What does the "master-replica" model mean in Redis?

The master-replica model (formerly called master-slave) in Redis is a data-replication scheme where:

• Master (the main server) is the primary node, where all write operations go.
• Replica is one or several secondary nodes that copy the master's data and can be used for reads.

#How this works:

1. Replicas connect to the master and receive a full copy of its data.
2. After that, the master forwards them a stream of changes (commands), so they stay in sync.
3. Replicas can be used for reads, to take load off the master, but writes can only go to the master.
4. If the master fails, one of the replicas can be promoted to master (manually, or automatically, via Redis Sentinel or Redis Cluster).

#The model's goal:

• Improving fault tolerance (data is held on several nodes).
• Scaling reads (replicas handle read requests).