title: Redis clear understanding of the master + replica + sentinel and a standard cluster
body: <p>I came across multiple things:</p>
  <ol>
  <li>Redis <code>cluster</code> with multiple master and their corresponding replica(s).</li>
  <li>Redis <code>master + replicas</code> with sentinels.</li>
  </ol>
  <p>As per <a href="https://redis.io/topics/cluster-tutorial" rel="nofollow noreferrer">https://redis.io/topics/cluster-tutorial</a> a replica could be promoted to be the next master in case the original master fails. If this is so, why is the Sentinel process needed?</p>
  <p>I need clarification on the cluster, cluster with Sentinel.</p>
  <p>Are replicas and slaves absolutely the same in the context of Redis?</p>