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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?
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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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Answer

During migration, a slot briefly exists in a special migrating or importing state on the source and destination nodes, and if a client requests a key that has already moved, the node responds with a redirect telling the client exactly which node now holds that key, allowing operations to continue correctly even while migration is actively in progress.
-- A client might receive an ASK or MOVED redirect
-- during an in progress slot migration
(error) ASK 3999 destinationnode:6379
Real-world example A properly built Redis Cluster client library automatically follows ASK and MOVED redirects during a live resharding operation, ensuring the application experiences no errors even while data is actively being redistributed in the background.

Common follow-ups: What is the difference between an ASK redirect and a MOVED redirect?;Do all Redis client libraries handle these redirects automatically?

Clustering;Connection Pooling & Client Libraries
All 7 Cluster Sharding & Hash Slots questions