to that role, and is possibly made up of higher-grade components that tolerate long periods of availability.
Of course, in either case, appropriate software is the core of the system. When people refer to a piece of a hardware as "a server," they typically mean that it is running one or more pieces of server software, may or may not be dedicated to that role, and is possibly made up of higher-grade components that tolerate long periods of availability.
Requests are sent to a server by a "client." A good example of a client is the garden variety Web browser typically used as client to a Web server. Networks are the backbone of client/server relationships, and the rise of the Internet and local area networking (Intranets) has seen the evolution of a wide variety of servers.
Many servers in use today are rooted in historical models that have been a part of computing for years; others are evolutions of these models, often tagged with new, marketing-friendly names. The upside of this market flood is an atmosphere of competition and improvement; the downside can be confusion about category names that are sometimes invented to help differentiate a product from its closest peers and competitors.
In this tutorial, we'll survey the wide variety of servers available, from familiar Web servers to so-called "application servers", proxy servers, e-mail servers, as well as DHCP servers, firewalls, and fax servers. We've broken the server types to be discussed into four categories: Web servers, "children" of Web servers, intranet-level servers, and on-demand Internet servers.
For the most part, a Web server is much like a robotic dumbwaiter. The client asks it for something - a file - and the Web servers gets the file and sends it to the client. In most cases the Web server does not read or otherwise process this file, but simply hands it off to the client that asked for it.
Over the years, the "robotic" part of the Web server model has grown more sophisticated. In terms of efficiency, for example, Web servers have become more adept at handling many requests simultaneously and delivering quicker on these requests. But Web servers have also evolved to having the capability to process requests in a manner more sophisticated than simply handing over a document. As a result, Web servers have crossed the blurry line into a new territory to be marketed as "application servers" or "information servers."
More information about Web servers can be found in Serving Up Web Server Basics
Web servers grew "brains" by relying on additional technologies so they could process pages before delivering final results to the client. The common gateway interface, or CGI, was the first popular technology that allowed a Web server to interact with an external computer program that could crunch data and deliver the results back to the Web server, before reaching the client. Embedded server-side scripting technologies such as Microsoft's Active Server Pages (ASP) and the open-source PHP begot another evolution, allowing developers to insert their processing code directly into the Web page itself.
The "application server," is an umbrella term that really encompasses all of these skills. Because these servers possess an extensive ability to process information, interaction with the client become like an application with give-and-take between the user and computer, rather than like a book, where the reader is limited to flipping through pages.
The achievements of an application server can certainly be enjoyed with an intelligent combination of existing technologies. For example, an open source developer might connect an Apache Web server with the PHP scripting language. In effect, this is an application server. But as we see the term used in the marketplace, an "application server" is typically a bundled solution offered by a vendor that contains all the component technologies needed. For some organizations, the bundled approach of a self-proclaimed application server eases development by unifying development models and centralizing support.
A more complex type of wireless server is the wireless gateway. Basically, a wireless gateway is a middleman that accepts requests from wireless devices and routes those requests to traditional Web servers. Most wireless gateways are operated by service providers specializing in wireless access, which can limit subscribers to the services these gateways support.
Wireless "application servers," or software suites designed to deliver dynamic data to wireless devices, are currently boxed into a rather small market segment. Still, products such as Lutris' Enhydra Wireless, IBM's WebSphere Everyplace and Bluestone's Total-e-Mobile are battling for early ground in a market that is hoped to explode rapidly.
But a proxy server can do more than just cache frequently accessed data. Because the proxy server "hears" and services requests from its clients, the clients themselves do not necessarily need any direct Internet access at all. Consequently, the proxy server became a very popular way to bridge a local network, also known as a LAN or intranet, to an external network such as the Internet -- as long as one computer running the proxy server could hear both networks simultaneously.
As the availability of broadband connections has grown rapidly, so has the thirst for proxy servers. This primarily because these connections offer enough bandwidth to support several machines simultaneously. The proxy server solutions require only one of these machines to maintain a connection to the broadband service, though. This can preserve IP address space and also save enterprises money on service subscription costs.
Proxy servers can be configured to allow or deny particular types of network requests, either from the LAN to the Internet, or from the Internet to the LAN. In this way, the proxy server becomes a firewall. A firewall, as its combative name implies, is a security measure that functions much like a border guard: It inspects each piece of data that tries to pass through the boundary.
Depending on the sophistication level of the firewall, it may be configurable to make a wide variety of distinctions between different types of incoming and outgoing data. Whether this data is allowed to pass into or out of the internal network depends on certain conditions, such as the originating IP address of said data. Good firewalls also provide extensive logging capabilities because a "paper trail" of network activity is key to investigating incidents or attempted incidents.
