Basics tutorial

This tutorial provides a basic Ruby 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 server and client code using the protocol buffer compiler.
• Use the Ruby gRPC API to write a simple client and server 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.

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.

Example code and setup

The example code for our tutorial is in grpc/grpc/examples/ruby/route_guide. To download the example, clone the grpc repository by running the following command:

git clone -b v1.66.0 --depth 1 --shallow-submodules https://github.com/grpc/grpc
cd grpc

Then change your current directory to examples/ruby/route_guide:

cd examples/ruby/route_guide

You also should have the relevant tools installed to generate the server and client interface code - if you don’t already, follow the setup instructions in Quick start.

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. You can see the complete .proto file in examples/protos/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 and server code

Next we need to generate the gRPC client and server interfaces from our .proto service definition. We do this using the protocol buffer compiler protoc with a special gRPC Ruby plugin.

If you want to run this yourself, make sure you have installed gRPC and protoc.

Once that’s done, the following command can be used to generate the ruby code.

grpc_tools_ruby_protoc -I ../../protos --ruby_out=../lib --grpc_out=../lib ../../protos/route_guide.proto

Running this command regenerates the following files in the lib directory:

• lib/route_guide.pb defines a module Examples::RouteGuide
  • This contain all the protocol buffer code to populate, serialize, and retrieve our request and response message types
• lib/route_guide_services.pb, extends Examples::RouteGuide with stub and service classes
  • a class Service for use as a base class when defining RouteGuide service implementations
  • a class Stub that can be used to access remote RouteGuide instances

Creating the server

First let’s look at how we create a RouteGuide server. If you’re only interested in creating gRPC clients, you can skip this section and go straight to Creating the client (though you might find it interesting anyway!).

There are two parts to making our RouteGuide service do its job:

• Implementing the service interface generated from our service definition: doing the actual “work” of our service.
• Running a gRPC server to listen for requests from clients and return the service responses.

You can find our example RouteGuide server in examples/ruby/route_guide/route_guide_server.rb. Let’s take a closer look at how it works.

Implementing RouteGuide

As you can see, our server has a ServerImpl class that extends the generated RouteGuide::Service:

# ServerImpl provides an implementation of the RouteGuide service.
class ServerImpl < RouteGuide::Service

ServerImpl implements all our service methods. Let’s look at the simplest type first, GetFeature, which just gets a Point from the client and returns the corresponding feature information from its database in a Feature.

def get_feature(point, _call)
  name = @feature_db[{
    'longitude' => point.longitude,
    'latitude' => point.latitude }] || ''
  Feature.new(location: point, name: name)
end

The method is passed a _call for the RPC, the client’s Point protocol buffer request, and returns a Feature protocol buffer. In the method we create the Feature with the appropriate information, and then return it.

Now let’s look at something a bit more complicated - a streaming RPC. ListFeatures is a server-side streaming RPC, so we need to send back multiple Features to our client.

# in ServerImpl

  def list_features(rectangle, _call)
    RectangleEnum.new(@feature_db, rectangle).each
  end

As you can see, here the request object is a Rectangle in which our client wants to find Features, but instead of returning a simple response we need to return an Enumerator that yields the responses. In the method, we use a helper class RectangleEnum, to act as an Enumerator implementation.

Similarly, the client-side streaming method record_route uses an Enumerable, but here it’s obtained from the call object, which we’ve ignored in the earlier examples. call.each_remote_read yields each message sent by the client in turn.

call.each_remote_read do |point|
  ...
end

Finally, let’s look at our bidirectional streaming RPC route_chat.

def route_chat(notes)
  RouteChatEnumerator.new(notes, @received_notes).each_item
end

Here the method receives an Enumerable, but also returns an Enumerator that yields the responses. 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.

