配布されたMinervaプロセス間の通信

Minervaは、配布されたMinervaプログラム間の通信をサポートするための高レベルのインタフェースを提供します。

ソケットを介して、配布されたMinervaプロセス間での通信をサポートする述語があります。

この述語は、分散アプリケーションのクライアント/サーバ・アーキテクチャを実装するのに役立ちます。 クライアントとサーバ間のチャネルを作り、これらのチャネルを介して、MINERVA項を送出し/受取ることができます。
Minervaによるクライアント/サーバ・アーキテクチャは、以下の構造を持ちます：

サーバ

main(..) :-
	...
	server_start(Port, Backlog, connection_request(_, ...), Server).


connection_request(Socket, ...) :-
	...
	open(Socket, read, Input, [socket,buffered,binary]),
	open(Socket, write, Output, [socket,buffered,binary]).
	listener_start(Input, handle_input(_,_,Output,...), Listener).


handle_input(Request, Listener, Output, ...) :-
	...
	% optionally: send(Output, ...)
	.

同期通信によるクライアント

main(..) :-
	...
	client_start(Hostname, Port, Socket),
	communication_loop(Socket, ...),
	client_stop(Socket).


communication_loop(Socket, ...) :-
	open(Socket, read, Input, [socket,buffered,binary]),
	open(Socket, write, Output, [socket,buffered,binary]).
	repeat,
		...
		send(Output, ...),
		...
		receive(Input, ...)
		...
	.

非同期通信によるクライアント

main(..) :-
	...
	client_start(Hostname, Port, Socket),
	open(Socket, read, Input, [socket,buffered,binary]),
	open(Socket, write, Output, [socket,buffered,binary]).
	listener_start(Input, handle_input(_,_,Output,...), Listener).


% handle_output has to be a callback predicate called by some
% thread (for example by some gui-request)
handle_output(Output, ...) :-
	...
	send(Output, ...).


handle_input(Request, Listener, Output, ...) :-
	...
	% optionally: send(Output, ...)
	.

例題：

単純な黒板システムです。 


main(['-server']) :- !,
	run_server(1254).
main(['-client', Username]) :- !,
	run_client(localhost, 1254, Username).


% ***** SERVER ***************************************************
% should be started with: minerva -daemon -l socktest -server
% You have to start exactly one server. The server has to be active
% before any client may be activated.


run_server(Port) :- !,
	% create a blackboard, which holds the names of the connected clients
	bb_create(login_names),
	println([ready]),
	% starts the server listening on port 'Port'.
	% if there is a connection request, the predicate user_login/1
	% will be called
	server_start(Port, 3, user_login(_), _).


% handle a connection request from a client
user_login(Socket) :-
	% open binary input and output streams to the client
	open_connection(Socket, Input, Output),
	% wait for input from the client.
	% If a client sends some term with send/2, than the
	% predicate user_io/3 will be called (the 1st argument will
	% be set to the transmitted term and the 2nd argument will
	% be set to the listener object). 
	listener_start(Input, user_io(_,_,Output), _).


% handle a client request
user_io(login(Username), Listener, _) :- !,
	% login request: register the client name
	println(['login: ', Username]),
	bb_put(login_names, Listener, Username).
user_io(logout, Listener, Output) :- !,
	% logout request
	bb_get(login_names, Listener, Username),
	println(['logout: ', Username]),
	% unregister the client ...
	bb_remove(login_names, Output),
	% ... and close the connection.
	listener_stop(Listener).
user_io(request(Goal), Listener, Output) :-
	% command request:
	bb_get(login_names, Listener, Username),
	println(['request from ', Username, ': ', quoted(Goal)]),
	% interpret the sended term as goal, execute it ...
	catch(Goal, _, fail), !,
	% ... and send the result back to the client.
	send(Output, Goal).
user_io(request(_), _, Output) :- !,
	% command request: inform the client about the failed command execution
	send(Output, request_failed).


% ***** CLIENT ***************************************************
% should be started with: minerva -l socktest -client 
% The server can handle more than one client at a time, therefore
% you may start many clients.
%
% example:
% minerva -l socktest -client john
% ?- assertz(likes(john, anna)).
% ...
% minerva -l socktest -client anna
% ?- likes(Who, anna).
% : likes(john, anna)
% ...


run_client(Hostname, Port, Username) :-
	% starts a connection to the server
	% (which has been started on host 'Hostname' and is listening
	% on port 'Port' for a connection).
	client_start(Hostname, Port, Socket),
	% open binary input and output streams to the server
	open_connection(Socket, Input, Output),
	% start the communication loop ...
	communicate(Input, Output, Username),
	% ... until the client want's to stop the communication
	client_stop(Socket).


open_connection(Socket, Input, Output) :-
	open(Socket, read, Input, [socket,buffered,binary]),
	open(Socket, write, Output, [socket,buffered,binary]).


communicate(Input,Output,Username) :-
	% send a login command, announcing the client name to the server
	send(Output, login(Username)),
	repeat,
		% read some goal from standard input
		write('?- '), flush_output,
		read(Request),
		% send the request to the server
		evaluate_request(Request, Input, Output),
	Request == end_of_file,
	!.


evaluate_request(end_of_file,_,Output) :- !,
	% send a logout command.
	send(Output, logout).
evaluate_request(Request,Input,Output) :- !,
	% send a user command ...
	send(Output, request(Request)),
	% ... wait for the answer ...
	receive(Input, Answer),
	% ... and print it to standard output.
	println([': ', quoted(Answer)]).

client_start/3
client_stop/1
listener_start/3
listener_stop/3
receive/2
send/2
server_start/4
server_stop/1

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