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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. It can be shared by mods across many stories. In addition to the capabilities provided to all modular components via fuchsia::modular::ComponentContext, an Agent is given additional capabilities via fuchsia::modular::AgentContext as an incoming service.

Agents must expose the fuchsia::modular::Agent service to receive new connections and provide services. An Agent component may implement the fuchsia::modular::Lifecycle service to receive 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 agent(context->outgoing(), [&loop] { loop.Quit(); });
  return 0;

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


fuchsia::modular::AgentContext is a protocol that is exposed to all Agent components. 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 other modular components, it needs to expose it as an agent service. modular::Agent::AddService<>() provides a way to do this:

  class SimpleImpl : Simple {

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

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

  int main(int /*argc*/, const char** /*argv*/) {
    modular::Agent agent(context->outgoing(), [&loop] { loop.Quit(); });

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


In the code above, SimpleAgent adds the Simple service as an agent service. Now, when a component connects to the SimpleAgent, it will be able to connect to the Simple interface and call methods on it. Those method calls will be delegated to the simple_impl object.

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.