How Clustering Works

In a two-cluster node Active / Active setup, if any one of the nodes fail, then the another active node will take over the active resources of the failed instance. It is always preferred while creating two-node cluster using Windows 2000 Advanced Server and Microsoft Clustering Service, that each node be connected to a shared disk array using either fiber channel or SCSI cables.

The shared data in the cluster must be stored on shared disks, otherwise, when a failover occurs; the node which is taking over in the cluster pack cannot access it. As we are already aware, clustering does not help protect data or the shared disk array that it is stored on. So it is very important that you select a shared disk array that is very reliable and includes fault tolerance.

Both nodes of the cluster are also connected to each other via a private network. This private network is used for each node to keep track of the status of the other node. For example, if one of the node experiences a hardware failure, the other node will detect this and will automatically initiate a failover.

When clients initiate a connection, how will they know what to do when a failover occurs? This is the most intelligent part of Microsoft Cluster Services. When a user establishes a connection with SQL Server, it is through SQL Server’s own virtual name and virtual TCP/IP address. This name and address are shared by both of the servers in the cluster. In other words, both nodes can be defined as preferred owners of this virtual name and TCP/IP address.

Usually, a client will connect to the SQL Server cluster using the virtual name used by the cluster. And as far as a client is concerned, there is only one physical SQL Server, not two. Assuming that the X node of the SQL Server cluster is the node running SQL Server ‘A’ in an Active/Active cluster design, then the X node will respond to the client’s requests. But if the X node fails, and failover to the next node Y occurs, the cluster will still retain the same SQL Server virtual name and TCP/IP address ‘A’, although now a new physical server will be responding to client’s requests.

During the failover period, which can last up to several minutes, clients will be unable to access SQL Server, so there is a small amount of downtime when failover occurs. The exact amount of time depends on the number and sizes of the databases on SQL Server, and how active they are.

Chapter 5 – Clustering Terms

Cluster Nodes

A cluster node is a server within a cluster group. A cluster node can be Active or it can be Passive as per SQL Server Instance installation.


The heartbeat is a checkup mechanism arranged between two nodes using a private network set up to see whether a node is up and running. This occurs at regular intervals known as time slices. A failover is initiated, if heartbeat is not functioning, and another node in the cluster will take over the active resources.

Private Network

The Private Network is available among cluster nodes only. Every node will have a Private Network IP address, which can be ping from one node to another. This is to check the heartbeat between two nodes.

Public Network

The Public Network is available for external connections. Every node will have a Public Network IP address, which can be connected from any client within the network.


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