Clustering is where two servers share a quorum disk on the backend. The normal setup is the active-passive configuration, as shown in Figure 14, and it is commonly used for email or database servers:

Figure 14: Clustering.

In the preceding diagram, the request comes to the Virtual IP that sends the request to the active node that fulfils the request. In the background, the passive node is also connected to the shared disk, but has a heartbeat polling the active node. If the active node fails, the passive takes over.

The other mode of clustering is where both nodes are active in an active-active configuration. With this configuration, ...

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