Defining Virtual Clusters

Virtualization is also an effective way of using a cluster of physical machines. A cluster can essentially act as a server farm for virtualization; in fact, the student networks described previously could run on the various nodes in a cluster. The physical cluster can serve as physical hardware for one or more virtual clusters. Listing 6 shows a project file that automatically creates a virtual cluster of 36 nodes.

This project uses the autoenum plugin (included with MLN) to automatically generate virtual machines with successive IP addresses and service hosts taken from the list specified in the plugin's service_hosts substanza.

Listing 6 creates 36 hosts on the basis of the settings in the cluster-node superclass, and the list of service hosts comes from an external file referenced with the #include directive.

The superclass definition also introduces two more configuration file entries that specify the required amount of free space within the filesystem in which the VM image will be stored (another way of specifying its size) and the default gateway (router) IP for a network interface.

Plugins for Special-Purpose Configuration Options

The preceding example illustrated the use of a plugin designed for a specific configuration task: in this case, assigning sequential IP addresses and service hosts.

The MLN configuration language has an open architecture that simplifies the task of writing plugins. MLN plugins are Perl routines that are invoked within the global stanza of a project file.

Listing 7 shows two more examples of plugins. One (apache) sets various configuration parameters for the Apache web server within a virtual machine. Another (sshkey) installs the specified SSH key file(s) into the designated user account. This technique is very practical because it allows you to build virtual machines that can be automatically accessed from other systems, such as by a monitoring tool like Nagios.

The apache plugin will cause MLN to set the Apache document root location and the maximum number of simultaneous connections (once again, the directives in the global stanza and the host stanza are merged).

The sshkey plugin will copy the public key from the local nagios user account to the .ssh/authorized_keys file in the home directory of the user nagios in the virtual machine web14 (creating the file and directory as needed).

Configuring MLN Defaults

Within the main configuration file, /etc/mln/mln.conf, you can set many MLN defaults. The installed version of this file contains extensive comments documenting the available entries. Some of the most useful and important settings are shown in Table 2.

Note that the -P, -T, and -F options to the mln command, which override default directory locations, appear before the desired subcommand (for example, mln -P /mln/projects start ldaptest).