Database / ValKey Interview questions
Explain the internal working of Valkey's hash slot mechanism in cluster mode?
Each of the 16384 slots is owned by exactly one primary at a time, and the current ownership map, along with a configuration epoch used to resolve conflicting claims, is gossiped continuously between nodes over the cluster bus.
flowchart LR
subgraph Migration
S1[Slot on Source Node] -->|marked MIGRATING| MOVE[Atomic key migration]
MOVE -->|marked IMPORTING| S2[Slot on Destination Node]
end
Gossip[Cluster Bus Gossip] --> S1
Gossip --> S2
Multi-key commands are only allowed when every key involved maps to the same slot, which application code can force using a hash tag like {user1000}.profile. When rebalancing moves a slot to a new node, Valkey historically moved its keys one at a time in a move-then-delete sequence; Valkey 9.0 introduced atomic slot migration, moving all of a slot's keys in a single atomic step instead, avoiding the partial-migration edge cases the older approach could hit.
Related keys can be forced onto the same hash slot for multi-key operations using...
Valkey 9.0's atomic slot migration primarily improves on the older approach by...
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