Message Passing

Message passing in CAF is always asynchronous. Further, CAF neither guarantees message delivery nor message ordering in a distributed setting. CAF uses TCP per default, but also enables nodes to send messages to other nodes without having a direct connection. In this case, messages are forwarded by intermediate nodes and can get lost if one of the forwarding nodes fails. Likewise, forwarding paths can change dynamically and thus cause messages to arrive out of order.

The messaging layer of CAF has three primitives for sending messages: send, request, and delegate. The former simply enqueues a message to the mailbox the receiver. The latter two are discussed in more detail in Requests and Delegating Messages.

Structure of Mailbox Elements

When enqueuing a message to the mailbox of an actor, CAF wraps the content of the message into a mailbox_element (shown below) to add meta data and processing paths.

UML class diagram for ``mailbox_element``\

UML class diagram for mailbox_element

The sender is stored as a strong_actor_ptr (see Pointer) and denotes the origin of the message. The message ID is either 0—invalid—or a positive integer value that allows the sender to match a response to its request. The stages vector stores the path of the message. Response messages, i.e., the returned values of a message handler, are sent to stages.back() after calling stages.pop_back(). This allows CAF to build pipelines of arbitrary size. If no more stage is left, the response reaches the sender. Finally, content() grants access to the type-erased tuple storing the message itself.

Mailbox elements are created by CAF automatically and are usually invisible to the programmer. However, understanding how messages are processed internally helps understanding the behavior of the message passing layer.

It is worth mentioning that CAF usually wraps the mailbox element and its content into a single object in order to reduce the number of memory allocations.

Copy on Write

CAF allows multiple actors to implicitly share message contents, as long as no actor performs writes. This allows groups (see Group Communication) to send the same content to all subscribed actors without any copying overhead.

Actors copy message contents whenever other actors hold references to it and if one or more arguments of a message handler take a mutable reference.

Requirements for Message Types

Message types in CAF must meet the following requirements:

  1. Serializable or inspectable (see Type Inspection (Serialization and String Conversion))
  2. Default constructible
  3. Copy constructible

A type is serializable if it provides free function serialize(Serializer&, T&) or serialize(Serializer&, T&, const unsigned int). Accordingly, a type is inspectable if it provides a free function inspect(Inspector&, T&).

Requirement 2 is a consequence of requirement 1, because CAF needs to be able to create an object of a type before it can call serialize or inspect on it. Requirement 3 allows CAF to implement Copy on Write (see Copy on Write).

Default and System Message Handlers

CAF has three system-level message types (down_msg, exit_msg, and error) that all actor should handle regardless of there current state. Consequently, event-based actors handle such messages in special-purpose message handlers. Additionally, event-based actors have a fallback handler for unmatched messages. Note that blocking actors have neither of those special-purpose handlers (see Blocking Actors).

Down Handler

Actors can monitor the lifetime of other actors by calling self->monitor(other). This will cause the runtime system of CAF to send a down_msg for other if it dies. Actors drop down messages unless they provide a custom handler via set_down_handler(f), where f is a function object with signature void (down_message&) or void (scheduled_actor*, down_message&). The latter signature allows users to implement down message handlers as free function.

Exit Handler

Bidirectional monitoring with a strong lifetime coupling is established by calling self->link_to(other). This will cause the runtime to send an exit_msg if either this or other dies. Per default, actors terminate after receiving an exit_msg unless the exit reason is exit_reason::normal. This mechanism propagates failure states in an actor system. Linked actors form a sub system in which an error causes all actors to fail collectively. Actors can override the default handler via set_exit_handler(f), where f is a function object with signature void (exit_message&) or void (scheduled_actor*, exit_message&).

Error Handler

Actors send error messages to others by returning an error (see Errors) from a message handler. Similar to exit messages, error messages usually cause the receiving actor to terminate, unless a custom handler was installed via set_error_handler(f), where f is a function object with signature void (error&) or void (scheduled_actor*, error&). Additionally, request accepts an error handler as second argument to handle errors for a particular request (see Error Handling in Requests). The default handler is used as fallback if request is used without error handler.

Default Handler

The default handler is called whenever the behavior of an actor did not match the input. Actors can change the default handler by calling set_default_handler. The expected signature of the function object is result<message> (scheduled_actor*, message_view&), whereas the self pointer can again be omitted. The default handler can return a response message or cause the runtime to skip the input message to allow an actor to handle it in a later state. CAF provides the following built-in implementations: reflect, reflect_and_quit, print_and_drop, drop, and skip. The former two are meant for debugging and testing purposes and allow an actor to simply return an input. The next two functions drop unexpected messages with or without printing a warning beforehand. Finally, skip leaves the input message in the mailbox. The default is print_and_drop.

