Question: What is the difference between a Redis cluster and a replication group?

Answer

Redis Cluster and a Replication Group are two different ways to achieve high availability and partition tolerance in Redis, but they do so in slightly different ways.

Redis Cluster:

Redis Cluster is a distributed version of Redis which includes automatic partitioning across multiple Redis nodes. It's a full mesh where every node is connected to every other. Data is automatically split among the available nodes, with each node holding a subset of the total data.

It has built-in replication and provides a level of availability during network partitions. This means that if one part of the network becomes unavailable, the system continues to operate.

Example of creating a Redis cluster:

redis-cli --cluster create 127.0.0.1:7000 127.0.0.1:7001 \
 127.0.0.1:7002 127.0.0.1:7003 127.0.0.1:7004 127.0.0.1:7005 \
 --cluster-replicas 1

This command creates a new cluster with six nodes, each one running on a different port, and configures one replica for each master.

Replication Group:

On the other hand, a Redis replication group is a set-up where some Redis servers (the slaves) are exact copies of another Redis server (the master). This results in high read scalability (you can scale your reads by adding more slaves) and data redundancy.

If the master fails, one of the slaves can be promoted to master and continue serving requests. Note that write operations can only be performed on the master, and then the changes are propagated to the slaves.

Example of setting up a replication group:

redis-cli -h master_host -p master_port SLAVEOF slave_host slave_port

This command tells the Redis server running on slave_host:slave_port to become a replica of the server at master_host:master_port.

The choice between using Redis Cluster or a replication group will depend on your specific requirements. If you need automatic sharding and higher availability, then Redis Cluster might be your best option. On the other hand, if you're looking for read scalability, a replication group could be more appropriate.