Suggest an editImprove this articleRefine the answer for “What does the Sentinel system consist of?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)The Redis Sentinel system consists of three parts: the Sentinels themselves (watchers that monitor the master's and replicas' state and jointly decide on failures), the Redis master and replicas (the servers being watched), and clients, which connect not directly to the master but through Sentinel. **Key point:** Sentinel handles monitoring (`PING`), voting (a quorum) to confirm a failure, failover (electing a new replica), and notifying clients via Pub/Sub (`+switch-master`, `+failover-end`, etc.) - forming a self-healing loop.Shown above the full answer for quick recall.Answer (EN)ImageThe **Redis Sentinel** system consists of three main components that work together, providing monitoring, failover, and configuration management for Redis. ### 1. Sentinels (the watchers) These are separate processes (Sentinel nodes) that: - watch the state of the master and all its replicas, - exchange information with each other, - jointly decide when there's been a failure, - trigger a failover when needed. **Main role:** to be the "watchers" over the Redis instances. > A minimum of 3 Sentinels is needed for fault tolerance (a quorum). ### 2. The Redis master and replicas These are the **Redis servers** themselves, which Sentinel monitors. - The master is the primary source of data. - Replicas are copies that receive data from the master. Sentinel tracks their availability and the connections between them. **Main role:** to hold the data Sentinel is watching over. ### 3. Clients (applications using Redis) Sentinel doesn't just manage Redis, it also serves as **the entry point for clients**: - clients connect not directly to the Redis master, but **through Sentinel**, - Sentinel tells them who the current master is, - during a failover, clients are automatically redirected to the new master. **Main role:** to get up-to-date information about the master. ### Sentinel's additional internal mechanisms - **Monitoring:** regular `PING` requests to every Redis node. - **Voting (a quorum):** several Sentinels agreeing on a failure. - **Failover:** electing a new replica and reassigning roles. - **Configuration:** forwarding notifications to clients (`+switch-master`, `+failover-end`, etc.) over Pub/Sub. ### Summary The Sentinel system = **(several Sentinel processes)** + **a set of Redis servers (master + replicas)** + **clients that know how to listen to Sentinels**. It forms a self-healing loop: Sentinel watches → detects a failure → picks a new role → notifies the clients.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.