How-To: Write an fuchsia::modular::Agent in C++


An fuchsia::modular::Agent is a Peridot component that runs without any direct user interaction. The lifetime of a single fuchsia::modular::Agent instance is bounded by its session. It can be shared by mods across many stories. In addition to the capabilities provided to all Peridot components via fuchsia::modular::ComponentContext, an fuchsia::modular::Agent is given additional capabilities via its fuchsia::modular::AgentContext.

See the simple directory for a complete implementation of the fuchsia::modular::Agent described here.


SimpleAgent is an fuchsia::modular::Agent that periodically writes a simple message to a fuchsia::modular::MessageQueue (a common communication channel).

SimpleAgent implements the Simple FIDL protocol which exposes the ability to control which fuchsia::modular::MessageQueue the messages will be sent to.

library simple;

protocol Simple {
  // Provides the Simple protocol with a message queue to which
  // messages will be written periodically.
  1: SetMessageQueue(string queue_token);

fuchsia::modular::Agent Initialization

The first step to writing an fuchsia::modular::Agent is implementing the initializer:

#include <lib/app_driver/cpp/agent_driver.h>

class SimpleAgent {
    SimpleAgent(AgentHost* const agent_host) {

The AgentHost parameter provides the fuchsia::modular::Agent with an StartupContext and an fuchsia::modular::AgentContext.


StartupContext gives the fuchsia::modular::Agent access to services provided to it by other system components via incoming_services. It also allows the fuchsia::modular::Agent to provide services to other components via outgoing_services.


fuchsia::modular::AgentContext is a protocol that is exposed to all Agents. For example, it allows agents to schedule Tasks that will be executed at specific intervals.

fuchsia::modular::AgentContext also gives fuchsia::modular::Agents access to fuchsia::modular::ComponentContext which is a protocol that is exposed to all Peridot components (i.e. fuchsia::modular::Agent and Module). For example, fuchsia::modular::ComponentContext provides access to Ledger, Peridot's cross-device storage solution.

Advertising the Simple Protocol

In order for the SimpleAgent to advertise the Simple protocol to the system it needs to provide it in its outgoing_services.

  SimpleAgent(AgentHost* const agent_host) {
        [this](fidl::InterfaceRequest<Simple> request) {

  void Connect(
      fidl::InterfaceRequest<fuchsia::sys::ServiceProvider> outgoing_services) {

  // The services namespace that the `Simple` service is added to.
  component::ServiceNamespace services_;

  // The implementation of the Simple service.
  std::unique_ptr<SimpleImpl> simple_impl_;

In the initializer above, SimpleAgent adds the Simple service to its ServiceNamespace. When SimpleAgent receives a Connect call, it needs to bind the handle held by request to the concrete implementation in services_. It does this by calling AddBinding(std::move(outgoing_services)).

Now, when a component connects to the SimpleAgent, it will be able to connect to the SimpleInterface and call methods on it. Those method calls will be delegated to the simple_impl_ per the callback given to AddService().

class SimpleImpl : Simple {

  void Connect(fidl::InterfaceRequest<Simple> request);

  std::string message_queue_token() const { return token_; }

  // |Simple| protocol method.
  void SetMessageQueue(fidl::StringPtr queue_token);

  // The bindings to the Simple service.
  fidl::BindingSet<Simple> bindings_;

  // The current message queue token.
  std::string token_;

The SimpleImpl could be part of the SimpleAgent class, but it's good practice to separate out the implementation of an fuchsia::modular::Agent from the implementation(s) of the protocol(s) it provides.

Running the fuchsia::modular::Agent

int main(int /*argc*/, const char** /*argv*/) {
  async::Loop loop(&kAsyncLoopConfigAttachToThread);
  auto context = component::StartupContext::CreateFromStartupInfo();
  modular::AgentDriver<simple_agent::SimpleAgent> driver(
      context.get(), [&loop] { loop.Quit(); });
  return 0;

AgentDriver is a helper class that helps manage the fuchsia::modular::Agent lifecycle. Here it is given a newly created StartupContext and a callback that will be executed when the fuchsia::modular::Agent exits. AgentDriver requires SimpleAgent to implement three methods, one of which is Connect which was shown above.

The other two are:

void RunTask(const fidl::StringPtr& task_id,
             fit::function<void()> done) {

void Terminate(fit::function<void()> done) { done(); }

RunTask is called when a task, scheduled through fuchsia::modular::AgentContext's ScheduleTask, is triggered.

Terminate is called when the framework requests fuchsia::modular::Agent to exit gracefully.

Connecting to SimpleAgent

To connect to the SimpleAgent from a different component:

// The agent is guaranteed to stay alive as long as |agent_controller| stays in scope.
fuchsia::modular::AgentControllerPtr agent_controller;
fuchsia::sys::ServiceProviderPtr agent_services;
SimpleServicePtr agent_service;
ConnectToService(agent_services.get(), agent_service.NewRequest());

Here the component context is asked to connect to the fuchsia::modular::Agent at agent_url, and is given a request for the services that the SimpleAgent will provide via agent_services, and a controller for the fuchsia::modular::Agent via agent_controller.

Then the client connects to the Simple protocol by invoking ConnectToService with a request for a new SimpleServicePtr. This protocol pointer can be used immediately to provide the agent with the token for the fuchsia::modular::MessageQueue to send messages to.

See the SimpleModule guide for a more in-depth example.