What's new in rotor v0.09

rotor is a non-intrusive event loop friendly C++ actor micro framework, similar to its elder brothers like caf and sobjectizer. The new release came out under the flag of pluginization, which affects the entire lifetime of an actor.

Actor Linking

The actor system is all about interactions between actors, i.e. sending messages to each other (and producing side effects for the outer world or listening to messages it produces). However, to let a message be delivered to the final actor, the actor should be alive (1); in other words, if actor A is going to send message M to actor B, A should somehow be sure that actor B is online and will not go offline while M is routing.

Before rotor v0.09, that kind of warranty was only available due to child-parent relations, i.e. between supervisor and its child-actor. In this case, an actor was guaranteed that a message would be delivered to its supervisor because the supervisor owned the actor and said supervisor's lifetime covered the respective actor's lifetime. Now, with the release of v0.09, it is possible to link actor A with actor B that are not parent- or child-related to one another and to make sure that all messages will be delivered after successful linking .

So, linking actors is performed somewhat along these lines:

namespace r = rotor;

void some_actor_t::on_start() noexcept override {
    request<payload::link_request_t>(b_address).send(timeout);
}

void some_actor_t::on_link_response(r::message::link_response_t &response) noexcept {
    auto& ec = message.payload.ec;
    if (!ec) {
        // successful linking
    }
}

However, code like this should not be used directly as is… because it is inconvenient. It becomes more obvious if you try linking actor A with 2 or more actors (B1, B2, etc.), since some_actor_t should keep an internal count of how many target actors are waiting for (successful) link responses. And here the pluginization system featured in the v0.09 release comes to the rescue:

namespace r = rotor;

void some_actor_t::configure(r::plugin::plugin_base_t &plugin) noexcept override {
    plugin.with_casted<r::plugin::link_client_plugin_t>(
        [&](auto &p) {
            p.link(B1_address);
            p.link(B2_address);
        }
    );
}

Now, this is much more convenient, since link_client_plugin_t is included out of the box with the rotor::actor_base_t. Nevertheless, it's still not enough, because it does not answer a few important questions, such as: 1. When is actor linking performed (and a "by-question": when is actor unlinking performed)? 2. What happens if the target actor (aka "server") does not exist or rejects linking? 3. What happens if the target actor decides to self-shutdown when there are "clients" still linked to it?

To provide answers to these questions, the concept of actor lifetime should be revisited.

Async Actor Initialization And Shutdown

Represented in a simplified manner is, here is how an actor’s state usually changes: new (constructor) -> initializing -> initialized -> operational -> shutting down -> shut down

The main job is performed in the operational state, and it is up to the user to define what an actor is to do in its up-and-running mode.

In the I-phase (i.e. initializing -> initialized), the actor should prepare itself for further functioning: locate and link with other actors, establish connection to the database, acquire whichever resources it needs to be operational. The key point of rotor is that I-phase is asynchronous, so an actor should notify its supervisor when it is ready (2).

The S-phase (i.e. shutting down -> shut down) is complementary to the I-phase, i.e. the actor is being asked to shut down, and, when it is done, it should notify its supervisor.

While it sounds easy, the tricky bit lies in the composability of actors, when they form Erlang-like hierarchies of responsibilities (see my article on trees of Supervisors). In other words, any actor can fail during its I-phase or S-phase, and that can lead to asynchronous collapse of the entire hierarchy, regardless of the failed actor's location within it. Essentially, the entire hierarchy of actors becomes operational, or, if something happens, the entire hierarchy becomes shut down.

rotor seems unique with its init/shutdown approach. There is nothing similar in caf;
in sobjectizer, there is a shutdown helper, which
carries a function similar to the S-phase above; however, it is limited to one actor only and offers no I-phase because sobjectizer has no concept of hierarchies (see update below).

While using rotor, it was discovered that the progress of the I-phase (S-phase) may potentially require many resources to be acquired (or released) asynchronously, which means that no single component, or actor, is able, by its own will, to answer the question of whether it has or has not completed the current phase. Instead, the answer comes as a result of collaborative efforts, handled in the right order. And this is where plugins come into play; they are like pieces, with each one responsible for a particular job of initialization/shutdown.

So, here are the promised answers related to link_client_plugin_t:

• Q: When is the actor linking or unlinking performed? A: When the actor state is initializing or shutting down respectively.
• Q: What happens if the target actor (aka "server") does not exist or rejects linking? A: Since this happens when the actor state is initializing, the plugin will detect the fail condition and will trigger client-actor shutdown. That may trigger a cascade effect, i.e. its supervisor will be triggered to shut down, too.
• Q: What happens if the target actor decides to self-shutdown when there are "clients" still linked to it? A: The "server-actor" will ask its clients to unlink, and once all "clients" have confirmed unlinking, the "server-actor" will continue the shutdown procedure (3).

