How-To: Write an Agent in C++

DEPRECATION WARNING: The Modular framework is being deprecated in favor of the Session Framework.


An Agent is a component that runs without any direct user interaction. The lifetime of an Agent component instance is bounded by its session. A single agent instance is shared by all components that ask for services from them.

Agents provide services to other components via their outgoing directory.

For legacy reasons, Agents can optionally expose the fuchsia.modular.Agent service to receive new connections and provide services.

Agent components should implement the fuchsia.modular.Lifecycle service to receive graceful termination signals and voluntarily exit.


fuchsia::modular::Agent Initialization

The first step to writing an Agent is setting up the scaffolding using the modular::Agent utility class.

#include <lib/modular/cpp/agent.h>

int main(int /*argc*/, const char** /*argv*/) {
  async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
  auto context = sys::ComponentContext::CreateAndServeOutgoingDirectory();
  // modular::Agent provides an implementation of fuchsia.modular.Lifecycle.
  // The agent can perform graceful teardown tasks in the callback.
  modular::Agent agent(context->outgoing(), [&loop] { loop.Quit(); });
  return 0;

The modular::Agent utility above implements and exposes fuchsia.modular.Lifecycle.

Advertising the Simple Protocol

In order for the SimpleAgent to advertise the Simple protocol to other modular components, it needs to expose it as an agent service. sys::ComponentContext::outgoing() provides a way to do this:

  class SimpleImpl : Simple {

    void AMethod(AMethodResult reuslt) override{
      // do stuff
      result("all done");

  int main(int /*argc*/, const char** /*argv*/) {
    auto context = sys::ComponentContext::CreateAndServeOutgoingDirectory();

    SimpleImpl simple_impl;
    fidl::BindingSet<Simple> simple_bindings;


In the code above, SimpleAgent adds the Simple service as an outgoing service. Now, when a component asks for the Simple service (see below), it will be served by SimpleAgent.

Connecting to SimpleAgent

To connect to the SimpleAgent from a different component, a service mapping must be added to the Modular config for your product:

  agent_service_index = [
      "service_name": "Simple",
      "agent_url": "fuchsia-pkg://url/to/your/agent"

Then, in the component's implementation (i.e.,

auto sys_component_context = sys::ComponentContext::CreateAndServeOutgoingDirectory();
SimplePtr simple = sys_component_context->svc()->Connect<Simple>();

When your component asks for the Simple service, the agent_service_index is consulted. The agent listed there is launched and is asked to provide the Simple service.

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