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Connecting components

A protocol handle is a well-known object that provides an implementation of a FIDL protocol that is discoverable using component namespaces. The component framework facilitates protocol discovery between components using capabilities. Capability routing describes which component should act as the provider for any given client. Once the proper components are identified, the Component Manager initiates connections between components using handles found in each component's namespace.

Consider the following example for a fuchsia.example.Foo protocol:

Diagram showing how connecting components is a combination of capability
routing and protocol serving. Components must serve the implementation of a
protocol they offer to other components.

The diagram highlights the main elements involved in performing the connection:

  1. The provider component statically declares the protocol in the capabilities section of the manifest. This enables the component framework to perform capability routing.
  2. A client component statically requests the protocol in the use section of the manifest. This creates the /svc/fuchsia.example.Foo protocol entry in the client's namespace if capability routing is successful.
  3. The provider code publishes the implementation at runtime. This creates a protocol entry at /svc/fuchsia.example.Foo in the provider's outgoing directory.
  4. The client code connects to the protocol handle at runtime. This opens a FIDL connection to the implementation running in the provider component.

Publishing a protocol implementation

Components that implement a FIDL protocol declare and expose that protocol as a capability in their component manifest. This enables the component framework to perform capability routing from this component to others in the topology that request the capability.

{
    // ...
    capabilities: [
        { protocol: "fuchsia.example.Foo" },
    ],
    expose: [
        {
            protocol: "fuchsia.example.Foo",
            from: "self",
        },
    ],
}

Capability routing describes the access rights for the protocol, but it does not establish the necessary endpoints for a connection. Components must publish the implementation as an /svc/ handle in the outgoing directory using the fuchsia.io protocol. The generated FIDL bindings wrap this handle and enable the provider to connect a request handle to begin receiving FIDL messages.

// Serve the protocol
FooImplementation instance;
fidl::Binding<fuchsia::example::Foo> binding(&instance);
instance.event_sender_ = &binding.events();
fidl::InterfaceRequestHandler<fuchsia::example::Foo> handler =
    [&](fidl::InterfaceRequest<fuchsia::example::Foo> request) {
      binding.Bind(std::move(request));
    };
context->outgoing()->AddPublicService(std::move(handler));

Connecting to a protocol implementation

Client components declare the protocol as a required capability in their component manifest. This allows the component framework to determine whether the component has the rights to access protocol implementation. If a valid route exists, the component's namespace contains a corresponding /svc/ handle.

{
    // ...
    use: [
        { protocol: "fuchsia.example.Foo" },
    ],
}

The client component uses the fuchsia.io protocol to establish a connection to the protocol implementation and open a channel. The generated FIDL bindings wrap this channel and enable the client to begin sending messages to the provider.

// Connect to FIDL protocol
fuchsia::example::FooSyncPtr proxy;
auto context = sys::ComponentContext::Create();
context->svc()->Connect(proxy.NewRequest());

Exercise: Echo server and client

In this section, you'll use the generated FIDL bindings for fidl.examples.routing.echo to implement client and server components.

Start the emulator

If you do not already have an instance running, start FEMU with networking support:

ffx emu start --headless workstation.qemu-x64

Once the emulator is running, register your local package server with the emulator instance:

ffx target repository register -r fuchsiasamples.com

Create the server component

Begin by creating a new component project to implement the echo server. This component will serve the Echo protocol and handle incoming requests.

Create a new project directory in your Bazel workspace for a new component called echo_server:

mkdir -p fuchsia-codelab/echo-server

This component project should have the following directory structure:

//fuchsia-codelab/echo-server
                  |- BUILD.bazel
                  |- meta
                  |   |- echo_server.cml
                  |
                  |- main.cc

Declare the Echo protocol as a capability provided by the server component, and expose it for use by the parent realm:

echo-server/meta/echo_server.cml:

{
    include: [
        "syslog/client.shard.cml",
        "inspect/client.shard.cml",
    ],

    // Information about the program to run.
    program: {
        // Use the built-in ELF runner.
        runner: "elf",

        // The binary to run for this component.
        binary: "bin/echo_server_cpp",
    },

    // Capabilities provided by this component.
    capabilities: [
        { protocol: "fidl.examples.routing.echo.Echo" },
    ],
    expose: [
        {
            protocol: "fidl.examples.routing.echo.Echo",
            from: "self",
        },
    ],
}

