An Algolia cluster is a set of three servers. All servers are equal: each is equally capable of responding to every request. For this to be possible, each server must have the same data, index settings, and overall system configuration, enabling the cluster to behave as a single server.
The benefit of clusters is redundancy: the service is still available if one or two servers go down. This redundancy guarantees an SLA of 99.99% availability.
You can read more about clusters in our docs.