Every Redis Cluster node requires two TCP connections open. The normal Redis TCP port used to serve clients, for example 6379, plus the port obtained by adding 10000 to the data port, so 16379 in the example.
Redis Cluster data sharding
Redis Cluster does not use consistent hashing, but a different form of sharding where every key is conceptually part of what we call a hash slot.
There are 16384 hash slots in Redis Cluster, and to compute what is the hash slot of a given key, we simply take the CRC16 of the key modulo 16384.
Every node in a Redis Cluster is responsible for a subset of the hash slots, so for example you may have a cluster with 3 nodes, where:
- Node A contains hash slots from 0 to 5500.
- Node B contains hash slots from 5501 to 11000.
- Node C contains hash slots from 11001 to 16383.
Because moving hash slots from a node to another does not require to stop operations, adding and removing nodes, or changing the percentage of hash slots hold by nodes, does not require any downtime.
In our example cluster with nodes A, B, C, if node B fails the cluster is not able to continue, since we no longer have a way to serve hash slots in the range 5501-11000.
However when the cluster is created (or at a later time) we add a slave node to every master, so that the final cluster is composed of A, B, C that are master nodes, and A1, B1, C1 that are slave nodes. This way, the system is able to continue if node B fails.
Node B1 replicates B, and B fails, the cluster will promote node B1 as the new master and will continue to operate correctly.
However, note that if nodes B and B1 fail at the same time, Redis Cluster is not able to continue to operate.
Redis Cluster is not able to guarantee strong consistency.