What is the shard-per-core architecture in ScyllaDB?

Shard-per-core means each CPU core on a node runs an independent, single-threaded execution engine that owns a dedicated slice of RAM, a dedicated set of data (by token range), and its own network connections. Instead of one process sharing memory across threads with locks, ScyllaDB partitions everything up front so cores rarely need to coordinate.

Node with 8 cores
 -> 8 independent shards
 -> each shard owns: its own memtable, its own CPU, its own connections
 -> cross-shard work happens via explicit message passing, not shared memory locks

This avoids the lock contention and cache-line bouncing that plague traditional multi-threaded databases as core counts grow. A shard-aware driver takes this further by routing each request directly to the specific shard that owns the relevant data, skipping an extra network hop inside the node entirely.

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