Requests

A main feature of CAF is its ability to couple input and output types via the type system. For example, a typed_actor<replies_to<int>::with<int>> essentially behaves like a function. It receives a single int as input and responds with another int. CAF embraces this functional take on actors by simply creating response messages from the result of message handlers. This allows CAF to match request to response messages and to provide a convenient API for this style of communication.

Sending Requests and Handling Responses

Actors send request messages by calling request(receiver, timeout, content...). This function returns an intermediate object that allows an actor to set a one-shot handler for the response message. Event-based actors can use either request(...).then or request(...).await. The former multiplexes the one-shot handler with the regular actor behavior and handles requests as they arrive. The latter suspends the regular actor behavior until all awaited responses arrive and handles requests in LIFO order. Blocking actors always use request(...).receive, which blocks until the one-shot handler was called. Actors receive a sec::request_timeout (see System Error Codes) error message (see Error Handler) if a timeout occurs. Users can set the timeout to infinite for unbound operations. This is only recommended if the receiver is running locally.

In our following example, we use the simple cell actors shown below as communication endpoints.

using cell = typed_actor<reacts_to<put_atom, int>,
                         replies_to<get_atom>::with<int>>;

struct cell_state {
  int value = 0;
};

cell::behavior_type cell_impl(cell::stateful_pointer<cell_state> self, int x0) {
  self->state.value = x0;
  return {
    [=](put_atom, int val) {
      self->state.value = val;
    },
    [=](get_atom) {
      return self->state.value;
    }
  };
}

The first part of the example illustrates how event-based actors can use either then or await.

void waiting_testee(event_based_actor* self, vector<cell> cells) {
  for (auto& x : cells)
    self->request(x, seconds(1), get_atom::value).await([=](int y) {
      aout(self) << "cell #" << x.id() << " -> " << y << endl;
    });
}

void multiplexed_testee(event_based_actor* self, vector<cell> cells) {
  for (auto& x : cells)
    self->request(x, seconds(1), get_atom::value).then([=](int y) {
      aout(self) << "cell #" << x.id() << " -> " << y << endl;
    });
}

The second half of the example shows a blocking actor making use of receive. Note that blocking actors have no special-purpose handler for error messages and therefore are required to pass a callback for error messages when handling response messages.

void blocking_testee(blocking_actor* self, vector<cell> cells) {
  for (auto& x : cells)
    self->request(x, seconds(1), get_atom::value).receive(
      [&](int y) {
        aout(self) << "cell #" << x.id() << " -> " << y << endl;
      },
      [&](error& err) {
        aout(self) << "cell #" << x.id()
                   << " -> " << self->system().render(err) << endl;
      }
    );
}

We spawn five cells and assign the values 0, 1, 4, 9, and 16.

vector<cell> cells;
for (auto i = 0; i < 5; ++i)
  cells.emplace_back(system.spawn(cell_impl, i * i));

When passing the cells vector to our three different implementations, we observe three outputs. Our waiting_testee actor will always print:

cell #9 -> 16
cell #8 -> 9
cell #7 -> 4
cell #6 -> 1
cell #5 -> 0

This is because await puts the one-shots handlers onto a stack and enforces LIFO order by re-ordering incoming response messages.

The multiplexed_testee implementation does not print its results in a predicable order. Response messages arrive in arbitrary order and are handled immediately.

Finally, the blocking_testee implementation will always print:

cell #5 -> 0
cell #6 -> 1
cell #7 -> 4
cell #8 -> 9
cell #9 -> 16

Both event-based approaches send all requests, install a series of one-shot handlers, and then return from the implementing function. In contrast, the blocking function waits for a response before sending another request.

Error Handling in Requests

Requests allow CAF to unambiguously correlate request and response messages. This is also true if the response is an error message. Hence, CAF allows to add an error handler as optional second parameter to then and await (this parameter is mandatory for receive). If no such handler is defined, the default error handler (see Error Handler) is used as a fallback in scheduled actors.

