basic-tutorial.md

Basic Tutorial

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

It assumes that you have read the Overview 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 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.

Example code and setup

The example code for our tutorial is in grpc/grpc-swift/Sources/Examples/RouteGuide. To download the example, clone the latest release in grpc-swift repository by running the following command (replacing x.y.z with the latest release, for example 1.7.0):

$ git clone -b x.y.z https://github.com/grpc/grpc-swift

Then change your current directory to grpc-swift/Sources/Examples/RouteGuide:

$ cd grpc-swift/Sources/Examples/RouteGuide

Defining the service

Our first step (as you'll know from the Overview) is to define the gRPC service and the method request and response types using protocol buffers. You can see the complete .proto file in grpc-swift/Sources/Examples/RouteGuide/Model/route_guide.proto.

To define a service, we specify a named service in the .proto file:

service RouteGuide {
   ...
}

Then we define rpc methods inside our service definition, specifying their request and response types. gRPC lets you define four kinds of service methods, 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 two plugins: one providing protocol buffer support for Swift (via Swift Protobuf) and the other for gRPC. You need to use the proto3 compiler (which supports both proto2 and proto3 syntax) in order to generate gRPC services.

For simplicity, we've provided a Makefile in the grpc-swift directory that runs protoc for you with the appropriate plugin, input, and output (if you want to run this yourself, make sure you've installed protoc first):

$ make generate-route-guide

Running this command generates the following files in the Sources/Examples/RouteGuide/Model directory:

• route_guide.pb.swift, which contains the implementation of your generated message classes
• route_guide.grpc.swift, which contains the implementation of your generated service classes

Let's look at how to run the same command manually:

$ protoc Sources/Examples/RouteGuide/Model/route_guide.proto \
    --proto_path=Sources/Examples/RouteGuide/Model \
    --plugin=./.build/debug/protoc-gen-swift \
    --swift_opt=Visibility=Public \
    --swift_out=Sources/Examples/RouteGuide/Model \
    --plugin=./.build/debug/protoc-gen-grpc-swift \
    --grpc-swift_opt=Visibility=Public \
    --grpc-swift_out=Sources/Examples/RouteGuide/Model

We invoke the protocol buffer compiler protoc with the path to our service definition route_guide.proto as well as specifying the path to search for imports. We then specify the path to the Swift Protobuf plugin and any options. In our case the generated code is in a separate module to the client and server, so the generated code must have Public visibility. We also specified that the 'async' client and server should be generated. The 'async' versions use Swift concurrency features introduced in Swift 5.5. We then specify the directory into which the generated messages should be written. The remainder of the arguments are very similar but pertain to the generation of the service code and use the protoc-gen-grpc-swift plugin.

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 protocol 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 provider in grpc-swift/Sources/Examples/RouteGuide/Server/RouteGuideProvider.swift. Let's take a closer look at how it works.

Implementing RouteGuide

As you can see, our server has a RouteGuideProvider class that extends the generated Routeguide_RouteGuideAsyncProvider protocol:

final class RouteGuideProvider: Routeguide_RouteGuideAsyncProvider {
...
}

Simple RPC

RouteGuideProvider 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.

/// A simple RPC.
///
/// Obtains the feature at a given position.
///
/// A feature with an empty name is returned if there's no feature at the given position.
func getFeature(
  request point: Routeguide_Point,
  context: GRPCAsyncServerCallContext
) async throws -> Routeguide_Feature {
  return self.lookupFeature(at: point) ?? Routeguide_Feature.with {
    // No feature was found: return an unnamed feature.
    $0.name = ""
    $0.location = location
  }
}

/// Returns a feature at the given location or an unnamed feature if none exist at that location.
private func lookupFeature(
  at location: Routeguide_Point
) -> Routeguide_Feature? {
  return self.features.first(where: {
    return $0.location.latitude == location.latitude
      && $0.location.longitude == location.longitude
  })
}

getFeature(request:context:) takes two parameters:

• Routeguide_Point: the request
• GRPCAsyncServerCallContext: a context which exposes various pieces of information about the call.

To return our response to the client and complete the call:

1. We construct and populate a Routeguide_Feature response object to return to the client, as specified in our service definition. In this example, we do this in a separate private lookupFeature(at:) method.
2. We return the feature returned from lookupFeature(at:) or an unnamed one if there was no feature at the given location.

Server-side streaming RPC

Next let's look at one of our streaming RPCs. ListFeatures is a server-side streaming RPC, so we need to send back multiple Routeguide_Feature s to our client.