A Simplified Example

Let's assume that there is a database driver with async-interface with one of the available event-loops for rotor, and there will be TCP-clients connecting to our service. The database will be served by db_actor_t and the service for serving clients will be named acceptor_t. The database actor is going to look like this:

namespace r = rotor;

struct db_actor_t: r::actor_base_t {

    struct resource {
        static const constexpr r::plugin::resource_id_t db_connection = 0;
    }

    void configure(r::plugin::plugin_base_t &plugin) noexcept override {
        plugin.with_casted<r::plugin::registry_plugin_t>([this](auto &p) {
            p.register_name("service::database", this->get_address())
        });
        plugin.with_casted<r::plugin::resources_plugin_t>([this](auto &) {
            resources->acquire(resource::db_connection);
            // initiate async connection to database
        });
    }

    void on_db_connection_success() {
        resources->release(resource::db_connection);
        ...
    }

    void on_db_disconnected() {
        resources->release(resource::db_connection);
    }

    void shutdown_start() noexcept override {
        r::actor_base_t::shutdown_start();
        resources->acquire(resource::db_connection);
        // initiate async disconnection from database, e.g. flush data
    }
};

The inner namespace resource is used to identify the database connection as a resource. It is good practice, better than hard-coding magic numbers like 0. During the actor configuration stage (which is part of initialization), when registry_plugin_t is ready, it will asynchronously register the actor address under a symbolic name of service::database in the registry (will be shown further down below). Then, with the resources_plugin_t, it acquires the database connection resource, blocking any further initialization and launching connection to the database. When connection is established, the resource is released, and the db_actor_t becomes operational. The S-phase is symmetrical, i.e. it blocks shutdown until all data is flushed to DB and connection is closed; once this step is complete, the actor will continue its shutdown (4).

The client acceptor code should look like this:

namespace r = rotor;
struct acceptor_actor_t: r::actor_base_t {
    r::address_ptr_t db_addr;

    void configure(r::plugin::plugin_base_t &plugin) noexcept override {
        plugin.with_casted<r::plugin::registry_plugin_t>([](auto &p) {
            p.discover_name("service::database", db_addr, true).link();
        });
    }

    void on_start() noexcept override {
        r::actor_base_t::on_start();
        // start accepting clients, e.g.
        // asio::ip::tcp::acceptor.async_accept(...);
    }

    void on_new_client(client_t& client) {
        // send<message::log_client_t>(db_addr, client)
    }
};

The key point here is the configure method. When registry_plugin_t is ready, it is configured to discover the name service::database and, when found, store it in the db_addr field; it then links the actor to the db_actor_t. If service::database is not found, the acceptor shuts down (i.e. on_start is not invoked); if the linking is not confirmed, the acceptor shuts down, too. When everything is fine, the acceptor starts accepting new clients.

The operational part itself is missing for the sake of brevity because it hasn't changed in the new rotor version: there is a need to define payload and message (including request and response types), as well as define methods which will accept the messages and finally subscribe to them.

Let's bundle everything together in a main.cpp. Let's assume that the boost::asio event loop is used.

namespace asio = boost::asio;
namespace r = rotor;

...
asio::io_context io_context;
auto system_context = rotor::asio::system_context_asio_t(io_context);
auto strand = std::make_shared<asio::io_context::strand>(io_context);
auto timeout = r::pt::milliseconds(100);
auto sup = system_context->create_supervisor<r::asio::supervisor_asio_t>()
               .timeout(timeout)
               .strand(strand)
               .create_registry()
               .finish();

sup->create_actor<db_actor_t>().timeout(timeout).finish();
sup->create_actor<acceptor_actor_t>().timeout(timeout).finish();

sup->start();
io_context.run();

The builder pattern is actively used in the v0.09 rotor. Here, the root supervisor sup was created with 3 actors instantiated on it: the user defined db_actor_t and acceptor_actor_t and implicitly created a registry actor. As is typical for the actor system, all actors are decoupled from one another, only sharing message types (skipped here).

All actors are simply created here, and the supervisor does not know the relations between them because actors are loosely coupled and have become more autonomous since v0.09.

Runtime configuration can be completely different: actors can be created on different threads, different supervisors, and even using different event loops, but the actor implementation remains the same (5). In that case, there will be more than one root supervisor; however, to let them find each other, the registry actor address should be shared between them. This is also supported via the get_registry_address() method of supervisor_t.

Summary

The most important feature of rotor v0.09 is the pluginization of its core. Among other plugins, the most important are: the link_client_plugin_t plugin, which maintains kind of a "virtual connection" between actors; the registry_plugin_t, which allows registering and discovering actor addresses by their symbolic names; and the resources_plugin_t, which suspends actor init/shutdown until external asynchronous events occur.

There are a few less prominent changes in the release, such as the new non-public properties access and builder pattern for actor construction.

Any feedback on rotor is welcome!

PS. I'd like to say thanks to Crazy Panda for supporting me in my actor model research.

Notes

(1) Currently, it will lead to segfault upon attempt to deliver a message to an actor whose supervisor is already destroyed.

(2) If it does not notify, init-request timeout will occur, and the actor will be asked by its supervisor to shut down, i.e. bypass the operational state.

(3) You might ask: what happens if a client-actor does not confirm unlinking on time? Well, this is somewhat of a violation of contract, and the system_context_t::on_error(const std::error_code&) method will be invoked, which, by default, will print error to std::cerr and invoke std::terminate(). To avoid contract violation, shutdown timeouts should be tuned to allow client-actors to unlink on time.

(4) During shutdown, the registry_plugin_t will unregister all registered names in the registry.

(5) With the exception of when different event loops are used, when actors use the event loop API directly, they will, obviously, change following the event loop change, but that's beyond rotor.

Update

During discussings with sobjectizer author below, it was clarified sobjectizer shutdowner and stop guard offer "long lasting" shutdown actions, however it's main purpose to give some actors additional time for shutdown, even if on the Environment stop was invoked. The asynchronous shutdown (and initialization) similar to rotor I-phase and S-phase can be modeled via actor's states, if needed. This is, however, framework users responsibility, contrary to rotor, where it is the framework responsibility.