Jim Gettys has a nice overview of all the components and how they inter-relate. Here is the section on thin clients:
The X Window System, arguably, invented thin client computing. In their heyday, there were a significant number of hardware companies offering what were called “X Terminals” (e.g. NCD). Most/all of these are defunct, as they became less economic than commodity PC hardware, but the concept still has serious validity to reduce system management costs. Some thin client hardware for sale today can be used with X and X based applications as well, particularly since they make machines interchangable, have lower CPU and memory requirements, and can easily eliminate the disk and fans of conventional desktop machines at a somewhat lower price point. The combination can result in a much lower TCO, particularly using Linux, since Microsoft’s pricing of its software licenses actively discriminates against this computing model.
Also note the availability of RDP clients for open source systems as mentioned above, which allow Windows applications to display on X desktops via Windows Terminal Server.
LTSP – Linux Terminal Server Project
The Linux Terminal Server Project is building “thin client” versions of Linux. Most applications are run remotely on servers (via X’s network transparency), but there are also provisions for locally run clients. This provides much better manageability of desktops and is very cost effective in many environments, making machines truly interchangable and obviating the need for “sneaker-net” system management. One view is that this is a reinvention of X terminals, but as there is provision for local clients it goes beyond that, and can take advantage of commodity PC hardware, thin client hardware, and old systems you thought were just junk. The K12LTSP makes a version specifically targeted toward use in schools.
LTSP is in widespread use, has been through four major releases, and has an active developer and user community, and supports multiple Linux distributions.
Athena Computing Environment
A similar style of not-quite-so thin computing is typified by the Athena computing environment at MIT (where both the X Window System and Kerberos have their roots). In this model, machines are also interchangable, but rely on distributed file systems and good authentication to keep any permanent data off the local machine. Local disks are used as caches for files and for swap, but never for long term data storage. As an integrated whole, this is not seen much outside of MIT, though X11 and Kerberos are reasonably widespread. As a interesting historical note, Athena’s “Zephyr” system was arguably the first instant message system.
Java is available from Sun for Linux, and there are several static Java compilers (e.g. GCJ, part of the GCC compiler suite and Jikes from IBM), and may be preinstalled. The Blackdown project provides community source distributions of Sun’s Java for additional platforms that Sun may not. Java release 1.4.2 introduces Swing support based on GTK2 look and feel, which aids in the natural integration of GUI applications built with Java on the open source desktop. As this deploys widely over 2004, Java applications using Sun’s VM will share the look and feel of Gnome desktops.
VNC stands for Virtual Network Computing. It is remote control software which allows you to view and interact with one computer (the “server”) using a simple program (the “viewer”) on another computer anywhere on the Internet. The two computers don’t even have to be the same type, so for example you can use VNC to view an office Linux machine on your Windows PC at home. VNC is freely and publicly available and is in widespread active use by millions throughout industry, academia and privately.
Note that VNC can be trivially replaced with a simple X application along with “ssh -X -C” in concert with the Damage extension.
Adds HP’s Martin Fink in a News.com interview: “Linux on the desktop is definitely an area where hype is ahead of reality by orders of magnitude. There’s a sexiness around the idea of taking on Microsoft. The reality is that (desktop Linux) is still less than 2 percent of the market, but at the same time, we certify and sell a number of Linux desktop solutions. There are two areas of interest: the engineering desktop with folks like DreamWorks and Disney, and the application developer. We are certifying our notebooks with Linux, and the target there is big-time application developers. In the developing countries that don’t have the Windows legacy–like India, China, Asia Pacific and the Eastern bloc–we see some pretty significant volume there.”