<<../../../_common/fidl/_connecting_intro.md>>
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));
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());
In this section, you'll use the generated FIDL bindings for examples.routing.echo to implement client and server components.
<<../_common/_start_femu_with_packages.md>>
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
After you complete this section, the project should have the following directory structure:
//fuchsia-codelab/echo-server |- BUILD.bazel |- meta | |- echo_server.cml | |- main.cc
Create the echo-server/meta/echo_server.cml component manifest, 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:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/cpp/echo_server/meta/echo_server.cml" region_tag="example_snippet" adjust_indentation="auto" %}
Add the following BUILD.bazel rules to build and package the server component:
echo-server/BUILD.bazel:
{% set build_bazel_snippet %} {% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/cpp/echo_server/BUILD.bazel” region_tag=“imports” adjust_indentation=“auto” %}
{% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/cpp/echo_server/BUILD.bazel” region_tag=“cc_binary” adjust_indentation=“auto” %}
{% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/cpp/echo_server/BUILD.bazel” region_tag=“component” adjust_indentation=“auto” %} {% endset %}
{{ build_bazel_snippet|replace("//src/routing/fidl","//fuchsia-codelab/echo-fidl")|trim() }}
Open the main source file and replace the import statements with the following code:
echo-server/main.cc:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/cpp/echo_server/main.cc" region_tag="imports" adjust_indentation="auto" %}
Add the following code to implement the protocol request handler:
echo-server/main.cc:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/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.
Add the following code to main() to serve the Echo protocol:
echo-server/main.cc:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/cpp/echo_server/main.cc" region_tag="main_body" adjust_indentation="auto" %}
This code performs the following steps to serve the Echo protocol:
ComponentContext and add an entry under /svc/examples.routing.echo.Echo in the outgoing directory.EchoImplementation instance as a request handler for any matching Echo requests.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
After you complete this section, the project should have the following directory structure:
//fuchsia-codelab/echo-client |- BUILD.bazel |- meta | |- echo_client.cml | |- main.cc
Create the echo-client/meta/echo_client.cml component manifest and configure the client component to request the examples.routing.echo.Echo capability exposed by the server:
echo-client/meta/echo_client.cml:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/cpp/echo_client/meta/echo_client.cml" region_tag="example_snippet" adjust_indentation="auto" %}
Add the following BUILD.bazel rules to build and package the client component:
echo-client/BUILD.bazel:
{% set build_bazel_snippet %} {% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/cpp/echo_client/BUILD.bazel” region_tag=“imports” adjust_indentation=“auto” %}
{% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/cpp/echo_client/BUILD.bazel” region_tag=“cc_binary” adjust_indentation=“auto” %}
{% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/cpp/echo_client/BUILD.bazel” region_tag=“component” adjust_indentation=“auto” %} {% endset %}
{{ build_bazel_snippet|replace("//src/routing/fidl","//fuchsia-codelab/echo-fidl")|trim() }}
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:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/routing/cpp/echo_client/meta/echo_client.cml" region_tag="program_args" adjust_indentation="auto" highlight="9,10" %}
Open the main source file and replace the import statements with the following code:
echo-client/main.cc:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/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:
echo-client/main.cc:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/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.
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
After you complete this section, the project should have the following directory structure:
//fuchsia-codelab/echo-realm |- BUILD.bazel |- meta |- echo_realm.cml
Create a new component manifest file echo-realm/meta/echo_realm.cml with the following contents:
echo-realm/meta/echo_realm.cml:
{% includecode gerrit_repo="fuchsia/sdk-samples/getting-started" gerrit_path="src/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 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:
{% set build_bazel_snippet %} {% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/BUILD.bazel” region_tag=“imports” adjust_indentation=“auto” %}
{% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/BUILD.bazel” region_tag=“component” adjust_indentation=“auto” %}
{% includecode gerrit_repo=“fuchsia/sdk-samples/getting-started” gerrit_path=“src/routing/BUILD.bazel” region_tag=“package” adjust_indentation=“auto” %} {% endset %}
{{ build_bazel_snippet |replace("components_routing_example","echo-realm") |replace("//src/routing/cpp/echo_client","//fuchsia-codelab/echo-client") |replace("//src/routing/cpp/echo_server","//fuchsia-codelab/echo-server") |trim() }}
Build and publish the package to the fuchsiasamples.com repository:
bazel run //fuchsia-codelab/echo-realm:pkg.publish -- \ --repo_name fuchsiasamples.com
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://fuchsiasamples.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://fuchsiasamples.com/echo-realm#meta/echo_realm.cm Type: CML dynamic component Component State: Resolved Execution State: Stopped Merkle root: 666c40477785f89b0ace22b30d65f1338f1d308ecceacb0f65f5140baa889e1b
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 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 examples.routing.echo.Echo Execution State: Running Job ID: 474691 Process ID: 474712 Running for: 2026280474361 ticks Merkle root: 666c40477785f89b0ace22b30d65f1338f1d308ecceacb0f65f5140baa889e1b Outgoing Capabilities: diagnostics examples.routing.echo.Echo
Clean up the echo-realm instance using the following command:
ffx component destroy /core/ffx-laboratory:echo-realm