What is the difference between Redis Sentinel and Redis Cluster?

Redis Sentinel and Cluster are two different high availability solutions provided by Redis.

1. Redis Sentinel is a system used to monitor and manage Redis master-slave replication. It consists of one or more sentinel nodes that regularly check the status of the Redis master and slave nodes. In case of a master node failure, it will automatically elect a slave node as the new master and notify other slave nodes to switch to the new master for replication. Sentinel can also monitor the health of master and slave nodes, and perform failover and automatic recovery in case of node failures. Sentinel mode is suitable for simple master-slave replication setups, but does not support data sharding or horizontal scaling.
2. Redis Cluster is a distributed data storage solution designed to overcome the limitations of storage capacity and performance of single-node Redis. Data is spread across multiple nodes in a Redis Cluster, which offers automatic data sharding and replication mechanisms. In cluster mode, each node can store a portion of the data and can act as either a master or a slave node. Data is distributed through hash sharding in the cluster, and communication and data synchronization between nodes are achieved using the Gossip protocol. The cluster provides increased capacity and performance, as well as the ability to dynamically scale up or down nodes.

In summary, Redis Sentinel is suitable for simple master-slave replication scenarios with few nodes and data loads, while Redis Cluster is designed for large-scale data storage and high concurrency access, allowing for data sharding and horizontal scaling.