Add the following BUILD.bazel rules to build and package the server component:

echo-server/BUILD.bazel:

load(
    "@rules_fuchsia//fuchsia:defs.bzl",
    "fuchsia_cc_binary",
    "fuchsia_component",
    "fuchsia_component_manifest",
)

fuchsia_cc_binary(
    name = "echo_server_cpp",
    srcs = [
        "main.cc",
    ],
    deps = [
        "//fuchsia-codelab/echo-fidl:fidl.examples.routing.echo.fidl_cc",
        "@fuchsia_sdk//pkg/async-default",
        "@fuchsia_sdk//pkg/async-loop",
        "@fuchsia_sdk//pkg/async-loop-cpp",
        "@fuchsia_sdk//pkg/async-loop-default",
        "@fuchsia_sdk//pkg/fdio",
        "@fuchsia_sdk//pkg/inspect",
        "@fuchsia_sdk//pkg/sys_cpp",
        "@fuchsia_sdk//pkg/sys_inspect_cpp",
        "@fuchsia_sdk//pkg/syslog",
    ],
)

fuchsia_component_manifest(
    name = "manifest",
    src = "meta/echo_server.cml",
    component_name = "echo_server_component",
    includes = [
        "@fuchsia_sdk//pkg/syslog:client",
        "@fuchsia_sdk//pkg/inspect:client",
    ],
)

fuchsia_component(
    name = "echo_server_component",
    component_name = "echo_server_component",
    deps = [
        ':echo_server_cpp',
    ],
    manifest = ":manifest",
    visibility = ["//visibility:public"],
)

Implement the server

Open the main source file and replace the import statements with the following code:

echo-server/main.cc:

#include <fidl/examples/routing/echo/cpp/fidl.h>
#include <lib/async-loop/cpp/loop.h>
#include <lib/async-loop/default.h>
#include <lib/fidl/cpp/binding.h>
#include <lib/sys/cpp/component_context.h>
#include <lib/sys/inspect/cpp/component.h>

Add the following code to implement the protocol request handler:

echo-server/main.cc:

// Handler for incoming service requests
class EchoImplementation : public fidl::examples::routing::echo::Echo {
 public:
  void EchoString(fidl::StringPtr value, EchoStringCallback callback) override { callback(value); }
  fidl::examples::routing::echo::Echo_EventSender* event_sender_;
};

Each Echo protocol method has a corresponding override function (EchoString()) and includes a callback interface to send back the return value.

This implementation simply "echoes" the same string value from the request back in the response payload.

Add the following code to main() to serve the Echo protocol:

echo-server/main.cc:

int main(int argc, const char** argv) {
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
  auto context = sys::ComponentContext::CreateAndServeOutgoingDirectory();

  // Initialize inspect
  sys::ComponentInspector inspector(context.get());
  inspector.Health().StartingUp();

  // Serve the Echo protocol
  EchoImplementation echo_instance;
  fidl::Binding<fidl::examples::routing::echo::Echo> binding(&echo_instance);
  echo_instance.event_sender_ = &binding.events();
  fidl::InterfaceRequestHandler<fidl::examples::routing::echo::Echo> handler =
      [&](fidl::InterfaceRequest<fidl::examples::routing::echo::Echo> request) {
        binding.Bind(std::move(request));
      };
  context->outgoing()->AddPublicService(std::move(handler));

  // Component is serving and ready to handle incoming requests
  inspector.Health().Ok();

  return loop.Run();
}

This code performs the following steps to serve the Echo protocol:

  1. Initialize ComponentContext and add an entry under /svc/fidl.examples.routing.echo.Echo in the outgoing directory.
  2. Serve the directory and begin listening for incoming connections.
  3. Attach the EchoImplementation instance as a request handler for any matching Echo requests.

Create the client component

Create another new component project to implement the echo client. This component will connect to the protocol implementation and send requests.

