Architecture of the VCL Sample Clauses

The 'Architecture of the VCL' clause defines the structural framework and organizational principles governing the VCL (Varnish Configuration Language). It typically outlines the key components, such as subroutines, variables, and the flow of request processing, and may specify how these elements interact within the VCL environment. By establishing a clear architecture, this clause ensures that users and developers understand how to structure and maintain VCL configurations, promoting consistency and reducing errors in web caching setups.
Architecture of the VCL. The VCL is composed of one physical host and two virtualisation layers, as shown in Fig. 1. The host machine is the student’s computer, which runs an arbi- trary operating system, i.e., the host operating system. The first virtualisation layer creates the virtual host machine. It consists of virtualisation software such as VMware Player (freeware) or Oracle VM VirtualBox (open source), which runs on the host machine just like an ordinary application. Versions of this soft- ware are available for a large range of platforms. VirtualBox for instance runs on host machines with either Windows, Linux or Mac OS X. This first virtualisation layer therefore runs on nearly all student computers, regardless of the hardware and the host operating system. The virtual host machine runs the virtual host operating system. For the VCL we selected Linux, since it is open source and can be distributed to students without licensing costs. In fact, we selected Knoppix, a bootable live Linux system containing a collection of GNU/Linux applications and the KDE graphical desktop environment. The second virtualisation layer is a Linux application, called Netkit [17], that runs inside the virtual host machine. This second virtualisation layer allows to instantiate multiple virtual machines that all run Linux. Netkit applies virtu- alisation based upon User Mode Linux (UML) [33, 34] and allows to setup and configure UML virtual machines with virtual network interfaces, and to connect these into virtual networks. The hardware requirements for running the VCL are very modest. A few UML virtual machines can already be run smoothly on a PC with a Pentium- 4 processor and 256 MB memory. The VCL has been used successfully in our security courses by hundreds of students with only few minor problems.
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