Experimenting with Protobufs generated types in Rust

This past week I was thinking about what it would take to design and build a home heating monitoring and control system. A "normal" mid-week thought. I was imagining something like a small computer in each room that would have a thermometer on radiator and thermometer measuring the ambient air temperature. These smaller computers would send messages into a "central" computer that would record things and turn radiators on and off. These computers would be on a separate physical network from other devices in the house and would not be internet connected (because having your house not work when the internet is down sounds super frustrating).

These smaller computers will need to encode the information in some format and send it to the central computer. Being a web developer I reached for familiar tools and decided to make the systems communicate via HTTP, but since I wanted to try something new: instead of having the body of the HTTP messages be a plain text format I decided to try using Protobufs.

What is a Protobuf

Protobufs are short for "Protocol buffers" and are a Google created mechanism for serializing structured data into a binary format. They work be having defined message types in .proto files. These files can then be used by various programming languages to generate a language appropriate binding that can be used to encode and decode messages.

The documentation on why you might want to use Protobufs does a good job of explaining their advantages. Using Protobufs have some trade-offs that likely make them not the most appropriate format for a hobby home automation project (such as not being human readable), but here we are.

Generating Rust structs from a Protobuf

I selected the Prost library somewhat arbitrarily since, in retrospect, I didn't look into other options.

I started off by installing Prost by adding this to my Cargo.toml:

[dependencies]
prost = "0.7"

[build-dependencies]
prost-build = "0.7"

Then creating a minimal .proto file called src/messages.proto:

syntax = "proto3";

package messages;

message ThermostatState {
    string name = 1;
    double air_temp = 2;
    double rad_temp = 3;
}

After that followed the instructions for using prost-build to generate a struct from the Protobuf definition. That included adding the following to the Cargo.toml:

build = "src/build.rs"

Creating a src/build.rs file that looks like this:

extern crate prost_build;

fn main() {
    prost_build::compile_protos(&["src/messages.proto"], &["src/"]).unwrap()
}

This file compiles are of the Protobufs in the first argument and outputs the generated code to the second argument. That means that it will output a Rust file that looks like this:

#[derive(Clone, PartialEq, ::prost::Message)]
pub struct ThermostatState {
    #[prost(string, tag="1")]
    pub name: std::string::String,
    #[prost(double, tag="2")]
    pub air_temp: f64,
    #[prost(double, tag="3")]
    pub rad_temp: f64,
}

The project's compile step generates the new rust file but we need to import it to be useful. We do this by include! the file into the src/lib.rs file.

pub mod messages {
    // The name "messages" corresponds with the `package` name in the `.proto`
    include!(concat!(env!("OUT_DIR"), "/messages.rs"));
}

The struct can then be constructed an used as a return type normally:

pub fn create_thermostat_state(name: String) -> messages::ThermostatState {
    let mut state = messages::ThermostatState::default();
    state.name = name;
    state
}

Encoding and decoding

Now that we are able to generate Rust code from the Protobufs we next want to send them from one system to another.

Sending

To send a HTTP POST I reached for the reqwest package (again didn't do a ton of research, just picked a dependency that looked good enough). I was then able to construct a Protobuf, encode it, then send it as a request body:

use reqwest;
use tokio;

// This trait needs to be included in order to call `.encode`
use prost::Message;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let msg = home_auto::create_thermostat_state("foo".to_string());
    // This being fixed length could be improved
    let mut buf: Vec<u8> = Vec::with_capacity(200);

    msg.encode(&mut buf).unwrap();

    let client = reqwest::Client::new();
    let _resp = client
        .post("http://localhost:8080")
        .body(buf)
        .send()
        .await?;
    println!("sent");
    Ok(())
}

The documentation for encode can be found here..

Receiving

The next step was getting something serving HTTP requests running on localhost:8080. To do this I also picked a dependency somewhat arbitrarily and used warp.

use bytes;
use tokio;
use warp::Filter;

// This trait is required to call `ThermostatState::decode`
use prost::Message;

#[tokio::main]
async fn main() {
    let route = warp::body::content_length_limit(1024 * 32)
        .and(warp::body::bytes())
        .map(|bytes: bytes::Bytes| {
            println!("bytes = {:?}", bytes);
            let msg = home_auto::messages::ThermostatState::decode(bytes).unwrap();
            println!("msg = {:?}", msg);
            "ok"
        });

    warp::serve(route).run(([127, 0, 0, 1], 8080)).await
}

This will bind to port 8080 and start decoding all request bodies as if they are valid ThermostatState Protobufs (which seems like a dangerous assumption if this was meant to be long living code).

Results

Each time the first program is run the second program will print:

bytes = b"\n\x03foo"
msg = ThermostatState { name: "foo", air_temp: 0.0, rad_temp: 0.0 }

The first bit is the binary format sent over the wire and the second is the std::fmt::Debug of the generated struct.

Which is pretty neat!

Conclusions

This was a fun thing to try out and I feel like I learned a few things from the process:

1. You could generate Rust structs from preexisting Protobufs allowing a typed boundary between languages.
2. Tooling to this approach (as is) is imperfect since the generated file only exists at compile time.
3. Async/Await in rust is wonderful and I am grateful of people's hard work to make it happen.

I could picture reaching for Protobufs again in the future.