ADR 0001: WebAssembly Component Model and WIT Contract

• Status: Accepted
• Date: 2026-08-09

Context

Maskura needs a contract between the gateway host and tenant-supplied filter plugins. The options considered:

1. Core Wasm C ABI (pointer + length): Host and guest exchange raw linear memory pointers. Fragile, no type safety, manual memory management on the guest side.
2. WASI command model (wasm32-wasip1): Treats each invocation as a separate process with stdin/stdout. Can’t maintain state across records within a single object stream.
3. WebAssembly Component Model with WIT: Typed interface definitions, stateful sessions, automatic memory management via list<u8>.

Decision

Use the Component Model and a versioned WIT world (package s4:filter@0.1.0). The guest exports begin, transform, and finish functions with typed parameters. Stateful per-object sessions allow filters to accumulate context across records.

Consequences

• Host side uses wasmtime’s component model bindgen for typed function calls.
• Guest side uses wit-bindgen for idiomatic Rust trait implementations.
• Shared WIT file is the single source of truth for the data-plane contract.
• Fresh Store per object provides strong isolation between requests.