All 7 Cluster Sharding & Hash Slots questions
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What is sharding in a Redis Cluster, and why is it necessary for very large datasets? What are hash slots in Redis Cluster, and how many are there in total? What is a hash tag in Redis Cluster, and why would you use one to control which slot certain keys are assigned to? How do you check the current distribution of hash slots and keys across the nodes in a Redis Cluster to identify potential imbalances? How would you plan and execute resharding a Redis Cluster to add a new node and redistribute existing hash slots? What happens to client requests during a hash slot migration, and how does Redis Cluster handle keys that are in the middle of being moved?
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How does Redis Cluster determine which hash slot a specific key belongs to?

Intermediate

Answer

Redis Cluster applies a hashing algorithm called CRC16 to the key, then takes the result modulo sixteen thousand three hundred eighty four to determine the specific hash slot, ensuring keys are distributed pseudo randomly but consistently across the entire cluster's available slots.
-- The hash slot calculation happens automatically
-- CRC16(key) mod 16384 determines the slot
redis-cli cluster keyslot session:abc123
Real-world example A developer troubleshooting uneven load across cluster nodes checks the hash slot calculation for their most frequently accessed keys, discovering a naming pattern that was accidentally concentrating too many keys onto a single node.

Common follow-ups: What is a hash tag, and how does it let you force related keys into the same slot?;Can you manually move a hash slot from one node to another?

Clustering;Redis Performance Tuning & Benchmarking
All 7 Cluster Sharding & Hash Slots questions