Create a new project directory in your Bazel workspace for a new component called echo_client:

mkdir -p fuchsia-codelab/echo-client

This component project should have the following directory structure:

//fuchsia-codelab/echo-client
                  |- BUILD.bazel
                  |- meta
                  |   |- echo_client.cml
                  |
                  |- main.cc

Configure the client's component manifest to request the fidl.examples.routing.echo.Echo capability exposed by the server:

echo-client/meta/echo_client.cml:

{
    include: [
        "syslog/client.shard.cml",
    ],

    // Information about the program to run.
    program: {
        // Use the built-in ELF runner.
        runner: "elf",

        // The binary to run for this component.
        binary: "bin/echo_client_cpp",

        // Program arguments
        args: [ "Hello Fuchsia!" ],
    },

    // Capabilities used by this component.
    use: [
        { protocol: "fidl.examples.routing.echo.Echo" },
    ],
}

Add the following BUILD.bazel rules to build and package the server component:

echo-client/BUILD.bazel:

load(
    "@rules_fuchsia//fuchsia:defs.bzl",
    "fuchsia_cc_binary",
    "fuchsia_component",
    "fuchsia_component_manifest",
)

fuchsia_cc_binary(
    name = "echo_client_cpp",
    srcs = [
        "main.cc",
    ],
    deps = [
        "//fuchsia-codelab/echo-fidl:fidl.examples.routing.echo.fidl_cc",
        "@fuchsia_sdk//pkg/async-default",
        "@fuchsia_sdk//pkg/async-loop",
        "@fuchsia_sdk//pkg/async-loop-cpp",
        "@fuchsia_sdk//pkg/async-loop-default",
        "@fuchsia_sdk//pkg/fdio",
        "@fuchsia_sdk//pkg/sys_cpp",
        "@fuchsia_sdk//pkg/syslog",
    ],
)

fuchsia_component_manifest(
    name = "manifest",
    src = "meta/echo_client.cml",
    component_name = "echo_client_component",
    includes = ["@fuchsia_sdk//pkg/syslog:client"],
)

fuchsia_component(
    name = "echo_client_component",
    component_name = "echo_client_component",
    deps = [
        ':echo_client_cpp',
    ],
    manifest = ":manifest",
    visibility = ["//visibility:public"],
)

Implement the client

Similar to echo, the client passes the program arguments as a message to the server. Recall that the arguments are described in the program block of echo_client.cml:

echo-client/meta/echo_client.cml:

// Information about the program to run.
program: {
    // Use the built-in ELF runner.
    runner: "elf",

    // The binary to run for this component.
    binary: "bin/echo_client_cpp",

    // Program arguments
    args: [ "Hello Fuchsia!" ],
},

Open the main source file and replace the import statements with the following code:

echo-client/main.cc:

#include <fidl/examples/routing/echo/cpp/fidl.h>
#include <lib/fidl/cpp/string.h>
#include <lib/sys/cpp/component_context.h>
#include <lib/syslog/global.h>

#include <cstdlib>
#include <iostream>
#include <string>

Add the following code to main() to connect to the Echo protocol and send a request:

echo-client/main.cc:

int main(int argc, const char* argv[], char* envp[]) {
  // Connect to FIDL protocol
  fidl::examples::routing::echo::EchoSyncPtr echo_proxy;
  auto context = sys::ComponentContext::Create();
  context->svc()->Connect(echo_proxy.NewRequest());

  // Send messages over FIDL interface for each argument
  fidl::StringPtr response = nullptr;
  for (int i = 1; i < argc; i++) {
    ZX_ASSERT(echo_proxy->EchoString(argv[i], &response) == ZX_OK);
    if (!response.has_value()) {
      FX_LOG(INFO, "echo_client", "echo_string got empty result");
    } else {
      FX_LOGF(INFO, "echo_client", "Server response: %s", response->c_str());
    }
  }

  return 0;
}

The EchoSyncPtr provides a wrapper to connect to the exposed capability by name and returns a handle to the open proxy interface. This proxy contains the EchoString() FIDL protocol method.

Integrate the components

The capabilities provided by the server must be routed to the client through the component framework. To enable this, you will implement a realm component to act as the parent and manage capability routing.

