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request-pipelining.md

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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:

{% 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/dart/request_pipelining/server/lib/main.dart" region_tag="echo-impl" %}

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:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/request_pipelining/server/lib/main.dart" region_tag="launcher-impl" %}

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.

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

Build the server

Optionally, to 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/dart/request_pipelining/server

  2. Build the Fuchsia image:

    fx build

Implement the client

Note: Most of the client code in client/lib/main.dart should be familiar if you 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/dart/request_pipelining/client/lib/main.dart" region_tag="main" highlight="9,10,11,12,13,14,15,16" %}

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.

Note: You should prefer just using await on a future instead of chaining them together using combinators like then. In this case, this is necessary to demonstrate the order in which the futures get completed.

The pipelined code is much simpler:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/request_pipelining/client/lib/main.dart" region_tag="main" highlight="18,19,20,21,22,23,24,25,26" %}

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:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/request_pipelining/client/lib/main.dart" region_tag="main" highlight="28,29" %}

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/dart/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/dart/request_pipelining/client --with //examples/fidl/dart/request_pipelining/server --with //examples/fidl/dart/launcher_bin

  2. Run the example:

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

You should see output similar to the following in the QEMU console (or using 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.