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What are hash slots in Redis Cluster, and how many are there in total? 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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What is sharding in a Redis Cluster, and why is it necessary for very large datasets?

Beginner

Answer

Sharding splits your data across multiple Redis nodes instead of storing everything on a single server, letting you scale beyond the memory and processing limits of one machine by distributing both the data and the workload of serving requests across several nodes working together.
-- A Redis Cluster automatically distributes keys
-- across multiple nodes based on hash slots
redis-cli -c -h node1 set user:123 'John'
Real-world example A large social media platform shards its user session data across many Redis nodes, since a single server could never hold enough memory to store session data for millions of active users at once.

Common follow-ups: How does Redis Cluster decide which node a specific key belongs to?;What happens if you need to add more nodes to a cluster later?

Clustering;Redis Replication
All 7 Cluster Sharding & Hash Slots questions