A server “cluster” is a group of two or more physically separate servers that are running Microsoft Cluster Services and working collectively as a single system. This server cluster, in turn, provides high availability, scalability, and manageability for resources and applications. In other words, this group of servers is physically connected via communication hardware (network), shares storage (via SCSI or fibreChannel), and uses cluster services software to tie them all together into one managed resource.
Server clusters can preserve client access to applications and resources during failures and planned outages. If one of the servers in the cluster is unavailable due to failure or maintenance, resources and applications move to another available cluster node.
Clusters use an algorithm to detect a failure, and use failover policies to determine how to handle the work from a failed server. These policies also specify how a server is to be restored to the cluster when it becomes available again.
While clustering doesn't guarantee “continuous” operation, it does provide availability sufficient for most mission-critical applications and is the building blocks of numerous high availability solutions. Cluster services can monitor applications and resources, automatically recognizing and recovering from many failure conditions. This provides greater flexibility in managing the workload within a cluster, and improves overall availability of the system. If you are using a mechanism that is “cluster aware” such as SQL Server, Microsoft Message Queue (MSMQ), or file shares, they have already been programmed to work with MSCS. If you are writing a new application, you will have to factor in the needed “cluster aware” code to take advantage of this capability. There is a “Server Cluster API” that can be harnessed for applications or tools that need to directly interface with MSCS. An application is considered cluster-aware if it uses these server cluster APIs. For more information on the Windows Server clustering APIs, follow this URL link to MSDN at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/mscs.
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