Basics tutorial

This tutorial provides a basic Rust programmer’s introduction to working with gRPC.

By walking through this example you’ll learn how to:

• Define a service in a .proto file.
• Generate client code using the protocol buffer compiler.
• Use the Rust gRPC API to write a simple client for your service.

It assumes that you have read the Introduction to gRPC and are familiar with protocol buffers. Note that the example in this tutorial uses the proto3 version of the protocol buffers language: you can find out more in the proto3 language guide and the Rust generated code guide. Note that protobuf-Rust is also still in a preview state and their APIs may be unstable.

NOTE: server-side support is not yet available in this version of grpc-Rust. This guide will be updated when it is.

Why use gRPC?

Our example is a simple route mapping application that lets clients get information about features on their route, create a summary of their route, and exchange route information such as traffic updates with the server and other clients.

With gRPC we can define our service once in a .proto file and generate clients and servers in any of gRPC’s supported languages, which in turn can be run in environments ranging from servers inside a large data center to your own tablet — all the complexity of communication between different languages and environments is handled for you by gRPC. We also get all the advantages of working with protocol buffers, including efficient serialization, a simple IDL, and easy interface updating.

Setup

You should have already installed the tools needed to generate client and server interface code – if you haven’t, see the Prerequisites section of Quick start for setup instructions.

Get the example code

The example code is part of the grpc-rust repo.

1. Download the repo as a zip file and unzip it, or clone the repo:

   git clone -b grpc-v0.9.0 --depth 1 https://github.com/grpc/grpc-rust
   

2. Change to the examples directory:

   cd grpc-rust/examples
   

Defining the service

Our first step (as you’ll know from the Introduction to gRPC) is to define the gRPC service and the method request and response types using protocol buffers. For the complete .proto file, see routeguide/route_guide.proto.

To define a service, you specify a named service in your .proto file:

service RouteGuide {
   ...
}

Then you define rpc methods inside your service definition, specifying their request and response types. gRPC lets you define four kinds of service method, all of which are used in the RouteGuide service:

• A simple RPC where the client sends a request to the server using the stub and waits for a response to come back, just like a normal function call.

  // Obtains the feature at a given position.
  rpc GetFeature(Point) returns (Feature) {}
  

• A server-side streaming RPC where the client sends a request to the server and gets a stream to read a sequence of messages back. The client reads from the returned stream until there are no more messages. As you can see in our example, you specify a server-side streaming method by placing the stream keyword before the response type.

  // Obtains the Features available within the given Rectangle.  Results are
  // streamed rather than returned at once (e.g. in a response message with a
  // repeated field), as the rectangle may cover a large area and contain a
  // huge number of features.
  rpc ListFeatures(Rectangle) returns (stream Feature) {}
  

• A client-side streaming RPC where the client writes a sequence of messages and sends them to the server, again using a provided stream. Once the client has finished writing the messages, it waits for the server to read them all and return its response. You specify a client-side streaming method by placing the stream keyword before the request type.

  // Accepts a stream of Points on a route being traversed, returning a
  // RouteSummary when traversal is completed.
  rpc RecordRoute(stream Point) returns (RouteSummary) {}
  

• A bidirectional streaming RPC where both sides send a sequence of messages using a read-write stream. The two streams operate independently, so clients and servers can read and write in whatever order they like: for example, the server could wait to receive all the client messages before writing its responses, or it could alternately read a message then write a message, or some other combination of reads and writes. The order of messages in each stream is preserved. You specify this type of method by placing the stream keyword before both the request and the response.

  // Accepts a stream of RouteNotes sent while a route is being traversed,
  // while receiving other RouteNotes (e.g. from other users).
  rpc RouteChat(stream RouteNote) returns (stream RouteNote) {}
  

Our .proto file also contains protocol buffer message type definitions for all the request and response types used in our service methods - for example, here’s the Point message type:

// Points are represented as latitude-longitude pairs in the E7 representation
// (degrees multiplied by 10**7 and rounded to the nearest integer).
// Latitudes should be in the range +/- 90 degrees and longitude should be in
// the range +/- 180 degrees (inclusive).
message Point {
  int32 latitude = 1;
  int32 longitude = 2;
}

Generating client code

Next we need to generate the gRPC client code from our .proto service definition. We do this using the protocol buffer compiler protoc with a special gRPC Rust plugin and build.rs integration. This is similar to what we did in the Quick start – please refer to this guide for more information on how this works.

