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Clustering with OpenAIS and Corosync
Heartbeat
When you talk about clusters, you need to distinguish between the various types. Storage clusters support access to a single filesystem from multiple systems. This approach removes the need to synchronize the data between individual computers because they all share the same storage.
High-availability (HA) clusters bundle individual resources, such as filesystems and IP addresses, to provide a cluster service. If one resource fails, the cluster attempts to reactivate it. If the machine hosting the cluster service fails completely, the service is restarted on another computer, which ensures that the service (e.g., a web server) is available at all times.
Load distribution clusters hide multiple systems behind a single IP address and distribute the incoming requests to the back-end systems on the basis of a specific algorithm.
If one of these systems fails, obviously, no requests will be forwarded to it. Clusters of this kind are particularly popular with heavily frequented web servers for which a single server would be unable to handle the high volume of requests on its own.
Finally, high-performance clusters distribute complex calculations over the cluster nodes, thus boosting the computational capacity. They are often used in research and industry, for example, for performing crash test simulations in the automotive industry. In this article, I'll look primarily at high-availability and storage clusters.
Cluster Components
A cluster always comprises multiple components. The heart is the cluster manager – the communications system of the cluster that decides which systems belong to the cluster and which need to be removed from the cluster. Quorum rules form the basis for the decision. If a machine is not performing well, or at all, the Cluster Manager accesses another subsystem known as the Fencing System to remove the faulty
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