Protocol request pipelining in Dart

Prerequisites

This tutorial builds on the Dart getting started tutorials.

Overview

A common aspect of using FIDL on Fuchsia is passing protocols themselves across protocols. Many FIDL messages include either the client end or the server end of a channel, where the channel is used to communicate over a different FIDL protocol. In this case, client end means that the remote end of the channel implements the specified protocol, whereas server end means that the remote end is making requests for the specified protocol. An alternate set of terms for client end and server end are protocol and protocol request.

This tutorial covers:

The usage of these client and server ends, both in FIDL and in the Dart FIDL bindings.
The request pipelining pattern and its benefits.

The full example code for this tutorial is located at //examples/fidl/dart/request_pipelining.

The FIDL protocol

This tutorial implements the EchoLauncher protocol from the fuchsia.examples library:

@discoverable
protocol EchoLauncher {
    GetEcho(struct {
        echo_prefix string:MAX_STRING_LENGTH;
    }) -> (resource struct {
        response client_end:Echo;
    });
    GetEchoPipelined(resource struct {
        echo_prefix string:MAX_STRING_LENGTH;
        request server_end:Echo;
    });
};

This is a protocol that lets clients retrieve an instance of the Echo protocol. Clients can specify a prefix, and the resulting Echo instance adds that prefix to every response.

There are two methods that can be used to accomplish this:

GetEcho: Takes the prefix as a request, and responds with the client end of a channel connected to an implementation of the Echo protocol. After receiving the client end in the response, the client can start making requests on the Echo protocol using the client end.
GetEchoPipelined: Takes the server end of a channel as one of the request parameters and binds an implementation of Echo to it. The client that made the request is assumed to already hold the client end, and will start making Echo requests on that channel after calling GetEchoPipeliend.

As the name suggests, the latter uses a pattern called protocol request pipelining, and is the preferred approach. This tutorial implements both approaches.

Implement the server

Implement the Echo protocol

This implementation of Echo allows specifying a prefix in order to distinguish between the different instances of Echo servers:

// Implementation of Echo that responds with a prefix prepended to each response
class _EchoImpl extends fidl_echo.Echo {
  // The EchoBinding is added as a member to make serving the protocol easier.
  final _binding = fidl_echo.EchoBinding();
  late final String prefix;

  _EchoImpl({required this.prefix});

  void bind(fidl.InterfaceRequest<fidl_echo.Echo> request) {
    _binding.bind(this, request);
  }

  // Reply to EchoString with a possibly reversed string
  @override
  Future<String> echoString(String value) async {
    return prefix + value;
  }

  // SendString isn't used for the purposes of this example
  @override
  Future<void> sendString(String value) async {}

  // OnString isn't used for the purposes of this example, so just return an empty stream
  @override
  Stream<String> get onString => Stream.empty();
}

The SendString handler is empty as the client just uses EchoString.

Additionally, the class holds an EchoBinding property to simplify the process of binding the server to a channel.

Implement the EchoLauncher protocol

This class uses stores a list of all of the instances of Echo that it launches:

// Implementation of EchoLauncher that will launch an Echo instance that
// responds with the specified prefix.
class _EchoLauncherImpl extends fidl_echo.EchoLauncher {
  final List<_EchoImpl> servers = [];

  // For the non pipelined method, the server needs to create a channel pair,
  // bind an Echo server to the server end, then send the client end back to the
  // client
  @override
  Future<fidl.InterfaceHandle<fidl_echo.Echo>> getEcho(String prefix) async {
    final echoPair = fidl.InterfacePair<fidl_echo.Echo>();
    final serverEnd = echoPair.passRequest();
    final clientEnd = echoPair.passHandle();

    // Throw exception if serverEnd or clientEnd is null
    launchEchoServer(prefix, serverEnd!);
    return clientEnd!;
  }

  // For the pipelined method, the client provides the server end of the channel
  // so we can simply call launchEchoServer
  @override
  Future<void> getEchoPipelined(
      String prefix, fidl.InterfaceRequest<fidl_echo.Echo> serverEnd) async {
    launchEchoServer(prefix, serverEnd);
  }

