---
title: Basics tutorial
description: A basic tutorial introduction to gRPC in Go.
weight: 50
spelling: cSpell:ignore Fatalf GOPATH Mendham Println Sprintf struct waitc
---

This tutorial provides a basic Go 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 Go gRPC API to write a simple client and server for your service.

It assumes that you have read the [Introduction to gRPC](/docs/what-is-grpc/introduction/) 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: you can find out more in the [proto3 language
guide](https://developers.google.com/protocol-buffers/docs/proto3) and the
[Go generated code
guide](https://developers.google.com/protocol-buffers/docs/reference/go-generated).

### Why use gRPC?

{{< why-grpc >}}

### 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-go][] repo.

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

    ```sh
    $ git clone -b {{< param grpc_vers.go >}} https://github.com/grpc/grpc-go
    ```

 2. Change to the example directory:

    ```sh
    $ cd grpc-go/examples/route_guide
    ```

### Defining the service

Our first step (as you'll know from the [Introduction to gRPC](/docs/what-is-grpc/introduction/)) is to
define the gRPC *service* and the method *request* and *response* types using
[protocol buffers](https://developers.google.com/protocol-buffers/docs/overview).
For the complete `.proto` file, see
[routeguide/route_guide.proto](https://github.com/grpc/grpc-go/blob/master/examples/route_guide/routeguide/route_guide.proto).

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

```proto
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.

  ```proto
  // 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.

  ```proto
  // 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.

  ```proto
  // 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.

  ```proto
  // 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:

```proto
// 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 Go plugin. This is similar to what we did in the [Quick start][].

From the `examples/route_guide` directory, run the following command:

```sh
$ protoc --go_out=. --go_opt=paths=source_relative \
    --go-grpc_out=. --go-grpc_opt=paths=source_relative \
    routeguide/route_guide.proto
```

Running this command generates the following files in the
[routeguide](https://github.com/grpc/grpc-go/blob/master/examples/route_guide/routeguide) directory:

- `route_guide.pb.go`, which contains all the protocol buffer code to
  populate, serialize, and retrieve request and response message types.
- `route_guide_grpc.pb.go`, which contains the following:
  - An interface type (or *stub*) for clients to call with the methods defined in
    the `RouteGuide` service.
  - An interface type for servers to implement, also with the methods defined in
    the `RouteGuide` service.

### Creating the server {#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](#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 dispatch them to
  the right service implementation.

You can find our example `RouteGuide` server in
[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.

#### Implementing RouteGuide

As you can see, our server has a `routeGuideServer` struct type that implements
the generated `RouteGuideServer` interface:

```go
type routeGuideServer struct {
        ...
}
...

func (s *routeGuideServer) GetFeature(ctx context.Context, point *pb.Point) (*pb.Feature, error) {
        ...
}
...

func (s *routeGuideServer) ListFeatures(rect *pb.Rectangle, stream pb.RouteGuide_ListFeaturesServer) error {
        ...
}
...

func (s *routeGuideServer) RecordRoute(stream pb.RouteGuide_RecordRouteServer) error {
        ...
}
...

func (s *routeGuideServer) RouteChat(stream pb.RouteGuide_RouteChatServer) error {
        ...
}
...
```

##### Simple RPC

The `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`.

```go
func (s *routeGuideServer) GetFeature(ctx context.Context, point *pb.Point) (*pb.Feature, error) {
  for _, feature := range s.savedFeatures {
    if proto.Equal(feature.Location, point) {
      return feature, nil
    }
  }
  // No feature was found, return an unnamed feature
  return &pb.Feature{Location: point}, nil
}
```

The 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.

##### Server-side streaming RPC

Now 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.

```go
func (s *routeGuideServer) ListFeatures(rect *pb.Rectangle, stream pb.RouteGuide_ListFeaturesServer) error {
  for _, feature := range s.savedFeatures {
    if inRange(feature.Location, rect) {
      if err := stream.Send(feature); err != nil {
        return err
      }
    }
  }
  return nil
}
```

As 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.

In 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.

##### Client-side streaming RPC

Now 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.

```go
func (s *routeGuideServer) RecordRoute(stream pb.RouteGuide_RecordRouteServer) error {
  var pointCount, featureCount, distance int32
  var lastPoint *pb.Point
  startTime := time.Now()
  for {
    point, err := stream.Recv()
    if err == io.EOF {
      endTime := time.Now()
      return stream.SendAndClose(&pb.RouteSummary{
        PointCount:   pointCount,
        FeatureCount: featureCount,
        Distance:     distance,
        ElapsedTime:  int32(endTime.Sub(startTime).Seconds()),
      })
    }
    if err != nil {
      return err
    }
    pointCount++
    for _, feature := range s.savedFeatures {
      if proto.Equal(feature.Location, point) {
        featureCount++
      }
    }
    if lastPoint != nil {
      distance += calcDistance(lastPoint, point)
    }
    lastPoint = point
  }
}
```

In 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 error
returned from `Read()` 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.