/// A server-to-client streaming RPC.
///
/// 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.
func listFeatures(
  request: Routeguide_Rectangle,
  responseStream: GRPCAsyncResponseStreamWriter<Routeguide_Feature>,
  context: GRPCAsyncServerCallContext
) async throws {
  let longitudeRange = request.lo.longitude ... request.hi.longitude
  let latitudeRange = request.lo.latitude ... request.hi.latitude

  for feature in self.features where !feature.name.isEmpty {
    if feature.location.isWithin(latitude: latitudeRange, longitude: longitudeRange) {
      try await responseStream.send(feature)
    }
  }
}

Like the simple RPC, this method gets a request object (the Routeguide_Rectangle in which our client wants to find Routeguide_Features), a stream to write responses on and a context.

This time, we get as many Routeguide_Feature objects as we need to return to the client (in this case, we select them from the service's feature collection based on whether they're inside our request Routeguide_Rectangle), and write them each in turn to the response stream using send(_:) method on responseStream.

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 Routeguide_Points from the client and return a single Routeguide_RouteSummary with information about their trip.

/// A client-to-server streaming RPC.
///
/// Accepts a stream of Points on a route being traversed, returning a RouteSummary when traversal
/// is completed.
internal func recordRoute(
  requestStream points: GRPCAsyncRequestStream<Routeguide_Point>,
  context: GRPCAsyncServerCallContext
) async throws -> Routeguide_RouteSummary {
  var pointCount: Int32 = 0
  var featureCount: Int32 = 0
  var distance = 0.0
  var previousPoint: Routeguide_Point?
  let startTimeNanos = DispatchTime.now().uptimeNanoseconds

  for try await point in points {
    pointCount += 1

    if let feature = self.lookupFeature(at: point), !feature.name.isEmpty {
      featureCount += 1
    }

    if let previous = previousPoint {
      distance += previous.distance(to: point)
    }

    previousPoint = point
  }

  let durationInNanos = DispatchTime.now().uptimeNanoseconds - startTimeNanos
  let durationInSeconds = Double(durationInNanos) / 1e9

  return .with {
    $0.pointCount = pointCount
    $0.featureCount = featureCount
    $0.elapsedTime = Int32(durationInSeconds)
    $0.distance = Int32(distance)
  }
}

As you can see our method gets a GRPCAsyncServerCallContext parameter and a request stream of points and returns a summary.

In the method body we iterate over the asynchronous stream of points send by the client. For each point we:

• Check if there is a feature at that point.
• Calculate the distance between the point and the last point we saw.

After the client has finished sending points we populate and return a Routeguide_RouteSummary.

Bidirectional streaming RPC

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

func routeChat(
  requestStream: GRPCAsyncRequestStream<Routeguide_RouteNote>,
  responseStream: GRPCAsyncResponseStreamWriter<Routeguide_RouteNote>,
  context: GRPCAsyncServerCallContext
) async throws {
  for try await note in requestStream {
    let existingNotes = await self.notes.addNote(note, to: note.location)

    // Respond with all existing notes.
    for existingNote in existingNotes {
      try await responseStream.send(existingNote)
    }
  }
}

final actor Notes {
  private var recordedNotes: [Routeguide_Point: [Routeguide_RouteNote]] = [:]

  /// Record a note at the given location and return the all notes which were previously recorded
  /// at the location.
  func addNote(
    _ note: Routeguide_RouteNote,
    to location: Routeguide_Point
  ) -> ArraySlice<Routeguide_RouteNote> {
    self.recordedNotes[location, default: []].append(note)
    return self.recordedNotes[location]!.dropLast(1)
  }
}

Here we receive a request stream of Routeguide_RouteNotes and a response stream of Routeguide_RouteNotes as well as the GRPCAsyncServerCallContext we got in other RPCs.

For the route chat for iterate over the stream of notes sent by the client and for each note we add it to a Notes helper actor. When a note is added to the Notes actor all notes previously recorded at the same location are returned and are sent back to the client.

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:

// Create an event loop group for the server to run on.
let group = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
defer {
  try! group.syncShutdownGracefully()
}

// Read the feature database.
let features = try loadFeatures()

// Create a provider using the features we read.
let provider = RouteGuideProvider(features: features)

// Start the server and print its address once it has started.
let server = Server.insecure(group: group)
  .withServiceProviders([provider])
  .bind(host: "localhost", port: 0)

server.map {
  $0.channel.localAddress
}.whenSuccess { address in
  print("server started on port \(address!.port!)")
}

// Wait on the server's `onClose` future to stop the program from exiting.
_ = try server.flatMap {
  $0.onClose
}.wait()

As you can see, we configure and start our server using a builder.