  // Launches a new echo server that uses the specified prefix, and binds it to
  // the provided InterfaceRequest. Each launched server is stored in the
  // servers member so that it doesn't get garbage collected.
  void launchEchoServer(
      String prefix, fidl.InterfaceRequest<fidl_echo.Echo> serverEnd) {
    servers.add(_EchoImpl(prefix: prefix)..bind(serverEnd));
  }
}

Both of the EchoLauncher methods are handled by calling the launchEchoServer helper method on the server end of the channel. The difference is that in getEcho, the server is responsible for initializing the channel - it uses one end as the server end and sends the other end back to the client. In getEchoPipelined, the server end is provided as part of the request, so no additional work needs to be done by the server, and no response is necessary.

Serve the EchoLauncher protocol

The main loop should is the same as in the server tutorial but serves an EchoLauncher instead of Echo.

void main(List<String> args) {
  setupLogger(name: 'echo-launcher-server');
  final context = ComponentContext.create();
  final echoLauncher = _EchoLauncherImpl();
  final binding = fidl_echo.EchoLauncherBinding();

  log.info('Running EchoLauncher server');
  context.outgoing
    ..addPublicService<fidl_echo.EchoLauncher>(
        (fidl.InterfaceRequest<fidl_echo.EchoLauncher> serverEnd) =>
            binding.bind(echoLauncher, serverEnd),
        fidl_echo.EchoLauncher.$serviceName)
    ..serveFromStartupInfo();
}

Build the server

Optionally, to check that things are correct, try building the server:

Configure your GN build to include the server:

fx set core.x64 --with //examples/fidl/dart/request_pipelining/server:echo-server

Build the Fuchsia image:
```
fx build
```

Implement the client

After connecting to the EchoLauncher server, the client code connects to one instance of Echo using GetEcho and another using GetEchoPipelined and then makes an EchoString request on each instance.

This is the non-pipelined code:

Future<void> main(List<String> args) async {
  final context = ComponentContext.createAndServe();
  setupLogger(name: 'echo-launcher-client');

  // Connect to the EchoLauncher service
  final echoLauncher = fidl_echo.EchoLauncherProxy();
  context.svc.connectToService(echoLauncher);

  // Non pipelined case: wait for EchoLauncher to respond with a client end, then bind it to the
  // proxy and make an EchoString request
  final nonPipelinedFut =
      echoLauncher.getEcho('not pipelined: ').then((clientEnd) async {
    final nonPipelinedEcho = fidl_echo.EchoProxy()..ctrl.bind(clientEnd);
    final response = await nonPipelinedEcho.echoString('hello');
    log.info('Got echo response $response');
  });

  final pipelinedEcho = fidl_echo.EchoProxy();
  // Pipelined case: make a request with the server end of the proxy
  unawaited(echoLauncher.getEchoPipelined(
      'pipelined: ', pipelinedEcho.ctrl.request()));
  // Then, make an EchoString request with the proxy without needing to wait for
  // a response.
  final pipelinedFut = pipelinedEcho
      .echoString('hello')
      .then((response) => log.info('Got echo response $response'));

  // Run the two futures concurrently.
  await Future.wait([nonPipelinedFut, pipelinedFut]);
  await Future(() => exit(0));
}

This code chains together two futures. First, it makes the GetEcho request to the client. It then takes the result of that future (a channel), and binds it the non pipelined client object, calls EchoString on it, and then blocks on the result using await.

The pipelined code is much simpler:

Future<void> main(List<String> args) async {
  final context = ComponentContext.createAndServe();
  setupLogger(name: 'echo-launcher-client');

  // Connect to the EchoLauncher service
  final echoLauncher = fidl_echo.EchoLauncherProxy();
  context.svc.connectToService(echoLauncher);

