Sharded Blob Store

Sharding distributes data across multiple backends to improve scalability and balance load. In the sharded blob store, each blob’s key is hashed and a modulus operation selects one of the configured backends to hold or retrieve it. Each backend therefore owns a deterministic portion of the blob namespace, and reads and writes for a given blob always resolve to the same backend.

Enterprise feature

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

Configuration

The sharded blob backend is selected by choosing the Sharded variant on the BlobStore object (found in the WebUI under Settings › Storage › Blob Store). The variant exposes a single field:

• stores: list of two or more backend blob stores that make up the shard. Each entry selects one of the base blob-store variants (S3, Azure, filesystem, FoundationDB, PostgreSQL, or MySQL).

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 stored blobs unreachable. Plan capacity carefully before configuring a shard.