The middleman is the main component of the I/O module and enables distribution. It transparently manages proxy actor instances representing remote actors, maintains connections to other nodes, and takes care of serialization of messages. Applications install a middleman by loading caf::io::middleman as module . Users can include "caf/io/all.hpp" to get access to all public classes of the I/O module.

Class middleman

expected<uint16> open(uint16, const char*, bool) See .
expected<uint16> publish(T, uint16, const char*, bool) See .
expected<void> unpublish(T x, uint16) See .
expected<node_id> connect(std::string host, uint16_t port) See .
expected<T> remote_actor<T = actor>(string, uint16) See .
expected<T> spawn_broker(F fun, ...) See .
expected<T> spawn_client(F, string, uint16, ...) See .
expected<T> spawn_server(F, uint16, ...) See .

Publishing and Connecting

The member function publish binds an actor to a given port, thereby allowing other nodes to access it over the network.

template <class T>
expected<uint16_t> middleman::publish(T x, uint16_t port,
                                      const char* in = nullptr,
                                      bool reuse_addr = false);

The first argument is a handle of type actor or typed_actor<...>. The second argument denotes the TCP port. The OS will pick a random high-level port when passing 0. The third parameter configures the listening address. Passing null will accept all incoming connections (INADDR_ANY). Finally, the flag reuse_addr controls the behavior when binding an IP address to a port, with the same semantics as the BSD socket flag SO_REUSEADDR. For example, with reuse_addr = false, binding two sockets to and will fail with EADDRINUSE since includes With reuse_addr = true binding would succeed because and are not literally equal addresses.

The member function returns the bound port on success. Otherwise, an error is returned.

template <class T>
expected<uint16_t> middleman::unpublish(T x, uint16_t port = 0);

The member function unpublish allows actors to close a port manually. This is performed automatically if the published actor terminates. Passing 0 as second argument closes all ports an actor is published to, otherwise only one specific port is closed.

The function returns an error if the actor was not bound to given port.

template<class T = actor>
expected<T> middleman::remote_actor(std::string host, uint16_t port);

After a server has published an actor with publish, clients can connect to the published actor by calling remote_actor:

// node A
auto ping = spawn(ping);
system.middleman().publish(ping, 4242);

// node B
auto ping = system.middleman().remote_actor("node A", 4242);
if (! ping) {
  cerr << "unable to connect to node A: "
       << system.render(ping.error()) << std::endl;
} else {
  self->send(*ping, ping_atom::value);

There is no difference between server and client after the connection phase. Remote actors use the same handle types as local actors and are thus fully transparent.

The function pair open and connect allows users to connect CAF instances without remote actor setup. The function connect returns a node_id that can be used for remote spawning (see ).

Free Functions

The following free functions in the namespace caf::io avoid calling the middleman directly. This enables users to easily switch between communication backends as long as the interfaces have the same signatures. For example, the (experimental) OpenSSL binding of CAF implements the same functions in the namespace caf::openssl to easily switch between encrypted and unencrypted communication.

ll expected<uint16> open(actor_system&, uint16, const char*, bool) & See .
expected<uint16> publish(T, uint16, const char*, bool) & See .
expected<void> unpublish(T x, uint16) & See .
expected<node_id> connect(actor_system&, std::string host, uint16_t port) & See .
expected<T> remote_actor<T = actor>(actor_system&, string, uint16) & See .

Transport Protocols

CAF communication uses TCP per default and thus the functions shown in the middleman API above are related to TCP. There are two alternatives to plain TCP: TLS via the OpenSSL module shortly discussed in and UDP.

UDP is integrated in the default multiplexer and BASP broker. Set the flag middleman_enable_udp to true to enable it (see ). This does not require you to disable TCP. Use publish_udp and remote_actor_udp to establish communication.

Communication via UDP is inherently unreliable and unordered. CAF reestablishes order and drops messages that arrive late. Messages that are sent via datagrams are limited to a maximum of 65.535 bytes which is used as a receive buffer size by CAF. Note that messages that exceed the MTU are fragmented by IP and are considered lost if a single fragment is lost. Optional reliability based on retransmissions and messages slicing on the application layer are planned for the future.