From the examples/ directory, do one of the following:

1. If you installed protoc and protoc-gen-rust-grpc into your path:

GRPC_RUST_REGENERATE_PROTO=1 cargo build --bin grpc-routeguide-client

2. If you prefer to build these tools from source with a C++ compiler and CMake as described above:

GRPC_RUST_REGENERATE_PROTO=1 cargo build --bin grpc-routeguide-client --features grpc-protobuf-build/build-plugin

3. If you’d like to run protoc manually from the command-line:

protoc \
   --rust_out=src/grpc-routeguide/generated/ --rust_opt=experimental-codegen=enabled,kernel=upb \
   --rust-grpc_out=src/grpc-routeguide/generated/ --rust-grpc_opt=client_only=true \
   --proto_path=proto/routeguide/ \
   proto/routeguide/route_guide.proto

Doing any of these generates the following files in the routeguide directory:

• route_guide.u.pb.rs, which contains all the protocol buffer code to populate, serialize, and retrieve request and response message types.
• route_guide_grpc.pb.rs, which contains a wrapper type (or stub) for clients to call with the methods defined in the RouteGuide service.
• generated.rs, a helper for the protobuf message generated code.

Including the generated code in your application

To include this generated code in our application, add the following to your crate (often in the top-level mod.rs or lib.rs):

mod generated {
    pub mod routeguide {
        include!("generated/generated.rs"); // Contains messages
        include!("generated/route_guide_grpc.pb.rs"); // Contains grpc stubs
    }
}

Creating the client

In this section, we’ll look at creating a Rust client for our RouteGuide service. You can see our complete example client code in grpc-rust/examples/src/grpc-routeguide/client.rs.

Creating a stub

To call service methods, we first need to create a gRPC channel to communicate with the server. We create this by passing the server address and port number to Channel::new() as follows:

use grpc::client::Channel;

let channel = Channel::new(
    format!("dns:///{address}"),
    Arc::new(LocalChannelCredentials::new()),
    ChannelOptions::default(),
);

You can substitute the LocalChannelCredentials with another credentials implementation (for example, TLS, GCE credentials, or JWT credentials) when a service requires them. The RouteGuide service doesn’t require any credentials. “Local” credentials is a safe plaintext credential type that verifies it is connecting to a local address, or fails to establish the connection. You should always have a strong credential type when connecting to any remote peer.

Once the gRPC channel is set up, we need a client stub to perform RPCs. We get it using the RouteGuideClient::new method provided by the code generated from the example .proto file.

use crate::generated::routeguide::route_guide_client::RouteGuideClient;

let client = RouteGuideClient::new(channel);

Calling service methods

Now let’s look at how we call our service methods. Note that in gRPC-Rust, all RPCs and their operations are implemented with async functions, which means that you will typically .await them to wait for them to complete.

Simple RPC

Calling the simple RPC GetFeature is just like calling a local async method.

let feature = client
    .get_feature(proto!(Point{latitude: 409146138, longitude: -746188906}))
    .await?; // Optional: use `.map_err()` to transform errors before propagating.

As you can see, we call the method on the stub we got earlier. In our method parameter we create and populate a request protocol buffer object (in our case Point). If the call doesn’t return an error, then feature will be the response information from the server.

// This is sufficient to print feature's "debug" information but we implement
// prettier output in the example application.
println!("{feature:?}");

Server-side streaming RPC

Here’s where we call the server-side streaming method ListFeatures, which returns a stream of geographical Features.

// Initialize a Rectangle:
let rect = proto!(Rectangle{...});

// Start the RPC.
let mut response_stream = client.list_features(rect).await;

// Receive the response messages.
while let Some(feature) = response_stream.recv().await {
    print_feature(feature);
}

// Confirm the status.
let status = response_stream.status().await;
assert!(status.is_ok(), "{status:?}");

As in the simple RPC, we pass the method the request. However, instead of getting a response object back, we get back an instance of GrpcStreamingResponse<Feature>. We use the GrpcStreamingResponse’s recv() method to repeatedly read in the server’s responses to a response protocol buffer object (in this case a Feature) until there are no more messages, i.e. until None is returned. At that time, the client needs to check the status returned by the status() method. If it is .ok(), the RPC succeeded; otherwise it will contain the failure information.

