Connecting components

<<../../_common/fidl/_connecting_intro.md>>

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.

• {Rust}

  let mut service_fs = ServiceFs::new_local();
  
  // Serve the protocol
  service_fs.dir("svc").add_fidl_service(PROTOCOL_NAME);
  service_fs.take_and_serve_directory_handle().context("failed to serve outgoing namespace")?;
  

• {C++}

  // 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.

• {Rust}

  // Connect to FIDL protocol
  let protocol = connect_to_protocol::<FooMarker>().expect("error connecting to echo");
  

• {C++}

  // 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 in Rust.

<<../_common/_start_femu_with_packages.md>>

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 project scaffold for a new component called echo-server in the //vendor/fuchsia-codelab directory:

mkdir -p vendor/fuchsia-codelab/echo-server

Create the following file and directory structure in the new project directory:

• {Rust}

  //vendor/fuchsia-codelab/echo-server
                          |- BUILD.gn
                          |- meta
                          |   |- echo.cml
                          |
                          |- src
                              |- main.rs
  

• {C++}

  //vendor/fuchsia-codelab/echo-server
                          |- BUILD.gn
                          |- meta
                          |   |- echo.cml
                          |
                          |- main.cc
  

Add the following build rules to your BUILD.gn file to build and package the server component:

{% set gn_rust_binary %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_server/BUILD.gn” region_tag=“executable” adjust_indentation=“auto” %} {% endset %}

{% set gn_rust_component %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_server/BUILD.gn” region_tag=“component” adjust_indentation=“auto” %} {% endset %}

{% set gn_cpp_binary %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_server/BUILD.gn” region_tag=“executable” adjust_indentation=“auto” %} {% endset %}

{% set gn_cpp_component %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_server/BUILD.gn” region_tag=“component” adjust_indentation=“auto” %} {% endset %}

• {Rust}

  echo-server/BUILD.gn:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_server/BUILD.gn" region_tag="imports" adjust_indentation="auto" %}
  
  {{ gn_rust_binary|replace("echo_server_rust","echo-server")|replace("//examples/components/routing/fidl","//vendor/fuchsia-codelab/echo-fidl")|trim() }}
  
  {{ gn_rust_component|replace("echo_server_component","component")|trim() }}
  
  fuchsia_package("echo-server") {
    package_name = "echo-server"
    deps = [ ":component" ]
  }
  

• {C++}

  echo-server/BUILD.gn:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_server/BUILD.gn" region_tag="imports" adjust_indentation="auto" %}
  
  {{ gn_cpp_binary|replace("echo_server_cpp","echo-server")|replace("//examples/components/routing/fidl","//vendor/fuchsia-codelab/echo-fidl")|trim() }}
  
  {{ gn_cpp_component|replace("echo_server_component","component")|trim() }}
  
  fuchsia_package("echo-server") {
    package_name = "echo-server"
    deps = [ ":component" ]
  }
  

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

{% set cml_rust %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_server/meta/echo_server.cml” region_tag=“example_snippet” adjust_indentation=“auto” %} {% endset %}

{% set cml_cpp %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_server/meta/echo_server.cml” region_tag=“example_snippet” adjust_indentation=“auto” %} {% endset %}

• {Rust}

  echo-server/meta/echo_server.cml:

  {{ cml_rust|replace("echo_server_rust","echo-server")|trim() }}
  

• {C++}

  echo-server/meta/echo_server.cml:

  {{ cml_cpp|replace("echo_server_cpp","echo-server")|trim() }}
  

Implement the server

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

• {Rust}

  echo-server/src/main.rs:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_server/src/main.rs" region_tag="imports" adjust_indentation="auto" %}
  

• {C++}

  echo-server/main.cc:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_server/main.cc" region_tag="imports" adjust_indentation="auto" %}
  

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

• {Rust}

  echo-server/src/main.rs:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_server/src/main.rs" region_tag="main_body" adjust_indentation="auto" %}
  

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

  1. Initialize ServiceFs 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 handle_echo_request() function as a request handler for any matching Echo requests.

• {C++}

  echo-server/main.cc:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_server/main.cc" region_tag="main_body" adjust_indentation="auto" %}
  

  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.

Add the following code to implement the protocol request handler:

• {Rust}

  echo-server/src/main.rs:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_server/src/main.rs" region_tag="handler" adjust_indentation="auto" %}
  

  Each request in the EchoRequestStream is typed by the method name (EchoString) and includes a responder interface to send back the return value.

• {C++}

  echo-server/main.cc:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_server/main.cc" region_tag="handler" adjust_indentation="auto" %}
  

  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.

