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.
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.
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
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;
}
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 classesroute_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,AsyncClient=True,AsyncServer=True \
--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.
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.
As you can see, our server has a RouteGuideProvider
class that extends the
generated Routeguide_RouteGuideAsyncProvider
protocol:
final class RouteGuideProvider: Routeguide_RouteGuideAsyncProvider {
...
}
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 requestGRPCAsyncServerCallContext
: a context which exposes various pieces of information about the call.
To return our response to the client and complete the call:
- 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 privatelookupFeature(at:)
method. - We return the feature returned from
lookupFeature(at:)
or an unnamed one if there was no feature at the given location.
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_Feature
s),
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
.
Now let's look at something a little more complicated: the client-side streaming
method RecordRoute
, where we get a stream of Routeguide_Point
s 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
.
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_RouteNote
s and a response
stream of Routeguide_RouteNote
s 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.
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:
- Create an insecure server builder; it's insecure because it does not use TLS.
- Create an instance of our service implementation class
RouteGuideProvider
and configure the builder to use it withwithServiceProviders(_:)
, - 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).
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.
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)
Now let's look at how we call our service methods.
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.
Next, let's look at a server-side streaming call to ListFeatures
, which
returns a stream of geographical Feature
s:
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.
Now for something a little more complicated: the client-side streaming method
RecordRoute
, where we send a stream of Routeguide_Point
s 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.
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.
Follow the instructions in the Route Guide example directory README to build and run the client and server.