  // Non pipelined case: wait for EchoLauncher to respond with a client end, then bind it to the
  // proxy and make an EchoString request
  final nonPipelinedFut =
      echoLauncher.getEcho('not pipelined: ').then((clientEnd) async {
    final nonPipelinedEcho = fidl_echo.EchoProxy()..ctrl.bind(clientEnd);
    final response = await nonPipelinedEcho.echoString('hello');
    log.info('Got echo response $response');
  });

  final pipelinedEcho = fidl_echo.EchoProxy();
  // Pipelined case: make a request with the server end of the proxy
  unawaited(echoLauncher.getEchoPipelined(
      'pipelined: ', pipelinedEcho.ctrl.request()));
  // Then, make an EchoString request with the proxy without needing to wait for
  // a response.
  final pipelinedFut = pipelinedEcho
      .echoString('hello')
      .then((response) => log.info('Got echo response $response'));

  // Run the two futures concurrently.
  await Future.wait([nonPipelinedFut, pipelinedFut]);
  await Future(() => exit(0));
}

The call to pipelinedEcho.ctrl.request() creates a channel, binds the client object to one end, then returns the other. The return value in this case gets passed to the call to GetEchoPipelined. After the call to GetEchoPipelined, the client can immediately make the EchoString request.

Finally, the two futures corresponding to the non-pipelined and pipelined calls are run to completion concurrently, to see which one completes first:

Future<void> main(List<String> args) async {
  final context = ComponentContext.createAndServe();
  setupLogger(name: 'echo-launcher-client');

  // Connect to the EchoLauncher service
  final echoLauncher = fidl_echo.EchoLauncherProxy();
  context.svc.connectToService(echoLauncher);

  // Non pipelined case: wait for EchoLauncher to respond with a client end, then bind it to the
  // proxy and make an EchoString request
  final nonPipelinedFut =
      echoLauncher.getEcho('not pipelined: ').then((clientEnd) async {
    final nonPipelinedEcho = fidl_echo.EchoProxy()..ctrl.bind(clientEnd);
    final response = await nonPipelinedEcho.echoString('hello');
    log.info('Got echo response $response');
  });

  final pipelinedEcho = fidl_echo.EchoProxy();
  // Pipelined case: make a request with the server end of the proxy
  unawaited(echoLauncher.getEchoPipelined(
      'pipelined: ', pipelinedEcho.ctrl.request()));
  // Then, make an EchoString request with the proxy without needing to wait for
  // a response.
  final pipelinedFut = pipelinedEcho
      .echoString('hello')
      .then((response) => log.info('Got echo response $response'));

  // Run the two futures concurrently.
  await Future.wait([nonPipelinedFut, pipelinedFut]);
  await Future(() => exit(0));
}

Build the client

Optionally, to check that things are correct, try building the client:

Configure your GN build to include the client:

fx set core.x64 --with //examples/fidl/dart/request_pipelining/client:echo-client

Build the Fuchsia image:
```
fx build
```

Run the example code

For this tutorial, a realm component is provided to declare the appropriate capabilities and routes for fuchsia.examples.Echo and fuchsia.examples.EchoLauncher.

Configure your build to include the provided package that includes the echo realm, server, and client:
```
fx set core.x64 --with examples/fidl/dart:echo-launcher-dart --with-base //src/dart \
  --args='core_realm_shards += [ "//src/dart:dart_runner_core_shard" ]'
```
NOTE: The flag --with-base //src/dart adds the required dart runner to the base packages; and the core_realm_shards argument updates the laboratory-env component environment (the environment provided to the ffx-laboratory realm, used in ffx component start) to include the required dart runner.
Build the Fuchsia image:
```
fx build
```
Start or restart your device and package server (fx serve or fx serve-updates) to ensure the Dart runner package can be served.

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-launcher-dart#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 EchoLauncher protocol. You should see output similar to the following in the device logs (ffx log):

[echo-launcher-server][][I] Running EchoLauncher server
[echo-launcher-server][][I] Got echo response pipelined: hello
[echo-launcher-server][][I] Got echo response not pipelined: hello

Based on the print order, you can see that the pipelined case is faster. The echo response for the pipelined case arrives first, even though the non pipelined request is sent first, since request pipelining saves a roundtrip between the client and server. Request pipelining also simplifies the code.

For further reading about protocol request pipelining, including how to handle protocol requests that may fail, see the FIDL API rubric.

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

ffx component destroy /core/ffx-laboratory:echo_realm