    // Copyright (C) 2003  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #undef DLIB_SERVER_KERNEL_ABSTRACT_
    #ifdef DLIB_SERVER_KERNEL_ABSTRACT_

    #include "../threads/threads_kernel_abstract.h"
    #include "../sockets/sockets_kernel_abstract.h"
    #include <string>

    namespace dlib
    {
        class server
        {

            /*!
                INITIAL VALUE
                    get_listening_ip()           == ""
                    get_listening_port()         == 0
                    is_running()                 == false
                    get_max_connections()        == 1000
                    get_graceful_close_timeout() == 500

                CALLBACK FUNCTIONS
                on_connect():
                    To use this object inherit from it and define the pure virtual function
                    on_connect.  Inside this function is where you will handle each new
                    connection.  Note that the connection object passed to on_connect() should
                    NOT be closed, just let the function end and it will be gracefully closed
                    for you.  Also note that each call to on_connect() is run in its own
                    thread.  Also note that on_connect() should NOT throw any exceptions,
                    all exceptions must be dealt with inside on_connect() and cannot be
                    allowed to leave.

                on_listening_port_assigned():
                    This function is called to let the client know that the operating
                    system has assigned a port number to the listening port.  This
                    happens if a port number of zero was given.  Note that this
                    function does not need to be defined.  If you don't care then
                    don't define it and it will do nothing.  Note also that this function
                    is NOT called in its own thread.  Thus, making it block might hang the
                    server.

                WHAT THIS OBJECT REPRESENTS
                    This object represents a server that listens on a port and spawns new
                    threads to handle each new connection.

                    Note that the clear() function does not return until all calls to
                    on_connect() have finished and the start() function has been shutdown.
                    Also note that when clear() is called all open connection objects
                    will be shutdown().

                    A note about get_max_connections(): when the maximum number of connections
                    has been reached accept() will simply not be called until the number of
                    open connections drops below get_max_connections().  This means connections
                    will just wait to be serviced, rather than being outright refused.

                THREAD SAFETY
                    All member functions are thread-safe.
            !*/

            public:

                server(
                );
                /*!
                    ensures
                        - #*this is properly initialized
                    throws
                        - std::bad_alloc
                        - dlib::thread_error
                !*/

                virtual ~server(
                );
                /*!
                    requires
                        - is not called from any of server's callbacks
                    ensures
                        - all resources associated with *this have been released
                !*/

                void clear(
                );
                /*!
                    requires
                        - is not called from any of server's callbacks
                    ensures
                        - #*this has its initial value
                        - all open connection objects passed to on_connect() are shutdown()
                        - blocks until all calls to on_connect() have finished
                        - blocks until the start() function has released all its resources
                    throws
                        - std::bad_alloc
                            if this exception is thrown then the server object is unusable
                            until clear() is called and succeeds
                !*/

                void start (
                );
                /*!
                    requires
                        - is_running() == false
                    ensures
                        - starts listening on the port and ip specified by get_listening_ip()
                          and #get_listening_port() for new connections.
                        - if (get_listening_port() == 0) then
                            - a port to listen on will be automatically selected
                            - #get_listening_port() == the selected port being used
                        - if (get_listening_ip() == "" ) then
                            - all local IPs will be listened on
                        - blocks until clear() is called or an error occurs
                    throws
                        - dlib::socket_error
                            start() will throw this exception if there is some problem binding
                            ports and/or starting the server or if there is a problem
                            accepting new connections while it's running.
                            If this happens then
                                - All open connection objects passed to on_connect() are shutdown()
                                  and the exception will not be thrown until all on_connect() calls
                                  have terminated.
                                - The server will be cleared and returned to its initial value.
                        - dlib::thread_error
                            start() will throw this exception if there is a problem
                            creating new threads.  Or it may throw this exception if there
                            is a problem creating threading objects.
                            If this happens then
                                - All open connection objects passed to on_connect() are shutdown()
                                  and the exception will not be thrown until all on_connect() calls
                                  have terminated.
                                - The server will be cleared and returned to its initial value.
                        - std::bad_alloc
                            start() may throw this exception and if it does then the object
                            will be unusable until clear() is called and succeeds
                !*/

