olai-codegen

Proto-driven code generator: turns a compiled protobuf descriptor into idiomatic Rust server and client code, resource registries, and optional Python/Node.js/TypeScript bindings. The single source of truth is your annotated .proto files.

It's the engine behind olai-trestle's trestle generate; this crate is the library you embed if you need codegen in your own build tool. For how it fits the wider Trestle pipeline, see the architecture overview.

What it generates

proto/  ──────────────── trestle generate ────────────────▶  Rust / Python / TS
         (buf generate models · buf build descriptor · codegen)

Proto annotations

The generator reads standard Google API extensions — no custom annotations:

message Catalog {
  option (google.api.resource) = {
    type: "example.io/Catalog"
    pattern: "catalogs/{catalog}"
    plural: "catalogs"
    singular: "catalog"
  };
  string name = 1 [(google.api.field_behavior) = IDENTIFIER];   // maps to the store's Object.id
  string comment = 2 [(google.api.field_behavior) = OPTIONAL];
}
service CatalogService {
  rpc GetCatalog(GetCatalogRequest) returns (Catalog) {
    option (google.api.http) = { get: "/catalogs/{name}" };
  }
}

Routing & hierarchy model

Unlike Google AIP's nested collections (/catalogs/{c}/schemas/{s}/tables), Trestle follows the Databricks Unity Catalog design: every resource gets a flat, top-level route (/catalogs, /schemas, /tables) and the parent is named by a request field, not a URL segment. This keeps resources directly addressable and avoids the N+1 navigation of resolving every ancestor just to build a URL.

The logical hierarchy (Catalog → Schema → Table) is still encoded, but it is discovered from annotations rather than the URL: each child's List (and Create) request carries a parent-scoping field annotated google.api.resource_reference = { child_type: "<this service's resource>" }. The generator collects those edges across services and reconstructs the depth-ordered chain, driving parent_label/path_names in the resource registry and resource-scoped client navigation. See docs/codegen-design.md for the binding-mode details.

Usage

Codegen is driven by the trestle CLI. trestle generate runs the whole pipeline — it invokes buf generate (model plugins), buf build (the proto descriptor), then trestle's own codegen — so you only need one command:

cargo install olai-trestle              # installs the `trestle` binary
trestle generate --config trestle.yaml  # buf generate + buf build + codegen

This requires buf on your PATH. For environments where buf isn't available (e.g. a central/Bazel build produces the descriptor upstream), pass a pre-built FileDescriptorSet and the buf steps are skipped:

trestle generate --config trestle.yaml --descriptors path/to/api.bin

The config is trestle.yaml, a nested schema (proto library × servers × clients). See trestle config --help to author it, or the TrestleConfig documentation in the olai-trestle crate for the full field reference — this README intentionally does not duplicate the schema. trestle new databricks-app-rust scaffolds a project already wired in the recommended four-crate layout (common / server / client + proto) with a ready trestle.yaml, so you rarely author it by hand.

What you write vs. generate

You hand-write (once, then stable): RequestContext (impl FromRequestParts), your Error/Result types, and parse_error_response. Everything under a gen//_gen/ directory is overwritten each run — don't edit it. The generator emits one Axum handler fn per RPC but does not build the Router: you wire routes by hand, since only your app knows the URL prefixes and which routes mix in hand-written handlers.

Library API

If you embed generation directly, the pipeline is parse → configure → generate:

use olai_codegen::{parse_file_descriptor_set, generate_code, CodeGenConfig, CodeGenOutput};

let fds = /* protobuf::descriptor::FileDescriptorSet parsed from api.bin */;
let metadata = parse_file_descriptor_set(&fds)?;
let config = CodeGenConfig {
    context_type_path: "crate::RequestContext".into(),
    result_type_path: "crate::Result".into(),
    output: CodeGenOutput { common: Some("src/gen/common".into()), ..Default::default() },
    ..Default::default()
};
generate_code(&metadata, &config)?;

See the rustdoc for the full CodeGenConfig / CodeGenOutput field reference. A client-only crate sets common to None to skip the Axum extractors; the server output needs them.

WASM / browser clients

Generated clients are decoupled from their HTTP transport via transport_type_path (default [olai_http::CloudClient]). Set it to olai-http-wasm's WasmClient (or transport: wasm in trestle.yaml) to emit a wasm32-buildable browser client; add output.wasm + runtime: buffa to also emit #[wasm_bindgen] JS bindings and a client.d.ts. See the olai-http-wasm README for details.

REST client options

CodeGenConfig::rest_client (clients.rust in trestle.yaml) shapes the REST client. Every option is off by default.

Path values are percent-encoded (/ too, except in multi-segment patterns such as {name=catalogs/*}) and left out of the request body, as google.api.http specifies. With json_field_names: proto, an explicit json_name is kept, and json_field_names_by_package overrides the style per proto package.

Python API

BindingsConfig::python (clients.python in trestle.yaml) shapes the PyO3 API. The bindings crate and the models crate depend on pyo3 0.30 with the experimental-inspect feature, the bindings crate also on tokio, and the Rust client needs the error_module above: the Python exception classes raise its errors.

The generated API:

• Model classes are frozen, eq, with a repr() of Name(field=value, …) without the None fields. Only the models that a request takes get a keyword constructor.
• Enums are enum.StrEnum classes (Python 3.11+) in <package>/_enums.py; parameters take a member or a name. The zero *_UNSPECIFIED value is not a member; a field that holds it reads as None.
• The extension module is a declarative #[pymodule], codegen::extension_module, with the proto comments as docstrings. PyO3's introspection writes the .pyi from the built library: maturin generate-stubs (maturin 1.16+), or the pyo3-introspection crate. The module also has __version__, the version of the bindings crate. Its init puts the submodules in sys.modules; for a module made with wrap_pymodule!, call codegen::add_submodules_to_sys_modules.
• codegen/error.rs holds the PyO3 exception classes: a base class (error_base_class; bindings.error_base_class in trestle.yaml) with message, code, status and details, and one subclass per kind of HTTP error status.
• The methods block on the Tokio runtime of codegen/runtime.rs (rt(), made on first use; it panics in a forked child). With async_methods, the module installs it in pyo3-async-runtimes.

In trestle.yaml, package and module sit under clients.python and the rest under clients.python.options. tests/rest_client_e2e runs a client and Python bindings with all of these against a server.

Examples

The proto/ directory has fully-annotated example definitions and a compiled descriptor; integration tests in tests/ run the full pipeline against them and serve as executable documentation.

License

Apache-2.0