1#gRPC Basics: Go 2 3This tutorial provides a basic Go programmer's introduction to working with gRPC. By walking through this example you'll learn how to: 4 5- Define a service in a .proto file. 6- Generate server and client code using the protocol buffer compiler. 7- Use the Go gRPC API to write a simple client and server for your service. 8 9It assumes that you have read the [Getting started](https://github.com/grpc/grpc/tree/master/examples) guide and are familiar with [protocol buffers] (https://developers.google.com/protocol-buffers/docs/overview). Note that the example in this tutorial uses the proto3 version of the protocol buffers language, which is currently in alpha release:you can find out more in the [proto3 language guide](https://developers.google.com/protocol-buffers/docs/proto3) and see the [release notes](https://github.com/google/protobuf/releases) for the new version in the protocol buffers Github repository. 10 11This isn't a comprehensive guide to using gRPC in Go: more reference documentation is coming soon. 12 13## Why use gRPC? 14 15Our 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. 16 17With gRPC we can define our service once in a .proto file and implement clients and servers in any of gRPC's supported languages, which in turn can be run in environments ranging from servers inside Google 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. 18 19## Example code and setup 20 21The example code for our tutorial is in [grpc/grpc-go/examples/route_guide](https://github.com/grpc/grpc-go/tree/master/examples/route_guide). To download the example, clone the `grpc-go` repository by running the following command: 22```shell 23$ go get google.golang.org/grpc 24``` 25 26Then change your current directory to `grpc-go/examples/route_guide`: 27```shell 28$ cd $GOPATH/src/google.golang.org/grpc/examples/route_guide 29``` 30 31You 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 [the Go quick start guide](examples/). 32 33 34## Defining the service 35 36Our first step (as you'll know from the [quick start](http://www.grpc.io/docs/#quick-start)) is to define the gRPC *service* and the method *request* and *response* types using [protocol buffers] (https://developers.google.com/protocol-buffers/docs/overview). You can see the complete .proto file in [`examples/route_guide/proto/route_guide.proto`](examples/route_guide/proto/route_guide.proto). 37 38To define a service, you specify a named `service` in your .proto file: 39 40```proto 41service RouteGuide { 42 ... 43} 44``` 45 46Then 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: 47 48- 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. 49```proto 50 // Obtains the feature at a given position. 51 rpc GetFeature(Point) returns (Feature) {} 52``` 53 54- 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. 55```proto 56 // Obtains the Features available within the given Rectangle. Results are 57 // streamed rather than returned at once (e.g. in a response message with a 58 // repeated field), as the rectangle may cover a large area and contain a 59 // huge number of features. 60 rpc ListFeatures(Rectangle) returns (stream Feature) {} 61``` 62 63- 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. 64```proto 65 // Accepts a stream of Points on a route being traversed, returning a 66 // RouteSummary when traversal is completed. 67 rpc RecordRoute(stream Point) returns (RouteSummary) {} 68``` 69 70- 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. 71```proto 72 // Accepts a stream of RouteNotes sent while a route is being traversed, 73 // while receiving other RouteNotes (e.g. from other users). 74 rpc RouteChat(stream RouteNote) returns (stream RouteNote) {} 75``` 76 77Our .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: 78```proto 79// Points are represented as latitude-longitude pairs in the E7 representation 80// (degrees multiplied by 10**7 and rounded to the nearest integer). 81// Latitudes should be in the range +/- 90 degrees and longitude should be in 82// the range +/- 180 degrees (inclusive). 83message Point { 84 int32 latitude = 1; 85 int32 longitude = 2; 86} 87``` 88 89 90## Generating client and server code 91 92Next 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 Go plugin. 93 94For simplicity, we've provided a [bash script](https://github.com/grpc/grpc-go/blob/master/codegen.sh) that runs `protoc` for you with the appropriate plugin, input, and output (if you want to run this by yourself, make sure you've installed protoc and followed the gRPC-Go [installation instructions](https://github.com/grpc/grpc-go/blob/master/README.md) first): 95 96```shell 97$ codegen.sh route_guide.proto 98``` 99 100which actually runs: 101 102```shell 103$ protoc --go_out=plugins=grpc:. route_guide.proto 104``` 105 106Running this command generates the following file in your current directory: 107- `route_guide.pb.go` 108 109This contains: 110- All the protocol buffer code to populate, serialize, and retrieve our request and response message types 111- An interface type (or *stub*) for clients to call with the methods defined in the `RouteGuide` service. 112- An interface type for servers to implement, also with the methods defined in the `RouteGuide` service. 113 114 115<a name="server"></a> 116## Creating the server 117 118First 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](#client) (though you might find it interesting anyway!). 119 120There are two parts to making our `RouteGuide` service do its job: 121- Implementing the service interface generated from our service definition: doing the actual "work" of our service. 