RPC boundaries with id_effect (`@effect/rpc` parity)

Effect.ts @effect/rpc ties Schema, Layer, and HTTP so typed contracts cross process boundaries. In Rust there is no single blessed stack; this chapter documents patterns and the workspace id_effect_rpc crate for RPC-shaped HTTP on top of id_effect_axum and id_effect::schema.

Choosing a wire stack (tonic, tarpc, HTTP+JSON)

Approach	Strengths	Trade-offs
`tonic` + Protobuf	Mature gRPC ecosystem, streaming, codegen from `.proto`	Separate schema language; protobuf ↔ `Effect` error mapping is manual at generated boundaries
`tarpc`	Rust-native service traits, pluggable transports	Fewer batteries than tonic for cross-language clients; still need an explicit error wire type
HTTP + JSON + `Schema`	Same `Schema` as domain validation; easy to debug; fits Axum	No streaming parity on day one; discipline required for versioning and error envelopes

Recommendation for id_effect today: use HTTP + JSON for public APIs where you already host Axum, validate bodies with Schema::decode_unknown via id_effect_axum::json::decode_json_schema, and return structured failures with id_effect_rpc::RpcError. Add gRPC (tonic) when you need IDL-first codegen or cross-language streaming at scale.

Effect, `R`, and errors at the edge

Keep Effect<A, E, R> for domain logic; E stays rich inside the process.
At the Axum route, map E (and schema failures) into RpcError or 422 JSON from decode_json_schema — clients see a stable wire contract, not internal enums.
Propagate x-correlation-id (read or generate with id_effect_rpc::correlation) and attach it to responses so logs and traces line up across hops.

Tracing and OpenTelemetry

Use id_effect_rpc::span::rpc_request_span for a stable rpc.request span name and fields (http.method, http.route, rpc.operation, correlation.id). When your binary installs a subscriber with an OpenTelemetry layer (see Phase B docs), these fields map cleanly to semantic conventions without id_effect_rpc depending on OTEL crates.

Workspace crate: `id_effect_rpc`

Piece	Role
`RpcError` / `RpcEnvelope`	JSON body + HTTP status + `IntoResponse` for Axum
`correlation`	`x-correlation-id` read, generate, append
`span`	`tracing` helpers for RPC-shaped requests

API index: cargo doc -p id_effect_rpc --open.

Example: Axum + Schema + `RpcError`

Runnable example:

cargo run -p id_effect_rpc --example 010_json_greet

It accepts POST /greet with JSON {"name": string, "enthusiasm": i64}, validates with Schema, rejects bad enthusiasm with RpcError::invalid_argument, and echoes x-correlation-id.

Stage D3 — codegen (optional)

Proc-macros or build.rs stubs for service definitions are backlog until the D2 operational model is proven in production. See docs/effect-ts-parity/phases/phase-d-rpc.md slugs iep-d-030 / iep-d-031.

Typed Effects in Rust