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when I'm trying to read document about new system structure i have some problem to understand the servers requirements like the different between clustered, redundant, failover ?

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This is a bit vague, but for the three terms you gave:

Clustered: This means, at a basic level, that two or more servers are combined together to act as one in some way. This may involve a computing cluster, such as in "grid" computing, where all the cluster nodes are computing a portion of the workload in order to scale the power of the system. Alternatively (and more commonly), it may involve a failover cluster of nodes that are designed to take over for each other in the event that one (or more) fails. In that case, a particular service only runs on a single node at a time, but may be moved to another when needed.

Redundant: Redundant nodes are those in a failover cluster, where they are designed to take over operations for one another in the event of the failure of one of the nodes.

Failover: The term for when services on one node "fail over" to another node in the event of an emergency, or when manually moved.

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In simplistic terms, this is what those terms mean to me.

  • Clustered : a group of computers provide a service in a way that looks to the client like a single computer.

  • Redundant : using two computers when one would be sufficient. The redundant server is there in case the other computer fails.

  • Failover : the automatic transfer of workload from a failed computer to another computer.

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