Starting the server

Once we’ve implemented all our methods, we also need to start up a gRPC server so that clients can actually use our service. The following snippet shows how we do this for our RouteGuide service:

port = '0.0.0.0:50051'
s = GRPC::RpcServer.new
s.add_http2_port(port, :this_port_is_insecure)
GRPC.logger.info("... running insecurely on #{port}")
s.handle(ServerImpl.new(feature_db))
# Runs the server with SIGHUP, SIGINT and SIGQUIT signal handlers to
#   gracefully shutdown.
# User could also choose to run server via call to run_till_terminated
s.run_till_terminated_or_interrupted([1, 'int', 'SIGQUIT'])

As you can see, we build and start our server using a GRPC::RpcServer. To do this, we:

1. Create an instance of our service implementation class ServerImpl.
2. Specify the address and port we want to use to listen for client requests using the builder’s add_http2_port method.
3. Register our service implementation with the GRPC::RpcServer.
4. Call run on theGRPC::RpcServer to create and start an RPC server for our service.

Creating the client

In this section, we’ll look at creating a Ruby client for our RouteGuide service. You can see our complete example client code in examples/ruby/route_guide/route_guide_client.rb.

Creating a stub

To call service methods, we first need to create a stub.

We use the Stub class of the RouteGuide module generated from our .proto.

stub = RouteGuide::Stub.new('localhost:50051')

Calling service methods

Now let’s look at how we call our service methods. Note that the gRPC Ruby only provides blocking/synchronous versions of each method: this means that the RPC call waits for the server to respond, and will either return a response or raise an exception.

Simple RPC

Calling the simple RPC GetFeature is nearly as straightforward as calling a local method.

GET_FEATURE_POINTS = [
  Point.new(latitude:  409_146_138, longitude: -746_188_906),
  Point.new(latitude:  0, longitude: 0)
]
..
  GET_FEATURE_POINTS.each do |pt|
    resp = stub.get_feature(pt)
	...
    p "- found '#{resp.name}' at #{pt.inspect}"
  end

As you can see, we create and populate a request protocol buffer object (in our case Point), and create a response protocol buffer object for the server to fill in. Finally, we call the method on the stub, passing it the context, request, and response. If the method returns OK, then we can read the response information from the server from our response object.

Streaming RPCs

Now let’s look at our streaming methods. If you’ve already read Creating the server some of this may look very familiar - streaming RPCs are implemented in a similar way on both sides. Here’s where we call the server-side streaming method list_features, which returns an Enumerable of Features.

resps = stub.list_features(LIST_FEATURES_RECT)
resps.each do |r|
  p "- found '#{r.name}' at #{r.location.inspect}"
end

Non-blocking usage of the RPC stream can be achieved with multiple threads and the return_op: true flag. When passing the return_op: true flag, the execution of the RPC is deferred and an Operation object is returned. The RPC can then be executed in another thread by calling the operation execute function. The main thread can utilize contextual methods and getters such as status, cancelled?, and cancel to manage the RPC. This can be useful for persistent or long running RPC sessions that would block the main thread for an unacceptable period of time.

op = stub.list_features(LIST_FEATURES_RECT, return_op: true)
Thread.new do 
  resps = op.execute
  resps.each do |r|
    p "- found '#{r.name}' at #{r.location.inspect}"
  end
rescue GRPC::Cancelled => e
  p "operation cancel called - #{e}"
end

# controls for the operation
op.status
op.cancelled?
op.cancel # attempts to cancel the RPC with a GRPC::Cancelled status; there's a fundamental race condition where cancelling the RPC can race against RPC termination for a different reason - invoking `cancel` doesn't necessarily guarantee a `Cancelled` status

The client-side streaming method record_route is similar, except there we pass the server an Enumerable.

...
reqs = RandomRoute.new(features, points_on_route)
resp = stub.record_route(reqs.each)
...

Finally, let’s look at our bidirectional streaming RPC route_chat. In this case, we pass Enumerable to the method and get back an Enumerable.

sleeping_enumerator = SleepingEnumerator.new(ROUTE_CHAT_NOTES, 1)
stub.route_chat(sleeping_enumerator.each_item) { |r| p "received #{r.inspect}" }

Although it’s not shown well by this example, each enumerable is independent of the other - both the client and server can read and write in any order — the streams operate completely independently.

Try it out!

Work from the example directory:

cd examples/ruby

Build the client and server:

gem install bundler && bundle install

Run the server:

bundle exec route_guide/route_guide_server.rb ../python/route_guide/route_guide_db.json

From a different terminal, run the client:

bundle exec route_guide/route_guide_client.rb ../python/route_guide/route_guide_db.json