Sharded In-Memory Store

Sharding distributes data across multiple backends to improve throughput and balance load. In the sharded in-memory store, each key is hashed and a modulus operation selects one of the configured backends to hold or serve the associated value. Each backend owns a deterministic subset of keys, which avoids hotspots and allows capacity to scale horizontally.

Enterprise feature

This feature is available exclusively in the Enterprise Edition of Stalwart and is not included in the Community Edition.

Configuration

The sharded in-memory backend is selected by choosing the Sharded variant on the InMemoryStore object (found in the WebUI under Settings › Storage › In-Memory Store). The variant exposes a single field:

• stores: list of two or more backend in-memory stores that make up the shard. Each entry selects one of the base in-memory variants (Redis or Redis Cluster).

IMPORTANT: the order and number of backends in stores must remain stable once set. Reordering or adding or removing entries changes the hash-to-store mapping and leaves previously written data unreachable. Plan capacity carefully before configuring a shard.