CROSSSLOT error

A command used keys that belong to different hash slots.

A Redis cluster returns this error when one command uses keys that belong to different hash slots. A cluster splits keys into 16384 hash slots, and different nodes can serve different slots. A command that uses several keys, such as MSET or MGET, only works when all of its keys are in the same slot.

CROSSSLOT Keys in request don't hash to the same slot

In redis-py, this error is ClusterCrossSlotError: Keys in request don't hash to the same slot.

Example

The keys user:1 and user:2 are in different slots, so you can't read them with one MGET:

> CLUSTER KEYSLOT user:1
(integer) 10778
> CLUSTER KEYSLOT user:2
(integer) 6777
> MGET user:1 user:2
(error) CROSSSLOT Keys in request don't hash to the same slot

How to fix it

• Use hash tags to put related keys in the same slot. When a key name has a part in curly braces, Redis uses only that part to choose the slot. So user:{42}:profile and user:{42}:cart are always in the same slot:

  > CLUSTER KEYSLOT user:{42}:profile
  (integer) 8000
  > CLUSTER KEYSLOT user:{42}:cart
  (integer) 8000
  > MSET user:{42}:profile "..." user:{42}:cart "..."
  OK
  

  Put the braces around the part that the keys share, such as a user ID. Don't give too many keys the same tag. All keys with the same tag are stored on the same node. See Hash tags.

• Split the command. Send one command for each key, or one for each group of keys in the same slot. Some cluster clients can do this for you. For example, redis-py's RedisCluster has mget_nonatomic() and mset_nonatomic(). As the names say, the keys are then no longer read or written in one step.

Transactions and Lua scripts follow the same rule: all the keys they use must be in the same slot. See Multi-key operations.