122- Running a gRPC server to listen for requests from clients and dispatch them to the right service implementation. 123 124You can find our example `RouteGuide` server in [grpc-go/examples/route_guide/server/server.go](https://github.com/grpc/grpc-go/tree/master/examples/route_guide/server/server.go). Let's take a closer look at how it works. 125 126### Implementing RouteGuide 127 128As you can see, our server has a `routeGuideServer` struct type that implements the generated `RouteGuideServer` interface: 129 130```go 131type routeGuideServer struct { 132 ... 133} 134... 135 136func (s *routeGuideServer) GetFeature(ctx context.Context, point *pb.Point) (*pb.Feature, error) { 137 ... 138} 139... 140 141func (s *routeGuideServer) ListFeatures(rect *pb.Rectangle, stream pb.RouteGuide_ListFeaturesServer) error { 142 ... 143} 144... 145 146func (s *routeGuideServer) RecordRoute(stream pb.RouteGuide_RecordRouteServer) error { 147 ... 148} 149... 150 151func (s *routeGuideServer) RouteChat(stream pb.RouteGuide_RouteChatServer) error { 152 ... 153} 154... 155``` 156 157#### Simple RPC 158`routeGuideServer` 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`. 159 160```go 161func (s *routeGuideServer) GetFeature(ctx context.Context, point *pb.Point) (*pb.Feature, error) { 162 for _, feature := range s.savedFeatures { 163 if proto.Equal(feature.Location, point) { 164 return feature, nil 165 } 166 } 167 // No feature was found, return an unnamed feature 168 return &pb.Feature{"", point}, nil 169} 170``` 171 172The method is passed a context object for the RPC and the client's `Point` protocol buffer request. It returns a `Feature` protocol buffer object with the response information and an `error`. In the method we populate the `Feature` with the appropriate information, and then `return` it along with an `nil` error to tell gRPC that we've finished dealing with the RPC and that the `Feature` can be returned to the client. 173 174#### Server-side streaming RPC 175Now let's look at one of our streaming RPCs. `ListFeatures` is a server-side streaming RPC, so we need to send back multiple `Feature`s to our client. 176 177```go 178func (s *routeGuideServer) ListFeatures(rect *pb.Rectangle, stream pb.RouteGuide_ListFeaturesServer) error { 179 for _, feature := range s.savedFeatures { 180 if inRange(feature.Location, rect) { 181 if err := stream.Send(feature); err != nil { 182 return err 183 } 184 } 185 } 186 return nil 187} 188``` 189 190As you can see, instead of getting simple request and response objects in our method parameters, this time we get a request object (the `Rectangle` in which our client wants to find `Feature`s) and a special `RouteGuide_ListFeaturesServer` object to write our responses. 191 192In the method, we populate as many `Feature` objects as we need to return, writing them to the `RouteGuide_ListFeaturesServer` using its `Send()` method. Finally, as in our simple RPC, we return a `nil` error to tell gRPC that we've finished writing responses. Should any error happen in this call, we return a non-`nil` error; the gRPC layer will translate it into an appropriate RPC status to be sent on the wire. 193 194#### Client-side streaming RPC 195Now let's look at something a little more complicated: the client-side streaming method `RecordRoute`, where we get a stream of `Point`s from the client and return a single `RouteSummary` with information about their trip. As you can see, this time the method doesn't have a request parameter at all. Instead, it gets a `RouteGuide_RecordRouteServer` stream, which the server can use to both read *and* write messages - it can receive client messages using its `Recv()` method and return its single response using its `SendAndClose()` method. 196 197```go 198func (s *routeGuideServer) RecordRoute(stream pb.RouteGuide_RecordRouteServer) error { 199 var pointCount, featureCount, distance int32 200 var lastPoint *pb.Point 201 startTime := time.Now() 202 for { 203 point, err := stream.Recv() 204 if err == io.EOF { 205 endTime := time.Now() 206 return stream.SendAndClose(&pb.RouteSummary{ 207 PointCount: pointCount, 208 FeatureCount: featureCount, 209 Distance: distance, 210 ElapsedTime: int32(endTime.Sub(startTime).Seconds()), 211 }) 212 } 213 if err != nil { 214 return err 215 } 216 pointCount++ 217 for _, feature := range s.savedFeatures { 218 if proto.Equal(feature.Location, point) { 219 featureCount++ 220 } 221 } 222 if lastPoint != nil { 223 distance += calcDistance(lastPoint, point) 224 } 225 lastPoint = point 226 } 227} 228``` 229 230In the method body we use the `RouteGuide_RecordRouteServer`s `Recv()` method to repeatedly read in our client's requests to a request object (in this case a `Point`) until there are no more messages: the server needs to check the the error returned from `Recv()` after each call. If this is `nil`, the stream is still good and it can continue reading; if it's `io.EOF` the message stream has ended and the server can return its `RouteSummary`. If it has any other value, we return the error "as is" so that it'll be translated to an RPC status by the gRPC layer. 231 232#### Bidirectional streaming RPC 233Finally, let's look at our bidirectional streaming RPC `RouteChat()`. 