Brokers.rst

Network I/O with Brokers

When communicating to other services in the network, sometimes low-level socket I/O is inevitable. For this reason, CAF provides brokers. A broker is an event-based actor running in the middleman that multiplexes socket I/O. It can maintain any number of acceptors and connections. Since the broker runs in the middleman, implementations should be careful to consume as little time as possible in message handlers. Brokers should outsource any considerable amount of work by spawning new actors or maintaining worker actors.

Note that all UDP-related functionality is still experimental.

Spawning Brokers

Brokers are implemented as functions and are spawned by calling on of the three following member functions of the middleman.

template <spawn_options Os = no_spawn_options,
          class F = std::function<void(broker*)>, class... Ts>
typename infer_handle_from_fun<F>::type
spawn_broker(F fun, Ts&&... xs);

template <spawn_options Os = no_spawn_options,
          class F = std::function<void(broker*)>, class... Ts>
expected<typename infer_handle_from_fun<F>::type>
spawn_client(F fun, const std::string& host, uint16_t port, Ts&&... xs);

template <spawn_options Os = no_spawn_options,
          class F = std::function<void(broker*)>, class... Ts>
expected<typename infer_handle_from_fun<F>::type>
spawn_server(F fun, uint16_t port, Ts&&... xs);

The function spawn_broker simply spawns a broker. The convenience function spawn_client tries to connect to given host and port over TCP and returns a broker managing this connection on success. Finally, spawn_server opens a local TCP port and spawns a broker managing it on success. There are no convenience functions spawn a UDP-based client or server.

Class broker

void configure_read(connection_handle hdl, receive_policy::config config);

Modifies the receive policy for the connection identified by hdl. This will cause the middleman to enqueue the next new_data_msg according to the given config created by receive_policy::exactly(x), receive_policy::at_most(x), or receive_policy::at_least(x) (with x denoting the number of bytes).

void write(connection_handle hdl, size_t num_bytes, const void* buf)
void write(datagram_handle hdl, size_t num_bytes, const void* buf)

Writes data to the output buffer.

void enqueue_datagram(datagram_handle hdl, std::vector<char> buf);

Enqueues a buffer to be sent as a datagram. Use of this function is encouraged over write as it allows reuse of the buffer which can be returned to the broker in a datagram_sent_msg.

void flush(connection_handle hdl);
void flush(datagram_handle hdl);

Sends the data from the output buffer.

template <class F, class... Ts>
actor fork(F fun, connection_handle hdl, Ts&&... xs);

Spawns a new broker that takes ownership of a given connection.

size_t num_connections();

Returns the number of open connections.

void close(connection_handle hdl);
void close(accept_handle hdl);
void close(datagram_handle hdl);

Closes the endpoint related to the handle.

expected<std::pair<accept_handle, uint16_t>>
add_tcp_doorman(uint16_t port = 0, const char* in = nullptr,
                bool reuse_addr = false);

Creates new doorman that accepts incoming connections on a given port and returns the handle to the doorman and the port in use or an error.

expected<connection_handle>
add_tcp_scribe(const std::string& host, uint16_t port);

Creates a new scribe to connect to host:port and returns handle to it or an error.

expected<std::pair<datagram_handle, uint16_t>>
add_udp_datagram_servant(uint16_t port = 0, const char* in = nullptr,
                         bool reuse_addr = false);

Creates a datagram servant to handle incoming datagrams on a given port. Returns the handle to the servant and the port in use or an error.

expected<datagram_handle>
add_udp_datagram_servant(const std::string& host, uint16_t port);

Creates a datagram servant to send datagrams to host:port and returns a handle to it or an error.

Broker-related Message Types

Brokers receive system messages directly from the middleman for connection and acceptor events.

Note: brokers are required to handle these messages immediately regardless of their current state. Not handling the system messages from the broker results in loss of data, because system messages are not delivered through the mailbox and thus cannot be skipped.

struct new_connection_msg {
  accept_handle source;
  connection_handle handle;
};

Indicates that source accepted a new TCP connection identified by handle.

struct new_data_msg {
  connection_handle handle;
  std::vector<char> buf;
};

Contains raw bytes received from handle. The amount of data received per event is controlled with configure_read (see broker-class). It is worth mentioning that the buffer is re-used whenever possible.

struct data_transferred_msg {
  connection_handle handle;
  uint64_t written;
  uint64_t remaining;
};

This message informs the broker that the handle sent written bytes with remaining bytes in the buffer. Note, that these messages are not sent per default but must be explicitly enabled via the member function ack_writes.

struct connection_closed_msg {
  connection_handle handle;
};

struct acceptor_closed_msg {
  accept_handle handle;
};

A connection_closed_msg or acceptor_closed_msg informs the broker that one of its handles is no longer valid.

struct connection_passivated_msg {
  connection_handle handle;
};

struct acceptor_passivated_msg {
  accept_handle handle;
};

A connection_passivated_msg or acceptor_passivated_msg informs the broker that one of its handles entered passive mode and no longer accepts new data or connections trigger.

The following messages are related to UDP communication (see transport-protocols). Since UDP is not connection oriented, there is no equivalent to the new_connection_msg of TCP.

struct new_datagram_msg {
  datagram_handle handle;
  network::receive_buffer buf;
};

Contains the raw bytes from handle. The buffer always has a maximum size of 65k to receive all regular UDP messages. The amount of bytes can be queried via the .size() member function. Similar to TCP, the buffer is reused when possible---please do not resize it.

struct datagram_sent_msg {
  datagram_handle handle;
  uint64_t written;
  std::vector<char> buf;
};

This message informs the broker that the handle sent a datagram of written bytes. It includes the buffer that held the sent message to allow its reuse. Note, that these messages are not sent per default but must be explicitly enabled via the member function ack_writes.

struct datagram_servant_closed_msg {
  std::vector<datagram_handle> handles;
};

A datagram_servant_closed_msg informs the broker that one of its handles is no longer valid.

struct datagram_servant_passivated_msg {
  datagram_handle handle;
};

A datagram_servant_closed_msg informs the broker that one of its handles entered passive mode and no longer accepts new data trigger.

Manually Triggering Events ^experimental

Brokers receive new events as new_connection_msg and new_data_msg as soon and as often as they occur, per default. This means a fast peer can overwhelm a broker by sending it data faster than the broker can process it. In particular if the broker outsources work items to other actors, because work items can accumulate in the mailboxes of the workers.

Calling self->trigger(x,y), where x is a connection or acceptor handle and y is a positive integer, allows brokers to halt activities after y additional events. Once a connection or acceptor stops accepting new data or connections, the broker receives a connection_passivated_msg or acceptor_passivated_msg.

Brokers can stop activities unconditionally with self->halt(x) and resume activities unconditionally with self->trigger(x).