ADR-0022: Runtime-pluggable storage backends

Status: Accepted (2026-08-03)

Context

Compile-time storage features duplicated the native engine across client artifacts and required every engine layer to enumerate every driver. External authors could not add a backend without joining Corium's feature graph.

Decision

Storage drivers live in separate rlib/cdylib crates and enter a process-wide registry keyed by backend kind. Engine code uses redacted JSON configuration and trait objects. Dynamic Rust plugins use the versioned corium-store-abi contract, abi_stable layout checks, and async-ffi futures. Plugins own their async runtimes and loaded libraries are never unloaded.

Memory and filesystem remain in the core crate. Statically linked driver crates remain supported. This decision supersedes ADR-0007 only where that ADR treated later drivers as additions to the core crate; its blob/root contract and initial backend choice remain in force.

Consequences

• A backend can be installed without rebuilding the engine.
• Python uses one native engine and discovers driver libraries through entry points.
• ABI and dependency-version skew become explicit load-time failures.
• A loaded plugin is trusted native code and may receive storage credentials.
• Dynamic drivers duplicate runtime and TLS process-global state by design.