#rpc #axum #protobuf #connect


Connect-Web RPC code generator for axum-connect

6 releases

0.1.5 May 8, 2023
0.1.4 May 2, 2023
0.1.2 Apr 29, 2023
0.1.1 Mar 3, 2023

#579 in HTTP server

30 downloads per month


193 lines

Axum Connect-Web

Brings the protobuf-based Connect-Web RPC framework to Rust via idiomatic Axum.

Alpha software ⚠️

Project is under active development for internal use; minor revision bumps are often breaking.

Features 🔍

  • Integrates into existing Axum HTTP applications seamlessly
  • Closely mirrors Axum's API
    • Extract State and other parts that impl RpcFromRequestParts just like with Axum.
    • Return any type that impl RpcIntoResponse just like Axum.
  • Generated types and service handlers are strongly typed and...
  • Handlers enforce semantically correct HTTP 'parts' access.
  • Allows users to derive RpcIntoResponse and RpcFromRequestParts just like Axum.
    • Note: These must be derivatives of Axum's types because they are more restrictive; you're not dealing with arbitrary HTTP any more, you're speaking connect-web RPC over HTTP.
  • Wrap connect-web error handling in idiomatic Axum/Rust.
  • Codegen from *.proto files in a separate crate.
  • All the other amazing benefits that come with Axum, like the community, documentation and performance!

Getting Started 🤓

Prior knowledge with Protobuf (both the IDL and it's use in RPC frameworks) and Axum are assumed.

Dependencies 👀

You'll need 2 axum-connect crates, one for code-gen and one for runtime use. Because of how prost works, you'll also need to add it to your own project. You'll obviously also need axum and tokio.

# Note: axum-connect-build will fetch `protoc` for you.
cargo add --build axum-connect-build
cargo add axum-connect prost axum
cargo add tokio --features full

Protobuf File 🥱

Start by creating the obligatory 'hello world' proto service definition.


syntax = "proto3";

package hello;

message HelloRequest { string name = 1; }

message HelloResponse { string message = 1; }

service HelloWorldService {
  rpc SayHello(HelloRequest) returns (HelloResponse) {}

Codegen 🤔

Use the axum_connect_codegen crate to generate Rust code from the proto IDL.

Currently all codegen is done by having the proto files locally on-disk, and using a build.rs file. Someday I hope to support more of Buf's idioms like Remote Code Gen.


use axum_connect_build::{axum_connect_codegen, AxumConnectGenSettings};

fn main() {
    // This helper will use `proto` as the import path, and globs all .proto
    // files in the `proto` directory. You can build an AxumConnectGenSettings
    // manually too, if you wish.
    let settings = AxumConnectGenSettings::from_directory_recursive("proto")
        .expect("failed to glob proto files");

The Fun Part 😁

With the boring stuff out of the way, let's implement our service using Axum!

use std::net::SocketAddr;

use axum::{extract::Host, Router};
use axum_connect::prelude::*;
use proto::hello::*;

mod proto {
    pub mod hello {
        include!(concat!(env!("OUT_DIR"), "/hello.rs"));

async fn main() {
    // Build our application with a route. Note the `rpc` method which was added
    // by `axum-connect`. It expect a service method handler, wrapped in it's
    // respective type. The handler (below) is just a normal Rust function. Just
    // like Axum, it also supports extractors!
    let app = Router::new().rpc(HelloWorldService::say_hello(say_hello_success));

    // Axum boilerplate to start the server.
    let addr = SocketAddr::from(([127, 0, 0, 1], 3030));
    println!("listening on http://{}", addr);

async fn say_hello_success(
  Host(host): Host,
  request: HelloRequest
) -> HelloResponse {
    HelloResponse {
        message: format!(
            "Hello {}! You're addressing the hostname: {}.",
            request.name, host


To test it out, try hitting the endpoint manually.

curl --location 'http://localhost:3030/hello.HelloWorldService/SayHello' \
--header 'Content-Type: application/json' \
--data '{ "name": "Alec" }'

From here you can stand up a connect-web TypeScript/Go project to call your API with end-to-end typed RPCs.

Roadmap / Stated Non-Goals 🛣️

  • Binary proto encoding based on HTTP content-type
  • Streaming server RPC responses
  • Bring everything in-line with connect-web
  • Version checking between generated and runtime code
  • A plan for forward-compatibility
  • Comprehensive tests
  • A first-stable launch

More Distant Goals 🌜

  • I would love to also support a WASM-ready client library
  • Use buf.build to support remote codegen and streamlined proto handling
  • Support gRPC calls
    • I don't think this is hard to do, I just have no personal use-case for it
  • Possibly maybe-someday support BiDi streaming over WebRTC
    • This would require connect-web picking up support for the same
    • WebRTC streams because they are DTLS/SRTP and are resilient
  • Replace Prost (with something custom and simpler)

Non-goals 🙅

  • To support every feature gRPC does
    • You get a lot of this already with Axum, but gRPC is a monster that I don't wish to reproduce. That complexity is useful for Google, and gets in the way for pretty much everyone else.
  • To do everything and integrate with everything
    • I plan on keeping axum-connect highly focused. Good at what it does and nothing else.
    • This is idiomatic Rust. Do one thing well, and leave the rest to other crates.

Prost and Protobuf 📖

Protoc Version

The installed version of protoc can be configured in the AxumConnectGenSettings if you need/wish to do so. Setting the value to None will disable the download entirely.


Prost stopped shipping protoc binaries (a decision I disagree with) so axum-connect-build internally uses protoc-fetcher download and resolve a copy of protoc. This is far more turnkey than forcing every build environment (often Heroku and/or Docker) to have a recent protoc binary pre-installed. This behavior can be disabled if you disagree, you need to comply with corporate policy, or your build environment is offline.

I would someday like to replace all of it with a new 'lean and mean' protoc library for the Rust community. One with a built-in parser, that supports only the latest proto3 syntax as well as the canonical JSON serialization format and explicitly doesn't support many of the rarely used features. But that day is not today.

Versioning 🔢

axum-connect and axum-connect-build versions are currently not kept in lockstep. They will be once I get to beta. Right now the versions mean nothing more than 'Alec pushed a new change'.

License 🧾

Axum-Connect is dual licensed (at your option)


~529K SLoC