Basics tutorial
A basic tutorial introduction to gRPC in C++.
Basics tutorial
This tutorial provides a basic C++ 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 C++ gRPC API to write a simple client and server for your service.
It assumes that you have read the Introduction to gRPC 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 and C++ generated code 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 generate clients
and servers in any of gRPC’s supported languages, which in turn can be run in
environments ranging from servers inside a large data center 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 is part of the grpc
repo under examples/cpp/route_guide.
Get the example code and build gRPC:
Follow the Quick start instructions to build and locally install gRPC from source.
From the repo folder, change to the route guide example directory:
$ cd examples/cpp/route_guide
Run
cmake
$ mkdir -p cmake/build $ cd cmake/build $ cmake -DCMAKE_PREFIX_PATH=$MY_INSTALL_DIR ../..
Defining the service
Our first step (as you’ll know from the Introduction to gRPC) is to
define the gRPC service and the method request and response types using
protocol buffers.
You can see the complete .proto
file in
examples/protos/route_guide.proto
.
To define a service, you specify a named service
in your .proto
file:
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.
// 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
a special gRPC C++ plugin.
For simplicity, we’ve provided a CMakeLists.txt
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 and followed
the gRPC code installation instructions first):
$ make route_guide.grpc.pb.o
which actually runs:
$ protoc -I ../../protos --grpc_out=. --plugin=protoc-gen-grpc=`which grpc_cpp_plugin` ../../protos/route_guide.proto
$ protoc -I ../../protos --cpp_out=. ../../protos/route_guide.proto
Running this command generates the following files in your current directory:
route_guide.pb.h
, the header which declares your generated message classesroute_guide.pb.cc
, which contains the implementation of your message classesroute_guide.grpc.pb.h
, the header which declares your generated service classesroute_guide.grpc.pb.cc
, which contains the implementation of your service classes
These contain:
All the protocol buffer code to populate, serialize, and retrieve our request and response message types
A class called
RouteGuide
that contains- a remote interface type (or stub) for clients to call with the methods
defined in the
RouteGuide
service. - two abstract interfaces for servers to implement, also with the methods
defined in the
RouteGuide
service.
- a remote interface type (or stub) for clients to call with the methods
defined in the
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 interface 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
server in
examples/cpp/route_guide/route_guide_server.cc.
Let’s take a closer look at how it works.
Implementing RouteGuide
As you can see, our server has a RouteGuideImpl
class that implements the
generated RouteGuide::Service
interface:
class RouteGuideImpl final : public RouteGuide::Service {
...
}
In this case we’re implementing the synchronous version of RouteGuide
, which
provides our default gRPC server behaviour. It’s also possible to implement an
asynchronous interface, RouteGuide::AsyncService
, which allows you to further
customize your server’s threading behaviour, though we won’t look at this in
this tutorial.
RouteGuideImpl
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
.
Status GetFeature(ServerContext* context, const Point* point,
Feature* feature) override {
feature->set_name(GetFeatureName(*point, feature_list_));
feature->mutable_location()->CopyFrom(*point);
return Status::OK;
}
The method is passed a context object for the RPC, the client’s Point
protocol
buffer request, and a Feature
protocol buffer to fill in with the response
information. In the method we populate the Feature
with the appropriate
information, and then return
with an OK
status to tell gRPC that we’ve
finished dealing with the RPC and that the Feature
can be returned to the
client.
Note that all service methods can (and will!) be called from multiple threads at
the same time. You have to make sure that your method implementations are
thread safe. In our example, feature_list_
is never changed after
construction, so it is safe by design. But if feature_list_
would change during
the lifetime of the service, we would need to synchronize access to this member.
Now let’s look at something a bit more complicated - a streaming RPC.
ListFeatures
is a server-side streaming RPC, so we need to send back multiple
Feature
s to our client.