There are stand-alone firewall products, and there are proxy servers that include firewall capabilities. As a firewall is more like a moat between servers, one could argue that a firewall in and of itself is not a "server" in the traditional sense. In theory, there is nothing intrinsically superior about a dedicated firewall product vs. a proxy server with firewall capabilities. In practice, however, the latter category is finally catching up to dedicated firewall products. WinRoutePro is an example of such a product and may represent the newest generation of proxy servers with strong firewall capabilities.
The e-mail server also handles outgoing messages in a fashion similar to how the apartment building's mailman collects outgoing mail left by residents. Just as the mailman does not personally deliver each outgoing message to its final address, the e-mail server is configured to interact with other servers, or nodes, through which a message is passed until it reaches its own destination network. At that point, the destination network's e-mail server handles the delivery to the final mailbox.
Most Internet providers offer an e-mail server to their subscribers, so individual users rarely need to install their own e-mail servers. Organizations, regardless of their size, may benefit from installing their own e-mail server, as this can offer increased customizability compared to relying on an ISP's server. Obvious advantages of this include customization (e.g., the choice of mailbox names and behavioral characteristics like quotas, auto-replies, listserv management) and cost savings resulting from the ongoing cost of activating many e-mail addresses through an outside provider.
In some cases, DHCP server software is integrated into a hardware product. For example, several hybrid router/switches with DHCP servers have come on to the market from companies such as Linksys, Netgear, and D-Link. These products enable a LAN to share a single broadband connection to the Internet. Often, these devices can be configured to use their own internal DHCP server to assign IP addresses on the local network. This makes it easy to add and remove machines to the local network without any further configuration.
Similarly, a single host machine on a LAN can perform that role by running DHCP server software, such as Vicomsoft's DHCP server for Windows and Macintoshes, SimpleDNS Plus for Windows, IPWorks DHCP server for Solaris and Windows, and Moreton Bay's DHCP server for Linux and Windows.
Sophisticated FTP servers give the administrator increased control over who may connect and share files, what types of files they may share, and where those files can be placed. Configurable quotas on the number of connects to the server, the amount of data transferred, minimum transfer speed, and so on are becoming increasingly common to help boost the security of FTP servers.
In a sense, fax servers are a bridge between the old way of doing business and the new. But, as long as documents continue to stampede across this bridge, the fax server market continues to breathe life vigorously.
In many respects, a fax server is similar to the aforementioned e-mail server. Both types of servers are bridges between outgoing and incoming messages. Both must route incoming messages to a destination. In the case of e-mail servers, this destination is always an inbox for a particular user. Fax servers for small, single-user environments often assume that the receiving computer itself is the sole destination, so the "inbox model" does not apply. On the other hand, fax servers designed for corporate environments do indeed parallel the e-mail server model, delivering incoming faxes to particular destinations assigned to individual users.
A well designed fax server may offer extra conveniences for handling incoming faxes, such as direct-to-printer output. It may also provide outgoing specialities, such as scheduled broadcasts of a document to many recipients, and automated outgoing faxes triggered by incoming requests ("fax-back").
Corporate fax servers must also juggle numerous outgoing faxes, possibly queued up by a number of different users. How well fax server software can effectively manage a limited number of phone lines, so as to schedule both outgoing and incoming faxes without conflict, is a major selling point for costlier, "enterprise" level fax servers.
Sophisticated fax servers also feature strong integration with electronic messaging systems, including e-mail, Microsoft Exchange and Lotus Notes. Such features enable the fax server to become a "seamless" bridge between electronic documents and the anachronistic world of fax documents.
Fax servers range widely in capabilities, scaled to different environments, from the home or small office needs addressed by MightyFAX, and RelayFax to enterprise-level products, including RightFAX Enterprise, FAXport, and Faxination.
"Appliance" in marketing lingo doesn't really refer to a type of server, per se, but a type of packaging or bundling. In fact, the term "appliance server" simply refers to any type of server that is sold already set up and configured, and ready to be added into a network. So, when IBM markets its "eServer xSeries 130", described as a Web-hosting appliance server, it is really selling a Web server -- probably a dynamic Web server also known as an application server -- bundled as "plug-and-play" as is reasonable for installation into an existing network.
Similarly, IBM's "eServer xSeries 150" is called a storage appliance, which basically means it is a file server to and from which users can store files.
Presumably, the term "appliance" is supposed to connotate the idea of a server -- any type of server -- as ready-to-use, just as a kitchen appliance, such as a refrigerator or stove, is basically usable out of the box.