In this section, we’ll look at creating a Go client for our RouteGuide service. You can see our complete example client code in grpc-go/examples/route_guide/client/client.go.
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 grpc.Dial() as follows:
conn, err := grpc.Dial(*serverAddr)
if err != nil {
...
}
defer conn.Close()
You can use DialOptions to set the auth credentials (e.g., TLS, GCE credentials, JWT credentials) in grpc.Dial if the service you request requires that - however, we don’t need to do this for our RouteGuide service.
Once the gRPC channel is setup, we need a client stub to perform RPCs. We get this using the NewRouteGuideClient method provided in the pb package we generated from our .proto file.
client := pb.NewRouteGuideClient(conn)
Calling service methods
Now let’s look at how we call our service methods. Note that in gRPC-Go, RPCs operate in a blocking/synchronous mode, which means that the RPC call waits for the server to respond, and will either return a response or an error.
Simple RPC
Calling the simple RPC GetFeature is nearly as straightforward as calling a local method.
feature, err := client.GetFeature(ctx, &pb.Point{409146138, -746188906})
if err != nil {
...
}
As you can see, we call the method on the stub we got earlier. In our method parameters we create and populate a request protocol buffer object (in our case Point). We also pass a context.Context object which lets us change our RPC’s behaviour if necessary, such as time-out/cancel an RPC in flight. If the call doesn’t return an error, then we can read the response information from the server from the first return value.
log.Println(feature)
Server-side streaming RPC
Here’s where we call the server-side streaming method ListFeatures, which returns a stream of geographical Features. 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.
rect := &pb.Rectangle{ ... } // initialize a pb.Rectangle
stream, err := client.ListFeatures(ctx, rect)
if err != nil {
...
}
for {
feature, err := stream.Recv()
if err == io.EOF {
break
}
if err != nil {
log.Fatalf("%v.ListFeatures(_) = _, %v", client, err)
}
log.Println(feature)
}
As in the simple RPC, we pass the method a context and a request. However, instead of getting a response object back, we get back an instance of RouteGuide_ListFeaturesClient. The client can use the RouteGuide_ListFeaturesClient stream to read the server’s responses.
We use the RouteGuide_ListFeaturesClient’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: the client needs to check the error err returned from Recv() after each call. If nil, the stream is still good and it can continue reading; if it’s io.EOF then the message stream has ended; otherwise there must be an RPC error, which is passed over through err.
Client-side streaming RPC
The client-side streaming method RecordRoute is similar to the server-side method, except that we only pass the method a context and get a RouteGuide_RecordRouteClient stream back, which we can use to both write and read messages.
// Create a random number of random points
r := rand.New(rand.NewSource(time.Now().UnixNano()))
pointCount := int(r.Int31n(100)) + 2 // Traverse at least two points
var points []*pb.Point
for i := 0; i < pointCount; i++ {
points = append(points, randomPoint(r))
}
log.Printf("Traversing %d points.", len(points))
stream, err := client.RecordRoute(ctx)
if err != nil {
log.Fatalf("%v.RecordRoute(_) = _, %v", client, err)
}
for _, point := range points {
if err := stream.Send(point); err != nil {
log.Fatalf("%v.Send(%v) = %v", stream, point, err)
}
}
reply, err := stream.CloseAndRecv()
if err != nil {
log.Fatalf("%v.CloseAndRecv() got error %v, want %v", stream, err, nil)
}
log.Printf("Route summary: %v", reply)
The RouteGuide_RecordRouteClient 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 CloseAndRecv() on the stream to let gRPC know that we’ve finished writing and are expecting to receive a response. We get our RPC status from the err returned from CloseAndRecv(). If the status is nil, then the first return value from CloseAndRecv() will be a valid server response.
Bidirectional streaming RPC
Finally, let’s look at our bidirectional streaming RPC RouteChat(). As in the case of RecordRoute, we only pass the method a context object and get back a stream that we can use to both write and read messages. However, this time we return values via our method’s stream while the server is still writing messages to their message stream.
stream, err := client.RouteChat(ctx)
waitc := make(chan struct{})
go func() {
for {
in, err := stream.Recv()
if err == io.EOF {
// read done.
close(waitc)
return
}
if err != nil {
log.Fatalf("Failed to receive a note : %v", err)
}
log.Printf("Got message %s at point(%d, %d)", in.Message, in.Location.Latitude, in.Location.Longitude)
}
}()
for _, note := range notes {
if err := stream.Send(note); err != nil {
log.Fatalf("Failed to send a note: %v", err)
}
}
stream.CloseSend()
<-waitc
The syntax for reading and writing here is very similar to our client-side streaming method, except we use the stream’s CloseSend() method once we’ve finished our call. 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.