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 project scaffold for a new component called echo-client in the //vendor/fuchsia-codelab directory:

mkdir -p vendor/fuchsia-codelab/echo-client

Create the following file and directory structure in the new project directory:

• {Rust}

  //vendor/fuchsia-codelab/echo-client
                          |- BUILD.gn
                          |- meta
                          |   |- echo.cml
                          |
                          |- src
                              |- main.rs
  

• {C++}

  //vendor/fuchsia-codelab/echo-client
                          |- BUILD.gn
                          |- meta
                          |   |- echo.cml
                          |
                          |- main.cc
  

Add the following build rules to your BUILD.gn file to build and package the client component:

{% set gn_rust_binary %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_client/BUILD.gn” region_tag=“executable” adjust_indentation=“auto” %} {% endset %}

{% set gn_rust_component %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_client/BUILD.gn” region_tag=“component” adjust_indentation=“auto” %} {% endset %}

{% set gn_cpp_binary %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_client/BUILD.gn” region_tag=“executable” adjust_indentation=“auto” %} {% endset %}

{% set gn_cpp_component %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_client/BUILD.gn” region_tag=“component” adjust_indentation=“auto” %} {% endset %}

• {Rust}

  echo-client/BUILD.gn:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_client/BUILD.gn" region_tag="imports" adjust_indentation="auto" %}
  
  {{ gn_rust_binary|replace("echo_client_rust","echo-client")|replace("//examples/components/routing/fidl","//vendor/fuchsia-codelab/echo-fidl")|trim() }}
  
  {{ gn_rust_component|replace("echo_client_component","component")|trim() }}
  
  fuchsia_package("echo-client") {
    package_name = "echo-client"
    deps = [ ":component" ]
  }
  

• {C++}

  echo-client/BUILD.gn:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_client/BUILD.gn" region_tag="imports" adjust_indentation="auto" %}
  
  {{ gn_cpp_binary|replace("echo_client_cpp","echo-client")|replace("//examples/components/routing/fidl","//vendor/fuchsia-codelab/echo-fidl")|trim() }}
  
  {{ gn_cpp_component|replace("echo_client_component","component")|trim() }}
  
  fuchsia_package("echo-client") {
    package_name = "echo-client"
    deps = [ ":component" ]
  }
  

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

{% set cml_rust %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_client/meta/echo_client.cml” region_tag=“example_snippet” adjust_indentation=“auto” %} {% endset %}

{% set cml_cpp %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_client/meta/echo_client.cml” region_tag=“example_snippet” adjust_indentation=“auto” %} {% endset %}

• {Rust}

  echo-client/meta/echo_client.cml:

  {{ cml_rust|replace("echo_client_rust","echo-client")|trim() }}
  

• {C++}

  echo-client/meta/echo_client.cml:

  {{ cml_cpp|replace("echo_client_cpp","echo-client")|trim() }}
  

Implement the client

Similar to echo-args, the client passes the program arguments as a message to the server. Add the following program arguments to echo_client.cml:

{% set cml_rust %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/rust/echo_client/meta/echo_client.cml” region_tag=“program_args” adjust_indentation=“auto” highlight=“9,10” %} {% endset %}

{% set cml_cpp %} {% includecode gerrit_repo=“fuchsia/fuchsia” gerrit_path=“examples/components/routing/cpp/echo_client/meta/echo_client.cml” region_tag=“program_args” adjust_indentation=“auto” highlight=“9,10” %} {% endset %}

• {Rust}

  echo-client/meta/echo_client.cml:

  {{ cml_rust|replace("echo_client_rust","echo-client")|trim() }}
  

• {C++}

  echo-client/meta/echo_client.cml:

  {{ cml_cpp|replace("echo_client_cpp","echo-client")|trim() }}
  

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

• {Rust}

  echo-client/src/main.rs:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_client/src/main.rs" region_tag="imports" adjust_indentation="auto" %}
  

• {C++}

  echo-client/main.cc:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_client/main.cc" region_tag="imports" adjust_indentation="auto" %}
  

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

• {Rust}

  echo-client/src/main.rs:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/rust/echo_client/src/main.rs" region_tag="main_body" adjust_indentation="auto" %}
  

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

• {C++}

  echo-client/main.cc:

  {% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/cpp/echo_client/main.cc" region_tag="main_body" adjust_indentation="auto" %}
  

  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 product definition:

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

Create the following file and directory structure in the new project directory:

//vendor/fuchsia-codelab/echo-realm
                        |- BUILD.gn
                        |- meta
                        |   |- echo_realm.cml

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

echo-realm/meta/echo_realm.cml:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/components/routing/meta/echo_realm.cml" region_tag="example_snippet" adjust_indentation="auto" %}

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.gn file to create a build target for the realm component:

echo-realm/BUILD.gn:

import("//build/components.gni")

fuchsia_component("echo_realm") {
  manifest = "meta/echo_realm.cml"
}

fuchsia_package("echo-realm") {
  deps = [
    ":echo_realm",
    "//vendor/fuchsia-codelab/echo-server:component",
    "//vendor/fuchsia-codelab/echo-client:component",
  ]
}

Update the build configuration to include the new components:

fx set workstation_eng.x64 \
    --with //vendor/fuchsia-codelab/echo-server \
    --with //vendor/fuchsia-codelab/echo-client \
    --with //vendor/fuchsia-codelab/echo-realm

Run fx build again to build the components:

fx build

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 create:

ffx component create /core/ffx-laboratory:echo-realm \
    fuchsia-pkg://fuchsia.com/echo-realm#meta/echo_realm.cm

Then, resolve the echo-realm component with ffx component resolve:

ffx component resolve /core/ffx-laboratory: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://fuchsia.com/echo-realm#meta/echo_realm.cm
                  Type: CML dynamic component
       Component State: Resolved
       Execution State: Stopped
           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