234 235```go 236func (s *routeGuideServer) RouteChat(stream pb.RouteGuide_RouteChatServer) error { 237 for { 238 in, err := stream.Recv() 239 if err == io.EOF { 240 return nil 241 } 242 if err != nil { 243 return err 244 } 245 key := serialize(in.Location) 246 ... // look for notes to be sent to client 247 for _, note := range s.routeNotes[key] { 248 if err := stream.Send(note); err != nil { 249 return err 250 } 251 } 252 } 253} 254``` 255 256This time we get a `RouteGuide_RouteChatServer` stream that, as in our client-side streaming example, can be used to read and write messages. However, this time we return values via our method's stream while the client is still writing messages to *their* message stream. 257 258The syntax for reading and writing here is very similar to our client-streaming method, except the server uses the stream's `Send()` method rather than `SendAndClose()` because it's writing multiple 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. 259 260### Starting the server 261 262Once 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: 263 264```go 265flag.Parse() 266lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port)) 267if err != nil { 268 log.Fatalf("failed to listen: %v", err) 269} 270grpcServer := grpc.NewServer() 271pb.RegisterRouteGuideServer(grpcServer, &routeGuideServer{}) 272... // determine whether to use TLS 273grpcServer.Serve(lis) 274``` 275To build and start a server, we: 276 2771. Specify the port we want to use to listen for client requests using `lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))`. 2782. Create an instance of the gRPC server using `grpc.NewServer()`. 2793. Register our service implementation with the gRPC server. 2804. Call `Serve()` on the server with our port details to do a blocking wait until the process is killed or `Stop()` is called. 281 282<a name="client"></a> 283## Creating the client 284 285In 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](https://github.com/grpc/grpc-go/tree/master/examples/route_guide/client/client.go). 286 287### Creating a stub 288 289To 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: 290 291```go 292conn, err := grpc.Dial(*serverAddr) 293if err != nil { 294 ... 295} 296defer conn.Close() 297``` 298 299You 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. 300 301Once 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. 302 303```go 304client := pb.NewRouteGuideClient(conn) 305``` 306 307### Calling service methods 308 309Now 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. 310 311#### Simple RPC 312 313Calling the simple RPC `GetFeature` is nearly as straightforward as calling a local method. 314 315```go 316feature, err := client.GetFeature(context.Background(), &pb.Point{409146138, -746188906}) 317if err != nil { 318 ... 319} 320``` 321 322As 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. 323 324```go 325log.Println(feature) 326``` 327 328#### Server-side streaming RPC 329 330Here's where we call the server-side streaming method `ListFeatures`, which returns a stream of geographical `Feature`s. If you've already read [Creating the server](#server) some of this may look very familiar - streaming RPCs are implemented in a similar way on both sides. 331 332```go 333rect := &pb.Rectangle{ ... } // initialize a pb.Rectangle 334stream, err := client.ListFeatures(context.Background(), rect) 335if err != nil { 336 ... 337} 338for { 339 feature, err := stream.Recv() 340 if err == io.EOF { 341 break 342 } 343 if err != nil { 344 log.Fatalf("%v.ListFeatures(_) = _, %v", client, err) 345 } 346 log.Println(feature) 347} 348``` 349 350As 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. 351 352We 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`. 353 354#### Client-side streaming RPC 355 356The 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. 357 358```go 359// Create a random number of random points 360r := rand.New(rand.NewSource(time.Now().UnixNano())) 361pointCount := int(r.Int31n(100)) + 2 // Traverse at least two points 362var points []*pb.Point 363for i := 0; i < pointCount; i++ { 364 points = append(points, randomPoint(r)) 365} 366log.Printf("Traversing %d points.", len(points)) 367stream, err := client.RecordRoute(context.Background()) 368if err != nil { 369 log.Fatalf("%v.RecordRoute(_) = _, %v", client, err) 370} 371for _, point := range points { 372 if err := stream.Send(point); err != nil { 373 log.Fatalf("%v.Send(%v) = %v", stream, point, err) 374 } 375} 376reply, err := stream.CloseAndRecv() 377if err != nil { 378 log.Fatalf("%v.CloseAndRecv() got error %v, want %v", stream, err, nil) 379} 380log.Printf("Route summary: %v", reply) 381``` 382 383The `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. 384 385#### Bidirectional streaming RPC 386 387Finally, 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. 388 389```go 390stream, err := client.RouteChat(context.Background()) 391waitc := make(chan struct{}) 392go func() { 393 for { 394 in, err := stream.Recv() 395 if err == io.EOF { 396 // read done. 397 close(waitc) 398 return 399 } 400 if err != nil { 401 log.Fatalf("Failed to receive a note : %v", err) 402 } 403 log.Printf("Got message %s at point(%d, %d)", in.Message, in.Location.Latitude, in.Location.Longitude) 404 } 405}() 406for _, note := range notes { 407 if err := stream.Send(note); err != nil { 408 log.Fatalf("Failed to send a note: %v", err) 409 } 410} 411stream.CloseSend() 412<-waitc 413``` 414 415The 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. 416 417## Try it out! 418 419To compile and run the server, assuming you are in the folder 420`$GOPATH/src/google.golang.org/grpc/examples/route_guide`, simply: 421 422```sh 423$ go run server/server.go 424``` 425 426Likewise, to run the client: 427 428```sh 429$ go run client/client.go 430``` 431 432