Clustering is the most common technique to achieve High availability for any services by introducing redundancy in software, hardware and data. In a failure the clustering software immediately start the application on the standby system without requiring administrative intervention. Depending on the type of redundancy in software to be provided for High Availability, clusters can be configured in any of the following configurations:
- Cold Standby: The secondary node acts as backup of another identical primary system. It will be installed and configured only when the primary node breaks down for the first time. Subsequently, In the case of a failure in the primary the secondary node is powered on and the data restored before finally starting the failed component. Data from primary system can be backed up on a storage system and restored on secondary system as and when required. This generally provides a recovery time of a few hours.
- Warm Standby: The software component is installed and available on the secondary node. The secondary node is up and running. In the case of a failure on the primary node, these software components are started on the secondary node. This process is usually automated using a cluster manager.Data is regularly mirrored to secondary system using disk based replication or shared disk. This generally provides a recovery time of a few minutes.
- Hot Standby: Software components are installed and available on both primary and secondary nodes. The software components on the secondary system are up but will not process data or requests. Data is mirrored in near real time and both systems will have identical data. Data replication is typically done through the software’s capabilities. This generally provides a recovery time of a few seconds.
- Active-Active (Load Balanced): In this method both the primary and secondary systems are active and processing requests in parallel. Data replication happens through software capabilities and would be bi-directional. This generally provides a recovery time that is instantaneous.