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Architecture of the Linux Enterprise Cluster

I've introduced a definition of the term cluster that I will use throughout this article as a logical model for understanding how to build one. The definition I've provided in this introduction (the four basic properties of a Linux Enterprise Cluster) is the foundation upon which the ideas in this article are built. After I've introduced the methods for implementing these ideas. If you don't have any experience with Linux you will probably want to start reading about the basic components of the GNU/Linux operating system that make building a Linux Enterprise Cluster possible.

The Load Balancer

The load balancer sits between the users of the cluster and the "whole computers," which are the nodes that make up the cluster. The load balancer decides how best to distribute the incoming workload across all of the nodes.

In an enterprise cluster, user-based transactions make up the incoming workload. As users make connections to the cluster, such as HTTP web requests or telnet connections, they are assigned to cluster nodes based upon a load-balancing scheme.

The Shared Storage Device

The shared storage device acts as the single repository for the enterprise cluster's data, just as a disk drive does inside a single computer. Like a single disk drive in a computer, one of the most important features of this storage device is its ability to arbitrate access to the data so that two programs cannot modify the same piece of data at the same time. This feature is especially important in an enterprise cluster because two or more programs may be running on different cluster nodes at the same time. An enterprise cluster must therefore use a filesystem on all cluster nodes that can perform lock arbitration. That is, the filesystem must be able to protect the data stored on the shared storage device in order to prevent two applications from thinking they each have exclusive access to the same data at the same time.

The Print Server

The output device in this example is just one of many possible cluster output devices—a shared print server. A shared print server and other servers that offer services to all cluster nodes are located outside the cluster to arbitrate contention for things like printers, fax lines, and so on. Servers that arbitrate contention for output devices do not know they are servicing cluster nodes and can continue to use the classic client-server model for network communication. As we'll see in the next section, all of the server functions required to service the cluster nodes can be consolidated on to a single highly available server pair.
 

Linux Cluster running Ubuntu Server
 

The Cluster Node Manager

Servers outside the cluster may do a variety of other things as well, such as:

  • Act as license servers for applications running inside the cluster.

  • Act as the central user account database.

  • Provide other services such as DNS and email to all cluster nodes.

  • Monitor the health of the cluster.

Again, the applications running on these servers outside the cluster should not need to be modified to support the nodes inside the cluster. In other words, the applications offering services to clients that are cluster nodes need not be cluster aware.

These functions can be combined on a high-availability server pair that we will call the cluster node manager. The cluster node manager holds the master user account database (and uses NIS or LDAP to distribute user accounts to all nodes). The cluster node manager monitors the health of the cluster; it is also a central print spooler (using LPRng). The cluster node manager can also offer a variety of other services to the cluster using the classic client-server model, such as the ability to send faxes.

The Webmin and OSCAR packages both have methods of distributing user accounts to all cluster nodes that do not rely on NIS or LDAP, but these methods can still use the cluster node manager as the central user account database.

Posted on: 24/07/2009





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