We don't have planet clustering yet and it's December 21st, 2012! If the world is ending, stop reading this and save yourself. If not, carry on.


What is Clustering?


Clustering is a technology that automatically allows one physical server to take over the tasks of another failed physical server. The goal of clustering is to ensure that users running applications will have little or no downtime when a failure happens. What does this mean? Basically, if one server fails in a cluster, another one will automatically take over, keeping downtime to a minimum.


To be a bit more specific, clustering refers to a group of two or more nodes that work together and represent themselves as a single virtual server. This means that when a client connects to a cluster, the client observes only one SQL server, as opposed to many. When one of the nodes in the cluster fails, its duties are taken over by another server in that cluster. Theoretically, the user notices nothing when the failure occurs.

Clustering is not a complete backup system for your applications. It is only one part of the equation when it comes to ensuring minimum downtime. Clustering provides the ability to recover from failed server hardware and software. This is the main benefit of clustering.


Types of Clustering


  • Active/Active SQL Server cluster: An Active/Active SQL server cluster means that the server is running on both nodes of a two-way cluster. Each copy of the server acts on its own and users see two different servers. If one of the servers in the cluster fails, then the failed instance of the server will fail over to the remaining server. This means both instances of  the SQL server will be running on one physical server, instead of two.
  • Active/Passive SQL Server: An Active/Passive SQL server cluster refers to a SQL server cluster where only one instance of SQL server is running on one physical server in the cluster. The other physical server simply waits to takeover should the main node fail.



Generally speaking, the Active/Passive configuration will provide better performance.