As an example, we consider a simple divider that returns an error on a division by zero. This examples uses a custom error category (see Errors).

enum class math_error : uint8_t {
  division_by_zero = 1
};

error make_error(math_error x) {
  return {static_cast<uint8_t>(x), atom("math")};
}

using div_atom = atom_constant<atom("div")>;

using divider = typed_actor<replies_to<div_atom, double, double>::with<double>>;

divider::behavior_type divider_impl() {
  return {
    [](div_atom, double x, double y) -> result<double> {
      if (y == 0.0)
        return math_error::division_by_zero;
      return x / y;
    }
  };
}

When sending requests to the divider, we use a custom error handlers to report errors to the user.

scoped_actor self{system};
self->request(div, std::chrono::seconds(10), div_atom::value, x, y).receive(
  [&](double z) {
    aout(self) << x << " / " << y << " = " << z << endl;
  },
  [&](const error& err) {
    aout(self) << "*** cannot compute " << x << " / " << y << " => "
               << system.render(err) << endl;
  }
);

Delaying Messages

Messages can be delayed by using the function delayed_send, as illustrated in the following time-based loop example.

// uses a message-based loop to iterate over all animation steps
void dancing_kirby(event_based_actor* self) {
  // let's get it started
  self->send(self, step_atom::value, size_t{0});
  self->become (
    [=](step_atom, size_t step) {
      if (step == sizeof(animation_step)) {
        // we've printed all animation steps (done)
        cout << endl;
        self->quit();
        return;
      }
      // print given step
      draw_kirby(animation_steps[step]);
      // animate next step in 150ms
      self->delayed_send(self, std::chrono::milliseconds(150),
                         step_atom::value, step + 1);
    }
  );
}

Delegating Messages

Actors can transfer responsibility for a request by using delegate. This enables the receiver of the delegated message to reply as usual—simply by returning a value from its message handler—and the original sender of the message will receive the response. The following diagram illustrates request delegation from actor B to actor C.

A                  B                  C
|                  |                  |
| ---(request)---> |                  |
|                  | ---(delegate)--> |
|                  X                  |---\
|                                     |   | compute
|                                     |   | result
|                                     |<--/
| <-------------(reply)-------------- |
|                                     X
|---\
|   | handle
|   | response
|<--/
|
X

Returning the result of delegate(...) from a message handler, as shown in the example below, suppresses the implicit response message and allows the compiler to check the result type when using statically typed actors.

void actor_a(event_based_actor* self, const calc& worker) {
  self->request(worker, std::chrono::seconds(10), add_atom::value, 1, 2).then(
    [=](int result) {
      aout(self) << "1 + 2 = " << result << endl;
    }
  );
}

calc::behavior_type actor_b(calc::pointer self, const calc& worker) {
  return {
    [=](add_atom add, int x, int y) {
      return self->delegate(worker, add, x, y);
    }
  };
}

calc::behavior_type actor_c() {
  return {
    [](add_atom, int x, int y) {
      return x + y;
    }
  };
}

void caf_main(actor_system& system) {
  system.spawn(actor_a, system.spawn(actor_b, system.spawn(actor_c)));
}

Response Promises

Response promises allow an actor to send and receive other messages prior to replying to a particular request. Actors create a response promise using self->make_response_promise<Ts...>(), where Ts is a template parameter pack describing the promised return type. Dynamically typed actors simply call self->make_response_promise(). After retrieving a promise, an actor can fulfill it by calling the member function deliver(...), as shown in the following example.

// using add_atom = atom_constant<atom("add")>; (defined in atom.hpp)

using adder = typed_actor<replies_to<add_atom, int, int>::with<int>>;

// function-based, statically typed, event-based API
adder::behavior_type worker() {
  return {
    [](add_atom, int a, int b) {
      return a + b;
    }
  };
}

// function-based, statically typed, event-based API
adder::behavior_type calculator_master(adder::pointer self) {
  auto w = self->spawn(worker);
  return {
    [=](add_atom x, int y, int z) -> result<int> {
      auto rp = self->make_response_promise<int>();
      self->request(w, infinite, x, y, z).then([=](int result) mutable {
        rp.deliver(result);
      });
      return rp;
    }
  };
}

Message Priorities

By default, all messages have the same priority and actors ignore priority flags. Actors that should evaluate priorities must be spawned using the priority_aware flag, as shown in the following example. This flag causes the actor to use a priority-aware mailbox implementation. It is not possible to change this implementation dynamically at runtime.

#include "caf/all.hpp"

using std::endl;
using namespace caf;

behavior foo(event_based_actor* self) {
  self->send(self, "world");
  self->send<message_priority::high>(self, "hello");
  // when spawning `foo` with priority_aware flag, it will print "hello" first
  return {
    [=](const std::string& str) {
      aout(self) << str << endl;
    }
  };
}

void caf_main(actor_system& system) {
  scoped_actor self{system};
  aout(self) << "spawn foo" << endl;
  self->spawn(foo);
  self->await_all_other_actors_done();
  aout(self) << "spawn foo again with priority_aware flag" << endl;
  self->spawn<priority_aware>(foo);
}

CAF_MAIN()