Fuchsia

Passing FIDL protocols

Prerequisites

This tutorial builds on the HLCPP getting started tutorials.

Overview

A common aspect of using FIDL on Fuchsia is passing protocols themselves across protocols. More precisely, many messages include either the client end or the server end of a channel, where the channel is used to communicate over a specific 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 HLCPP FIDL bindings.
  • The request pipelining pattern and its benefits.

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

The FIDL protocol

To do so, this tutorial implements the EchoLauncher protocol from the fuchsia.examples library:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/echo.test.fidl" region_tag="launcher" %}

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:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/request_pipelining/server/main.cc" region_tag="echo-impl" %}

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

Implement the EchoLauncher protocol

This class uses a binding set to keep track of all of the instances of Echo that it launches:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/request_pipelining/server/main.cc" region_tag="launcher-impl" highlight="1,17,18" %}

The code explicitly specifies not just the protocol that the binding set is templated on, but also the pointer type of the bindings that it stores. The code uses unique_ptr instead of raw pointers so that the binding set owns the instances of EchoImpl.

This is the implentation of the two methods:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/request_pipelining/server/main.cc" region_tag="launcher-impl" highlight="3,4,5,6,7,8,9,11,12,13,14,15" %}

For GetEcho, the code first needs to instantiate both ends of the channel. It creates a Binding using the server end, and then sends a response back with the client end. For GetEchoPipelined, the client has already done the work of creating both ends of the channel. It keeps one end and has passed the other to the server, so all the code needs to do is bind it to an Echo implementation.

Serve the EchoLauncher protocol

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

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/request_pipelining/server/main.cc" region_tag="main" %}

Build the server

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

  1. Configure your GN build to include the server:

    fx set core.x64 --with //examples/fidl/hlcpp/request_pipelining/server

  2. Build the Fuchsia image:

    fx build

Implement the client

Note: Most of the client code in client/main.cc should be familiar after having followed the client tutorial. The different parts of the code are covered in more detail here.

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:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/request_pipelining/client/main.cc" region_tag="non-pipelined" %}

This code has two layers of callbacks:

  • The outer layer handles the launcher request.
  • The inner layer handles the EchoString request.

Also, the code instantiates the EchoPtr in the outer scope then Binds it inside of the callback instead of calling fidl::InterfaceRequest<T>::Bind. This is because the proxy needs to be in scope when the echo response is received, which will most likely be after the top level callback returns.

Despite having to initialize the channels, the pipelined code is much simpler:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/request_pipelining/client/main.cc" region_tag="pipelined" %}

Build the client

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

  1. Configure your GN build to include the client:

    fx set core.x64 --with //examples/fidl/hlcpp/request_pipelining/client

  2. Build the Fuchsia image:

    fx build

Run the example code

To run the example code:

  1. Configure your GN build as follows:

    fx set core.x64 --with //examples/fidl/hlcpp/request_pipelining/client --with //examples/fidl/hlcpp/request_pipelining/server --with //examples/fidl/test:echo-launcher

  2. Build the Fuchsia image:

    fx build

  3. Run the example:

    fx shell run fuchsia-pkg://fuchsia.com/echo-launcher#meta/launcher.cmx fuchsia-pkg://fuchsia.com/echo-launcher-hlcpp-client#meta/echo-client.cmx fuchsia-pkg://fuchsia.com/echo-launcher-hlcpp-server#meta/echo-server.cmx fuchsia.examples.EchoLauncher

You should see the following print output in the QEMU console (or using fx log):

[120106.044] 769545:769547> Got non pipelined request
[120106.044] 769545:769547> Got pipelined request
[120106.044] 769545:769547> Got echo request for prefix pipelined:
[120106.044] 769795:769797> Got non pipelined response
[120106.044] 769545:769547> Got echo request for prefix not pipelined:
[120106.044] 769795:769797> Got echo response pipelined: hello!
[120106.044] 769795:769797> 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.