Status ListFeatures(ServerContext* context, const Rectangle* rectangle,
ServerWriter<Feature>* writer) override {
auto lo = rectangle->lo();
auto hi = rectangle->hi();
long left = std::min(lo.longitude(), hi.longitude());
long right = std::max(lo.longitude(), hi.longitude());
long top = std::max(lo.latitude(), hi.latitude());
long bottom = std::min(lo.latitude(), hi.latitude());
for (const Feature& f : feature_list_) {
if (f.location().longitude() >= left &&
f.location().longitude() <= right &&
f.location().latitude() >= bottom &&
f.location().latitude() <= top) {
writer->Write(f);
}
}
return Status::OK;
}
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 ServerWriter
object. In the
method, we populate as many Feature
objects as we need to return, writing them
to the ServerWriter
using its Write()
method. Finally, as in our simple RPC,
we return Status::OK
to tell gRPC that we’ve finished writing responses.
If you look at the client-side streaming method RecordRoute
you’ll see it’s
quite similar, except this time we get a ServerReader
instead of a request
object and a single response. We use the ServerReader
s Read()
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 return
value of Read()
after each call. If true
, the stream is still good and it
can continue reading; if false
the message stream has ended.
while (stream->Read(&point)) {
...//process client input
}
Finally, let’s look at our bidirectional streaming RPC RouteChat()
.
Status RouteChat(ServerContext* context,
ServerReaderWriter<RouteNote, RouteNote>* stream) override {
RouteNote note;
while (stream->Read(¬e)) {
std::unique_lock<std::mutex> lock(mu_);
for (const RouteNote& n : received_notes_) {
if (n.location().latitude() == note.location().latitude() &&
n.location().longitude() == note.location().longitude()) {
stream->Write(n);
}
}
received_notes_.push_back(note);
}
return Status::OK;
}
This time we get a ServerReaderWriter
that can be used to read and write
messages. The syntax for reading and writing here is exactly the same as for our
client-streaming and server-streaming methods. 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.
Note that since received_notes_
is an instance variable and can be accessed by
multiple threads, we use a mutex lock here to guarantee exclusive access.
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:
void RunServer(const std::string& db_path) {
std::string server_address("0.0.0.0:50051");
RouteGuideImpl service(db_path);
ServerBuilder builder;
builder.AddListeningPort(server_address, grpc::InsecureServerCredentials());
builder.RegisterService(&service);
std::unique_ptr<Server> server(builder.BuildAndStart());
std::cout << "Server listening on " << server_address << std::endl;
server->Wait();
}
As you can see, we build and start our server using a ServerBuilder
. To do this, we:
- Create an instance of our service implementation class
RouteGuideImpl
. - Create an instance of the factory
ServerBuilder
class. - Specify the address and port we want to use to listen for client requests
using the builder’s
AddListeningPort()
method. - Register our service implementation with the builder.
- Call
BuildAndStart()
on the builder to create and start an RPC server for our service. - Call
Wait()
on the server to do a blocking wait until process is killed orShutdown()
is called.
Creating the client
In this section, we’ll look at creating a C++ client for our RouteGuide
service. You can see our complete example client code in
examples/cpp/route_guide/route_guide_client.cc.
Creating a stub
To call service methods, we first need to create a stub.
First we need to create a gRPC channel for our stub, specifying the server address and port we want to connect to - in our case we’ll use no SSL:
grpc::CreateChannel("localhost:50051", grpc::InsecureChannelCredentials());
Note
In order to set additional options for the channel, use thegrpc::CreateCustomChannel()
api with any special channel arguments - grpc::ChannelArguments
.Now we can use the channel to create our stub using the NewStub
method provided in the RouteGuide
class we generated from our .proto
.
public:
RouteGuideClient(std::shared_ptr<ChannelInterface> channel,
const std::string& db)
: stub_(RouteGuide::NewStub(channel)) {
...
}
Calling service methods
Now let’s look at how we call our service methods. Note that in this tutorial we’re calling the blocking/synchronous versions of each method: this means that the RPC call waits for the server to respond, and will either return a response or raise an exception.