                void start_async (
                );
                /*!
                    ensures
                        - starts listening on the port and ip specified by get_listening_ip()
                          and #get_listening_port() for new connections.
                        - if (get_listening_port() == 0) then
                            - a port to listen on will be automatically selected
                            - #get_listening_port() == the selected port being used
                        - if (get_listening_ip() == "" ) then
                            - all local IPs will be listened on
                        - does NOT block.  That is, this function will return right away and
                          the server will run on a background thread until clear() or this
                          object's destructor is called (or until some kind of fatal error
                          occurs).
                        - if an error occurs in the background thread while the server is
                          running then it will shut itself down, set is_running() to false, and
                          log the error to a dlib::logger object.
                        - calling start_async() on a running server has no effect.
                    throws
                        - dlib::socket_error
                            start_async() will throw this exception if there is some problem binding
                            ports and/or starting the server.
                            If this happens then
                                - The server will be cleared and returned to its initial value.
                !*/

                bool is_running (
                ) const;
                /*!
                    ensures
                        - returns true if start() is running
                        - returns false if start() is not running or has released all
                          its resources and is about to terminate
                    throws
                        - std::bad_alloc
                !*/

                int get_max_connections (
                ) const;
                /*!
                    ensures
                        - returns the maximum number of connections the server will accept
                          at a time.
                        - returns 0 if the server will accept any number of connections
                    throws
                        - std::bad_alloc
                !*/

                const std::string get_listening_ip (
                ) const;
                /*!
                    ensures
                        - returns the local ip to listen for new connections on
                        - returns "" if ALL local ips are to be listened on
                    throws
                        - std::bad_alloc
                !*/

                int get_listening_port (
                ) const;
                /*!
                    ensures
                        - returns the local port number to listen for new connections on
                        - returns 0 if the local port number has not yet been set
                    throws
                        - std::bad_alloc
                !*/

                void set_listening_port (
                    int port
                );
                /*!
                    requires
                        - port >= 0
                        - is_running() == false
                    ensures
                        - #get_listening_port() == port
                    throws
                        - std::bad_alloc
                !*/

                void set_listening_ip (
                    const std::string& ip
                );
                /*!
                    requires
                        - is_ip_address(ip) == true or ip == ""
                        - is_running() == false
                    ensures
                        - #get_listening_ip() == ip
                    throws
                        - std::bad_alloc
                !*/

                void set_max_connections (
                    int max
                );
                /*!
                    requires
                        - max >= 0
                    ensures
                        - #get_max_connections() == max
                    throws
                        - std::bad_alloc
                !*/

                void set_graceful_close_timeout (
                    unsigned long timeout
                );
                /*!
                    ensures
                        - #get_graceful_close_timeout() == timeout
                !*/

                unsigned long get_graceful_close_timeout (
                ) const;
                /*!
                    ensures
                        - When on_connect() terminates, it will close the connection using
                          close_gracefully().  This is done so that any data still in the
                          operating system's output buffers gets a chance to be properly
                          transmitted to the remote host.  Part of this involves waiting for
                          the remote host to close their end of the connection.  Therefore,
                          get_graceful_close_timeout() returns the timeout, in milliseconds,
                          that we wait for the remote host to close their end of the
                          connection.  This is the timeout value given to close_gracefully().
                !*/

            private:

                virtual void on_connect (
                    connection& new_connection
                )=0;
                /*!
                    requires
                        - on_connect() is run in its own thread
                        - is_running() == true
                        - the number of current connections < get_max_connection()
                        - new_connection == the new connection to the server which is
                          to be serviced by this call to on_connect()
                    ensures
                        - when new_connection is shutdown() on_connect() will terminate
                        - this function will not call clear()
                    throws
                        - does not throw any exceptions
                !*/

                // do nothing by default
                virtual void on_listening_port_assigned (
                ) {}
                /*!
                    requires
                        - is called if a listening port of zero was specified and
                          an actual port number has just been assigned to the server
                    ensures
                        - this function will not block
                        - this function will not call clear()
                    throws
                        - does not throw any exceptions
                !*/

                // restricted functions
                server(server&);        // copy constructor
                server& operator=(server&);    // assignment operator
        };

    }

    #endif // DLIB_SERVER_KERNEL_ABSTRACT_