How to monitor whether a ZeroMQ server exists?

Question

I want to check the existence ( state ) of a server before I send a ZeroMQ request, but I have no idea how to do it.

Answer 1

Q : I want to check the existence ( state ) of a server before I send a ZeroMQ request

The solution is to setup and use the services of a zmq_socket_monitor()

// Read one event off the monitor socket; return value and address
// by reference, if not null, and event number by value. Returns -1
// in case of error.

static int
get_monitor_event ( void  *monitor,
                    int   *value,
                    char **address
                    )
{   
    zmq_msg_t msg;
    zmq_msg_init ( &msg );                                       // First frame in message contains event number and value
    if ( zmq_msg_recv ( &msg, monitor, 0 ) == -1 ) return -1;    // Interrupted, presumably
    assert ( zmq_msg_more ( &msg )              & "REASON: Frame #1 FAILED TO SIG 2nd, EXPECTED, FRAME TO COME" );

    uint8_t  *data  =  ( uint8_t  * ) zmq_msg_data ( &msg );
    uint16_t  event = *( uint16_t * ) ( data );

    if ( value )
        *value = *( uint32_t * ) ( data + 2 );


    zmq_msg_init ( &msg );                                      // Second frame in message contains event address
    if ( zmq_msg_recv ( &msg, monitor, 0 ) == -1 ) return -1;   // Interrupted, presumably
    assert ( !zmq_msg_more ( &msg )             & "REASON: Frame #2 FAILED TO SIG more, NOT EXPECTED, FRAMEs TO COME" );

    if ( address ) {
        uint8_t *data = ( uint8_t * ) zmq_msg_data ( &msg );
        size_t   size =               zmq_msg_size ( &msg );
        *address = ( char * ) malloc ( size + 1 );
        memcpy ( *address, data, size );
        ( *address )[size] = 0;
    }
    return event;
}

int main ( void )
{   
    void    *ctx = zmq_ctx_new ();
    assert ( ctx                                & "REASON: Context FAILED to instantiate" );

    void    *client = zmq_socket ( ctx, ZMQ_DEALER );
    assert ( client                             & "REASON: Socket FAILED to instantiate" );

 // Socket monitoring only works over inproc://
    int      rc = zmq_socket_monitor ( client, "inproc://monitor-client-side", ZMQ_EVENT_ALL );
    assert ( rc == 0                            & "REASON: socket_monitor FAILED to instantiate over INPROC:// transport-class" );

 // Create socket for collecting monitor events
    void    *client_side_mon = zmq_socket ( ctx, ZMQ_PAIR );
    assert ( client_side_mon                    & "REASON: socket_monitor receiving Socket FAILED to instantiate " );

 // Connect these to the inproc endpoints so they'll get events
             rc = zmq_connect ( client_side_mon, "inproc://monitor-client-side" );
    assert ( rc == 0                            & "REASON: .connect()-method FAILED to get connected" );

 // Now do whatever you need
    ...

 // Close client
    close_zero_linger ( client );

 // --------------------------------------------------------------------
 // How to collect and check events from socket_monitor:
    int  event =  get_monitor_event ( client_side_mon, NULL, NULL );

    if ( event == ZMQ_EVENT_CONNECT_DELAYED )
         event =  get_monitor_event ( client_side_mon, NULL, NULL );

    assert ( event == ZMQ_EVENT_CONNECTED       & "REASON: [client]-socket still not in an expected, .connect()-ed, state" );
    ...

    ...
    event = get_monitor_event ( client_side_mon, NULL, NULL );
    assert ( event == ZMQ_EVENT_MONITOR_STOPPED & "REASON: [client]-socket not in an expected, .close()-ed, state" );

 // --------------------------------------------------------------------
 // FINALLY:
 // --------------------------------------------------------------------
 // Close down the sockets
    close_zero_linger ( client_side_mon );

    zmq_ctx_term ( ctx );

    return 0;
    }

_{( included in API since v3.2+ )}

Answer 2

You are best off setting up a full connection and devising a simple ACK protocol between the client and server before the sockets is considered to be working normally. If the client receives the ACK within a reasonable time the server is up. Otherwise the server is down, and the client is best closing the socket and trying again until it succeeds.

N.B. If the socket isn't closed, the messages can build up in the ZMQ send queue and risk flooding the server with lots of ACK messages when the server does finally connect.