Clustering
Load balancing does not require collaboration between the separate servers. When the servers are aware of each other and actively participate in distributing load, then they form a cluster. Clusters can be used for load balancing, in the case of active/active clusters. They can also be used for redundancy in the case of failure. These are called active/passive clusters, meaning that one server handles all the load until it fails and then the passive one takes over and becomes active.
Fully load-balanced farms scale close to linearly. Load-balanced clusters do not. Clusters incur some communication overhead in heartbeats and state synchronization. As a result of this overhead, the capacity of a cluster scales less than linearly and ...
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