This technology aims to increase the availability of applications which are critical and compatible with this one. The principle is to immediately have a second server in case of failure of the first one. There is an automatic switching over of the application to a redundant or standy server (Failover) through a process of mutual verification of proper operation if any component of the software or hardware supported and supervised by the clustering software were to fail. This mechanism allows to reduce or to make completely invisible to users a major failure of one of the systems. The level of availability is thus significantly increase. Once the component is replaced and the server is back in production, the restoration of a failed system component's share of a load to a replacement component after a failback event (failback) can be executed at administrator's request. We distinguish clustering solutions in "active-passive" or "active-active" mode. In the first case, one of the servers (or node) is standby and ready to support the application executed on the active node. In the other mode, both nodes or n -nodes are simultaneously active and share the application workloads. The level of availability is highter than an active-passive solution, however the cost and complexity of such a solution are much higher. It is also possible to implement extended clustering solution (stretched cluster). In this case, the cluster nodes are on geographically remote sites. Such a complex and onerous solution is for applications requiring very high availability of service.
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