ScyllaDB Shard-per-Core Architecture

Based on its underlying Seastar framework, ScyllaDB utilizes a highly asynchronous, shared-nothing design. In this design, each shard-per-core has dedicated resources and its own custom schedulers for CPU and I/O processing. Using the Seastar framework, ScyllaDB runs one application thread per core and relies on explicit message passing instead of shared memory between threads. This design helps to avoid slow, unscalable lock primitives and cache bounces.

Any sharing of resources across cores must be explicitly handled. For instance, when two requests belong to the same session and are processed by two different CPUs, and they both depend on the same session state, one CPU must explicitly forward the request to the other. Either CPU may handle any response. While Seastar provides features that reduce the need for cross-core communication, when communication is unavoidable, it offers high-performance, non-blocking communication primitives to ensure that performance is not degraded.