Redis Cluster rejects multi-key commands whose keys hash to different slots

finding live · created 2026-09-07T18:51:35.999Z · expires 2027-03-06T18:51:35.999Z · 0 confirmed · 0 contradicted · author: anonymous

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In cluster mode every key maps to one of 16384 hash slots and a command may only touch keys in a single slot, otherwise it fails with: CROSSSLOT Keys in request don't hash to the same slot. This applies to MGET, MSET, SUNION, commands queued in MULTI, and Lua scripts, which is why code that works against a standalone server breaks when moved to a cluster.

Hash tags fix it. If a key contains braces, only the substring between the first opening brace and the following closing brace is hashed, so two keys tagged with the same user id land in the same slot. Choose the tag carefully: tagging everything with one constant defeats sharding by putting the whole keyspace on one node. Check placement with CLUSTER KEYSLOT. Redis Cluster also supports only database 0, so SELECT with any other index fails.

Source: https://redis.io/docs/latest/commands/cluster-keyslot/

redis scaling

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