Simple RPC
Calling the simple RPC GetFeature
is nearly as straightforward as calling a
local method.
Point point;
Feature feature;
point = MakePoint(409146138, -746188906);
GetOneFeature(point, &feature);
...
bool GetOneFeature(const Point& point, Feature* feature) {
ClientContext context;
Status status = stub_->GetFeature(&context, point, feature);
...
}
As you can see, we create and populate a request protocol buffer object (in our
case Point
), and create a response protocol buffer object for the server to
fill in. We also create a ClientContext
object for our call - you can
optionally set RPC configuration values on this object, such as deadlines,
though for now we’ll use the default settings. Note that you cannot reuse this
object between calls. Finally, we call the method on the stub, passing it the
context, request, and response. If the method returns OK
, then we can read the
response information from the server from our response object.
std::cout << "Found feature called " << feature->name() << " at "
<< feature->location().latitude()/kCoordFactor_ << ", "
<< feature->location().longitude()/kCoordFactor_ << std::endl;
Streaming RPCs
Now let’s look at our streaming methods. 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. Here’s where we call the server-side
streaming method ListFeatures
, which returns a stream of geographical
Feature
s:
std::unique_ptr<ClientReader<Feature> > reader(
stub_->ListFeatures(&context, rect));
while (reader->Read(&feature)) {
std::cout << "Found feature called "
<< feature.name() << " at "
<< feature.location().latitude()/kCoordFactor_ << ", "
<< feature.location().longitude()/kCoordFactor_ << std::endl;
}
Status status = reader->Finish();
Instead of passing the method a context, request, and response, we pass it a
context and request and get a ClientReader
object back. The client can use the
ClientReader
to read the server’s responses. We use the ClientReader
s
Read()
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 return value of Read()
after each
call. If true
, the stream is still good and it can continue reading; if
false
the message stream has ended. Finally, we call Finish()
on the stream
to complete the call and get our RPC status.
The client-side streaming method RecordRoute
is similar, except there we pass
the method a context and response object and get back a ClientWriter
.
std::unique_ptr<ClientWriter<Point> > writer(
stub_->RecordRoute(&context, &stats));
for (int i = 0; i < kPoints; i++) {
const Feature& f = feature_list_[feature_distribution(generator)];
std::cout << "Visiting point "
<< f.location().latitude()/kCoordFactor_ << ", "
<< f.location().longitude()/kCoordFactor_ << std::endl;
if (!writer->Write(f.location())) {
// Broken stream.
break;
}
std::this_thread::sleep_for(std::chrono::milliseconds(
delay_distribution(generator)));
}
writer->WritesDone();
Status status = writer->Finish();
if (status.IsOk()) {
std::cout << "Finished trip with " << stats.point_count() << " points\n"
<< "Passed " << stats.feature_count() << " features\n"
<< "Travelled " << stats.distance() << " meters\n"
<< "It took " << stats.elapsed_time() << " seconds"
<< std::endl;
} else {
std::cout << "RecordRoute rpc failed." << std::endl;
}
Once we’ve finished writing our client’s requests to the stream using Write()
,
we need to call WritesDone()
on the stream to let gRPC know that we’ve
finished writing, then Finish()
to complete the call and get our RPC status.
If the status is OK
, our response object that we initially passed to
RecordRoute()
will be populated with the server’s response.
Finally, let’s look at our bidirectional streaming RPC RouteChat()
. In this
case, we just pass a context to the method and get back a ClientReaderWriter
,
which we can use to both write and read messages.
std::shared_ptr<ClientReaderWriter<RouteNote, RouteNote> > stream(
stub_->RouteChat(&context));
The syntax for reading and writing here is exactly the same as for our client-streaming and server-streaming methods. 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!
Build the client and server:
$ make
Run the server:
$ ./route_guide_server --db_path=path/to/route_guide_db.json
From a different terminal, run the client:
$ ./route_guide_client --db_path=path/to/route_guide_db.json