Handling multiple clients

Prerequisites

This tutorial builds on the HLCPP getting started tutorials.

Overview

This tutorial updates the Echo client from the getting started tutorials to make multiple connections to the server, and update the Echo server to handle multiple client connections. For running multiple instances of a server (or multiple FIDL protocols), see the tutorial on services.

The full example code for this tutorial is located at //examples/fidl/hlcpp/multiple_clients.

Implement the server

In the previous implementation, the main() function initialized a single fidl::Binding, and bound any incoming requests to it:

int main(int argc, const char** argv) {
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);

  EchoImpl impl;
  fidl::Binding<fuchsia::examples::Echo> binding(&impl);
  impl.event_sender_ = &binding.events();
  fidl::InterfaceRequestHandler<fuchsia::examples::Echo> handler =
      [&](fidl::InterfaceRequest<fuchsia::examples::Echo> request) {
        binding.Bind(std::move(request));
      };
  auto context = sys::ComponentContext::CreateAndServeOutgoingDirectory();
  context->outgoing()->AddPublicService(std::move(handler));

  printf("Running echo server\n");
  return loop.Run();
}

This means that if a second client tries to connect to the server at the same time, the second call to binding.Bind will overwrite the channel from the first client. To support multiple clients, keep track of multiple fidl::Bindings (one for each client) using a fidl::BindingSet:

int main(int argc, const char** argv) {
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);

  EchoImpl impl;
  fidl::BindingSet<fuchsia::examples::Echo> bindings;
  auto context = sys::ComponentContext::CreateAndServeOutgoingDirectory();
  context->outgoing()->AddPublicService(bindings.GetHandler(&impl));

  printf("Running echo server\n");
  return loop.Run();
}

A binding set also simplifies the code since it no longer needs to create a custom handler. The binding set has a GetHandler method, which returns a handler that creates a new Binding and stores it in a vector.

To use fidl::BindingSet, include lib/fidl/cpp/binding_set.h.

Implement the client

In order to manage multiple clients connected to a protocol, the FIDL HLCPP runtime library provides an anolog to fidl::BindingSet: the fidl::InterfacePtrSet. Use the class to write code that makes multiple connections to the same protocol:

int main(int argc, const char** argv) {
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);

  auto context = sys::ComponentContext::Create();

  fidl::InterfacePtrSet<fuchsia::examples::Echo> echoers;
  for (int i = 0; i < kNumClients; i++) {
    fuchsia::examples::EchoPtr proxy;
    context->svc()->Connect(proxy.NewRequest());
    proxy.set_error_handler([&loop](zx_status_t status) {
      std::cout << "Error reading incoming message: " << status << std::endl;
      loop.Quit();
    });
    echoers.AddInterfacePtr(std::move(proxy));
  }

  size_t responses = 0;
  for (auto& echoer : echoers.ptrs()) {
    (*echoer)->EchoString("Hello echoer " + std::to_string(responses++), [&](std::string response) {
      std::cout << "Got response " << response << std::endl;
      if (responses == echoers.size()) {
        loop.Quit();
      }
    });
  }

  loop.Run();
  return responses == kNumClients ? 0 : 1;
}

The code for setting up a proxy and making requests is the same as in the client tutorial except it uses an interface pointer set to simplify the process of broadcasting a message to a set of clients. An added benefit of using fidl::InterfacePtrSet and fidl::BindingSet is that any binding or interface pointer that experiences an error on its channel is automatically removed from the set.

To use fidl::InterfacePtrSet, include lib/fidl/cpp/interface_ptr_set.h.

Run the example

In order for the client and server to communicate using the Echo protocol, component framework must route the fuchsia.examples.Echo capability from the server to the client. For this tutorial, a realm component is provided to declare the appropriate capabilities and routes.

Configure your build to include the provided package that includes the echo realm, server, and client:
```
fx set core.x64 --with //examples/fidl/hlcpp:echo-hlcpp-multi-client
```
Build the Fuchsia image:
```
fx build
```

Run the echo_realm component. This creates the client and server component instances and routes the capabilities:

ffx component run /core/ffx-laboratory:echo_realm fuchsia-pkg://fuchsia.com/echo-hlcpp-multi-client#meta/echo_realm.cm

Start the echo_client instance:

ffx component start /core/ffx-laboratory:echo_realm/echo_client

The server component starts when the client attempts to connect to the Echo protocol. You should see output similar to the following in the device logs (ffx log):

[echo_server][][I] Running echo server
[echo_client][][I] Got response Hello echoer 0

Terminate the realm component to stop execution and clean up the component instances:

ffx component destroy /core/ffx-laboratory:echo_realm