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When I add a node in a redis cluster, it has 0 hash slots. Why redis cluster doesn't automatically does a resharding operation in order to make the new node fully functional?
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A shard (API/CLI: node group) is a collection of one to six Redis nodes. A Redis (cluster mode disabled) cluster will never have more than one shard. You can create a cluster with higher number of shards and lower number of replicas totaling up to 90 nodes per cluster.
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 the hash slot for a given key, we simply take the CRC16 of the key modulo 16384.
In Redis, data resides in one place in a cluster (Redis Cluster), and each node or shard has a portion of the keyspace. A cluster is divided up among 16,384 slots — the maximum number of nodes or shards in a Redis cluster.
As you can see here, redis supports now automatic partitioning.
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The process of adding a node consists of two steps:
CLUSTER MEET so that all the nodes start to communicate via cluster busCLUSTER ADDSLOTS or as a slave via CLUSTER REPLICATE
The separation helps to keep the commands simple.
Automatic resharding is part of the Redis 4.2 roadmap
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