Create a new project directory for the realm component definition:

mkdir -p fuchsia-codelab/echo-realm/meta

Create a new component manifest file meta/echo_realm.cml with the following contents:

echo-realm/meta/echo_realm.cml:

{
    // Two children: a server and client.
    children: [
        {
            name: "echo_server",
            url: "#meta/echo_server.cm",
        },
        {
            name: "echo_client",
            url: "#meta/echo_client.cm",
        },
    ],

    // Route Echo service from server to client.
    offer: [
        {
            protocol: "fidl.examples.routing.echo.Echo",
            from: "#echo_server",
            to: "#echo_client",
        },
        {
            protocol: "fuchsia.logger.LogSink",
            from: "parent",
            to: [
                "#echo_client",
                "#echo_server",
            ],
        },
    ],
}

This creates a component realm with the server and client as child components, and routes the fidl.examples.routing.echo.Echo protocol capability to the client.

Add a BUILD.bazel file to create a build target for the realm component and a Fuchsia package containing the server and client:

echo-realm/BUILD.bazel:

load(
    "@rules_fuchsia//fuchsia:defs.bzl",
    "fuchsia_component",
    "fuchsia_component_manifest",
    "fuchsia_package",
)

fuchsia_component_manifest(
    name = "manifest",
    src = "meta/echo_realm.cml",
    component_name = "echo_realm",
)

fuchsia_component(
    name = "echo_realm",
    component_name = "echo_realm",
    manifest = ":manifest",
)

fuchsia_package(
    name = "pkg",
    package_name = "echo-realm",
    visibility = ["//visibility:public"],
    deps = [
        ":echo_realm",
        "//fuchsia-codelab/echo-client:echo_client_component",
        "//fuchsia-codelab/echo-server:echo_server_component",
    ],
)

Run bazel build and verify that the build completes successfully:

bazel build --config=fuchsia_x64 //fuchsia-codelab/echo-realm:pkg \
     --publish_to=$HOME/.package_repos/sdk-samples

Add the components to the topology

You will add your component to the ffx-laboratory — a restricted collection used for development inside the product's core realm. Collections enable components to be dynamically created and destroyed at runtime.

Create the component instances by passing the echo-realm component URL and an appropriate moniker inside ffx-laboratory to ffx component run:

ffx component run fuchsia-pkg://fuchsiasamples.com/echo-realm#meta/echo_realm.cm \
    --name echo-realm

Verify that instances of the server and client were also created as child components using ffx component show:

ffx component show echo
               Moniker: /core/ffx-laboratory:echo-realm/echo_client
                   URL: #meta/echo_client.cm
                  Type: CML static component
       Component State: Unresolved
       Execution State: Stopped

               Moniker: /core/ffx-laboratory:echo-realm/echo_server
                   URL: #meta/echo_server.cm
                  Type: CML static component
       Component State: Unresolved
       Execution State: Stopped

               Moniker: /core/ffx-laboratory:echo-realm
                   URL: fuchsia-pkg://fuchsiasamples.com/echo-realm#meta/echo_realm.cm
                  Type: CML dynamic component
       Component State: Resolved
       Execution State: Running
           Merkle root: 666c40477785f89b0ace22b30d65f1338f1d308ecceacb0f65f5140baa889e1b

Verify the component interactions

Start the existing client component instance using ffx component start:

ffx component start /core/ffx-laboratory:echo-realm/echo_client

Open another terminal window and verify the log output from the client component:

ffx log --filter echo

You should see the following output in the device logs:

[echo_client][I] Server response: Hello, Fuchsia!

The server component starts once the client makes a connection to the fidl.examples.routing.echo.Echo capability and continues running to serve additional FIDL requests.

Use ffx component show the see the echo server running in the component instance tree:

ffx component show echo_server
               Moniker: /core/ffx-laboratory:echo-realm/echo_server
                   URL: #meta/echo_server.cm
                  Type: CML static component
       Component State: Resolved
 Incoming Capabilities: fuchsia.logger.LogSink
  Exposed Capabilities: diagnostics
                        fidl.examples.routing.echo.Echo
       Execution State: Running
                Job ID: 474691
            Process ID: 474712
           Running for: 2026280474361 ticks
           Merkle root: 666c40477785f89b0ace22b30d65f1338f1d308ecceacb0f65f5140baa889e1b
 Outgoing Capabilities: diagnostics
                        fidl.examples.routing.echo.Echo

Destroy the instance

Clean up the echo-realm instance using the following command:

ffx component destroy /core/ffx-laboratory:echo-realm