##### Bidirectional streaming RPC

Finally, let's look at our bidirectional streaming RPC `RouteChat()`.

```go
func (s *routeGuideServer) RouteChat(stream pb.RouteGuide_RouteChatServer) error {
  for {
    in, err := stream.Recv()
    if err == io.EOF {
      return nil
    }
    if err != nil {
      return err
    }
    key := serialize(in.Location)
                ... // look for notes to be sent to client
    for _, note := range s.routeNotes[key] {
      if err := stream.Send(note); err != nil {
        return err
      }
    }
  }
}
```

This 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.

The 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.

#### 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:

```go
flag.Parse()
lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", *port))
if err != nil {
  log.Fatalf("failed to listen: %v", err)
}
var opts []grpc.ServerOption
...
grpcServer := grpc.NewServer(opts...)
pb.RegisterRouteGuideServer(grpcServer, newServer())
grpcServer.Serve(lis)
```

To build and start a server, we:

1. Specify the port we want to use to listen for client requests using:<br>
   `lis, err := net.Listen(...)`.
2. Create an instance of the gRPC server using `grpc.NewServer(...)`.
3. Register our service implementation with the gRPC server.
4. Call `Serve()` on the server with our port details to do a blocking wait
   until the process is killed or `Stop()` is called.

### Creating the client {#client}

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](https://github.com/grpc/grpc-go/tree/master/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:

```go
var opts []grpc.DialOption
...
conn, err := grpc.Dial(*serverAddr, opts...)
if err != nil {
  ...
}
defer conn.Close()
```

You can use `DialOptions` to set the auth credentials (for example, TLS, GCE
credentials, or JWT credentials) in `grpc.Dial` when a service requires them.
The `RouteGuide` service doesn't require any credentials.

Once the gRPC *channel* is setup, we need a client *stub* to perform RPCs. We
get it using the `NewRouteGuideClient` method provided by the `pb` package
generated from the example `.proto` file.

```go
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.

```go
feature, err := client.GetFeature(context.Background(), &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
behavior 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.

```go
log.Println(feature)
```

##### Server-side streaming RPC

Here'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.

```go
rect := &pb.Rectangle{ ... }  // initialize a pb.Rectangle
stream, err := client.ListFeatures(context.Background(), 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.

```go
// 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(context.Background())
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.

```go
stream, err := client.RouteChat(context.Background())
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.

### Try it out!

Execute the following commands from the `examples/route_guide` directory:

 1. Run the server:

    ```sh
    $ go run server/server.go
    ```

 2. From another terminal, run the client:

    ```sh
    $ go run client/client.go
    ```

You'll see output like this:

```nocode
Getting feature for point (409146138, -746188906)
name:"Berkshire Valley Management Area Trail, Jefferson, NJ, USA" location:<latitude:409146138 longitude:-746188906 >
Getting feature for point (0, 0)
location:<>
Looking for features within lo:<latitude:400000000 longitude:-750000000 > hi:<latitude:420000000 longitude:-730000000 >
name:"Patriots Path, Mendham, NJ 07945, USA" location:<latitude:407838351 longitude:-746143763 >
...
name:"3 Hasta Way, Newton, NJ 07860, USA" location:<latitude:410248224 longitude:-747127767 >
Traversing 56 points.
Route summary: point_count:56 distance:497013163
Got message First message at point(0, 1)
Got message Second message at point(0, 2)
Got message Third message at point(0, 3)
Got message First message at point(0, 1)
Got message Fourth message at point(0, 1)
Got message Second message at point(0, 2)
Got message Fifth message at point(0, 2)
Got message Third message at point(0, 3)
Got message Sixth message at point(0, 3)
```

{{% alert title="Note" color="info" %}}
  We've omitted timestamps from the client and server trace output shown in this
  page.
{{% /alert %}}

[download]: https://github.com/grpc/grpc-go/archive/{{< param grpc_vers.go >}}.zip
[grpc-go]: https://github.com/grpc/grpc-go
[Prerequisites]: ../quickstart/#prerequisites
[Quick start]: ../quickstart/
