CertiGuide to A+ (A+ 4 Real)  9  Chapter 16: Network Load Balancing and Clustering       9  Clustering

How Clustering Works 1

Once the cluster resources have been created, they are linked together in cluster groups. The group is a collection of resources that belong together. For example, if you wished to cluster an IIS website, a cluster resource group may contain the following resources:

• A disk resource, on which the IIS website HTML files are stored
  
  
• An IP address, through which the website is accessed
  
  
• A NetBIOS/DNS name, associated with the IP address
  
  
• An IIS virtual site instance

After creating the resources in the resource group, the next step is to configure the dependencies. A dependency controls the order in which the resources in a group are brought online. In the above example, the dependency order should be as follows:

• The IIS site is dependant on the IP address
  
  
• The NetBIOS name is dependant on the IP address
  
  
• The IP address is dependant on the disk resource

The aim of the dependency configuration is to ensure that no resources are brought online prematurely. For example if the disk resource failed to come online, it makes little sense to bring the IP address or virtual site online.

At this point, any further required configuration can be completed, such as the preferred node to own the resource and the action taken when a resource in the group fails. If the cluster has been set up successfully, it should now be possible to move the resource group between the cluster nodes as required, with no visible difference (apart from a very short loss of service) to any clients using it. Once in production, if one of the cluster nodes should fail, the other node will take over ownership of the resources for almost seamless service uptime.

Clustering and load balancing are extremely interesting and complex enterprise technologies, and this author encourages you to learn and play with them as much as possible! A 24 hour, 7 day per week flawless availability is vital for any successful web service offered around the globe.

How Clustering Works 1