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High Availability Clusters |
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| High-availability clusters (also known as HA Clusters) are computer clusters that are implemented primarily for the purpose of improving the availability of services which the cluster provides. They operate by having redundant computers or nodes which are then used to provide service when system components fail. HA clusters are often used for - Key databases (Oracle, Sybase, DB2).
- File Servers (SAMBA, NFS)
- Directory Services(eDirectory, OpenLDAP).
- Web & Applications Servers ( Apache, Oracle , WebSphere,...)
- Mail Servers (Senmail, Qmail, Postfix ,...)
- Business Applications customer services such as electronic commerce websites.
HA cluster implementations attempt to build redundancy into a cluster to eliminate single points of failure, including multiple network connections and data storage which is multiply connected via Storage area networks. HA clusters usually use a heartbeat private network connection which is used to monitor the health and status of each node in the cluster. HA Cluster configurations |  | | The most common size for an HA cluster is two nodes, since that's the minimum required to provide redundancy, but many clusters consist of many more, sometimes dozens, of nodes. Such configurations can sometimes be categorized into one of the following models: | - Active/Active — traffic intended for the failed node is either passed onto an existing node or load balance across the remaining nodes. This is usually only possible when the nodes utilize a homogenous software configuration.
- Active/Passive — provides a fully redundant instance of each node, which is only brought online when its associated primary node fails. This configuration typically requires the most amount of extra hardware.
| | The term Logical host or Cluster logical host is used to describe the network identity which is used to provide services to those connecting to the servers. This logical host identity is not a physical system; it has a hostname and IP address and is visible on the network to customers, but the identity of that logical host is being run on one of the physical server nodes in the cluster at any given time. The active cluster nodes have both their own network identity and that of the logical host running; passive nodes have only their own network identity. | |
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