To do this, we:

1. Create an insecure server builder; it's insecure because it does not use TLS.
2. Create an instance of our service implementation class RouteGuideProvider and configure the builder to use it with withServiceProviders(_:),
3. Call bind(host:port:) on the builder with the address and port we want to use to listen for client requests, this starts the server.

Once the server has started succesfully we print out the port the server is listening on. We then wait() on the server's onClose future to stop the program from exiting (since close() is never called on the server).

Creating the client

In this section, we'll look at creating a Swift client for our RouteGuide service. You can see our complete example client code in grpc-swift/Sources/Examples/RouteGuide/Client/main.swift.

Creating a stub

To call service methods, we first need to create a stub. All generated Swift stubs are non-blocking/asynchronous.

First we need to create a gRPC channel for our stub, we're not using TLS so we use the .plaintext security transport and specify the server address and port we want to connect to:

let group = PlatformSupport.makeEventLoopGroup(loopCount: 1)
defer {
  try? group.syncShutdownGracefully()
}

let channel = try GRPCChannelPool.with(
  target: .host("localhost", port: port),
  transportSecurity: .plaintext,
  eventLoopGroup: group
)

let routeGuide = Routeguide_RouteGuideAsyncClient(channel: channel)

Calling service methods

Now let's look at how we call our service methods.

Simple RPC

Calling the simple RPC GetFeature is straightforward.

let point: Routeguide_Point = .with {
  $0.latitude = latitude
  $0.longitude = longitude
}

let feature = try await routeGuide.getFeature(point)

We create and populate a request protocol buffer object (in our case Routeguide_Point), pass it to the getFeature() method on our stub, and await the response Routeguide_Feature.

If an error occurs, it is encoded as a GRPCStatus and thrown whilst await-ing the response.

Server-side streaming RPC

Next, let's look at a server-side streaming call to ListFeatures, which returns a stream of geographical Features:

let rectangle: Routeguide_Rectangle = .with {
  $0.lo = .with {
    $0.latitude = numericCast(lowLatitude)
    $0.longitude = numericCast(lowLongitude)
  }
  $0.hi = .with {
    $0.latitude = numericCast(highLatitude)
    $0.longitude = numericCast(highLongitude)
  }
}

for try await feature in routeGuide.listFeatures(rectangle) {
  print("Received feature: \(feature)")
}

As you can see, it's very similar to the simple RPC we just looked at, except the listFeatures(_:) returns a stream of responses. Here we await each response on the stream, once we finish iterating the response stream the call is complete.

Client-side streaming RPC

Now for something a little more complicated: the client-side streaming method RecordRoute, where we send a stream of Routeguide_Points to the server and get back a single Routeguide_RouteSummary.

let recordRoute = routeGuide.makeRecordRouteCall()

for _ in 1 ... featuresToVisit {
  if let feature = features.randomElement() {
    let point = feature.location
    try await recordRoute.requestStream.send(point)
  }
}

try await recordRoute.requestStream.finish()
let summary = try await recordRoute.response

Here we we create a record route call. It has a request stream and a single await-able response for the Routeguide_RouteSummary.

We call recordRoute.requestStream.send(_:) for each point we want to send to the server and await for the call to accept the request.

Once we've finished writing points, we call recordRoute.requestStream.finish() to tell gRPC that we've finished writing on the client side. Once we're done, we await on the recordRoute.summary to check that the server responded with.

Bidirectional streaming RPC

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

let notes: [Routeguide_RouteNote] = ...

try await withThrowingTaskGroup(of: Void.self) { group in
  let routeChat = self.routeGuide.makeRouteChatCall()

  group.addTask {
    for note in notes {
      try await routeChat.requestStream.send(note)
    }
    try await routeChat.requestStream.finish()
  }

  group.addTask {
    for try await note in routeChat.responseStream {
      print("Received message '\(note.message)' at \(note.location)")
    }
  }

  try await group.waitForAll()
}

As with our client-side streaming example, we have a routeChat call object with a requestStream but a responseStream instead of a single await-able response. In this example we create a task group and create separate tasks for sending requests and receiving responses and await for both to complete.

Try it out!

Follow the instructions in the Route Guide example directory README to build and run the client and server.