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What is sharding in a Redis Cluster, and why is it necessary for very large datasets? How does Redis Cluster determine which hash slot a specific key belongs to? 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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Redis › Cluster Sharding & Hash Slots

What are hash slots in Redis Cluster, and how many are there in total?

Beginner

Answer

Redis Cluster divides its entire key space into sixteen thousand three hundred and eighty four hash slots, and every key is assigned to exactly one of these slots based on a hash calculation, with each node in the cluster responsible for owning and serving a specific range of these slots.
-- Check which hash slot a key belongs to
redis-cli cluster keyslot user:123
Real-world example A three node Redis Cluster might divide its sixteen thousand three hundred eighty four hash slots roughly evenly, with each node responsible for owning about five thousand four hundred slots and the keys that map to them.

Common follow-ups: How do you check which specific node currently owns a particular hash slot?;What happens to hash slots when a node is added or removed from the cluster?

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All 7 Cluster Sharding & Hash Slots questions