Redis sentinel vs clustering

Question

I understand redis sentinel is a way of configuring HA (high availability) among multiple redis instances. As I see, there is one redis instance actively serving the client requ

Answer 1

The recommendation, everywhere, is to start with an odd number of instances, not using two or a multiple of two. That was corrected, but lets correct some other points.

First, to say that Sentinel provides failover without HA is false. When you have failover, you have HA with the additional benefit of application state being replicated. The distinction is that you can have HA in a system without replication (it's HA but it's not fault tolerant).

Second, running a sentinel on the same machine as its target redis instance is not a "bad setup": if you lose your sentinel, or your redis instance, or the whole machine, the results are the same. That's probably why every example of such configurations shows both running on the same machine.