Client-side streaming RPC

The client-side streaming method RecordRoute is similar to the server-side method, except that we pass no parameters to the method and get a ClientStreamingCall back, which we can use to both write messages and read the final response message or the status if an error occurred.

// Create a random number of random points.
let point_count = rand::random_range(2..=30); // Traverse at least two points
let mut points = Vec::with_capacity(point_count);
for _ in 0..point_count {
    points.push(random_point());
}
println!("Traversing {point_count} points.");

// Start the RPC.
let mut stream = client.record_route().await;

// Send the request messages.
for point in points {
    if stream.send(&point).await.is_err() {
        // RPC terminated early; abort sending and the response stream
        // will provide the RPC status.
        break;
    }
}

// Receive the response or status.
let response = stream.close_and_recv().await.expect("RPC failed");
println!(
    "Route summary: Point count: {}, Distance: {}",
    response.point_count(),
    response.distance()
);

The ClientStreamingCall has a send() method that we can use to send requests to the server. Once we’ve finished writing our client’s requests to the stream using send(), we need to call close_and_recv() on the stream to let gRPC know that we’ve finished writing and are expecting to receive a response. We get our response message or the RPC status from that operation.

Bidirectional streaming RPC

Finally, let’s look at our bidirectional streaming RPC RouteChat(). As in the case of RecordRoute, we pass no parameters to the method when we call it. This time, however, we receive two streaming objects, GrpcStreamingRequest and GrpcStreamingResponse, that we can use to write and read messages, respectively. This allows us to send values via our method’s stream while the server is still sending messages to their message stream.

// Start the RPC.
let (mut tx, mut rx) = client.route_chat().await;

// Spawn a task to send the request messages asynchronously.
let handle = tokio::task::spawn(async move {
    for note in notes {
        if tx.send(note).await.is_err() {
            // RPC terminated early; abort sending and the response stream
            // will provide the RPC status.
            return;
        }
    }
    // Indicate to the server that we are done sending requests.  This also
    // triggers naturally if `tx` is dropped, which will happen automatically
    // at the end of this task.
    tx.close();
});

// Receive the response messages in the current task.
while let Some(response) = rx.recv().await {
    println!(
        "Got message {} at Point: {{{}, {}}}",
        response.message(),
        response.location().latitude(),
        response.location().longitude()
    );
}

// Confirm the status.
let status = rx.status().await;
assert!(status.is_ok(), "{status:?}");

// Wait for the spawned task to complete as part of proper resource cleanup.
handle.await.unwrap();

The syntax for reading and writing here is very similar to our client-side and server-side streaming methods, except we use the GrpcStreamingRequest’s close() method once we’ve finished our call. Note that this happens automatically via its Drop implementation as well. Although each side will always get the other’s messages in the order they were written, both the client and server can read and write in any order — the streams operate completely independently.

Try it out!

Execute the following commands from the examples/route_guide directory:

1. Run the tonic routeguide server:

   cargo run --bin routeguide-server
   

2. From another terminal, run the client:

   cargo run --bin grpc-routeguide-client
   

You’ll see output like this:

Connecting to [::1]:10000...
*** SIMPLE RPC ***
Response = Name = "Berkshire Valley Management Area Trail, Jefferson, NJ, USA", Point: {409146138, -746188906}
*** MISSING FEATURE ***
Response = Name = "", Point: {0, 0}
*** FEATURES IN RANGE ***
Response = Name = "Patriots Path, Mendham, NJ 07945, USA", Point: {407838351, -746143763}
Response = Name = "101 New Jersey 10, Whippany, NJ 07981, USA", Point: {408122808, -743999179}
...
*** RECORD ROUTE ***
Traversing 29 points.
Route summary: Point count: 29, Distance: 313071718
*** ROUTE CHAT ***
Got message Message One at Point: {0, 1}
Got message Message Two at Point: {0, 2}
Got message Message Three at Point: {0, 3}
Got message Message One at Point: {0, 1}
Got message Message Four at Point: {0, 1}
Got message Message One at Point: {0, 1}
Got message Message Four at Point: {0, 1}
Got message Message Five at Point: {0, 1}