Redis sentinel vs clustering

Question

I understand redis sentinel is a way of configuring HA (high availability) among multiple redis instances. As I see, there is one redis instance actively serving the client requ

Answer 1

Redis Sentinel performs the failover promoting replicas when they see a master is down. You typically want an odd number of sentinel nodes. For the example of one master and one replica, 3 sentinels should be used so there can be a consensus on the decision. Ideally the 3rd sentinel is on a 3rd server so the decision is not skewed (depending on failure). Sentinel takes care of changing the master/replica config settings on your nodes so that promotion and syncing occurs in the correct order and you don’t overwrite data by bringing on an old failed master that now contains older data.

Once you have your sentinel nodes set up to perform failovers, you need to ensure you are pointing to the correct instance. See an example of HAProxy configuration for this. HAProxy performs health checks and will point to the new master if a failure occurs.

Clustering will allow you to scale horizontally and can help handle high loads. It does take a bit of work to set up and configure up front.

There is an open source fork of Redis, “KeyDB” that has eliminated the need for sentinel nodes with an active-replica option. This allows the replica node to accept reads and writes. When a failover occurs HAProxy stops reads/writes with the failed node and just uses the remaining active node which is already sync’d. Timestamping enables the failed nodes to rejoin automatically and resync without losing data when they come back online. Setup is simple and for higher traffic you don’t need special upfront setup to direct reads to the replica node and read/writes to the master. See example of active replication here. KeyDB is also multi-threaded which for some applications might be an alternative to clustering, but really depends on what your needs are.

There is also an example of setting up clustering manually and with the create-cluster tool. These are the same steps if you are using Redis (replace 'keydb' with 'redis' in instruction)