7 The stream_pool library

The library(streampool) library dispatches input from multiple streams based on wait_for_input/3. It is part of the clib package as it is used most of the time together with the library(socket) library. On non-Unix systems it often can only be used with socket streams.

With SWI-Prolog 5.1.x, multi-threading often provides a good alternative to using this library. In this schema one thread watches the listening socket waiting for connections and either creates a thread per connection or processes the accepted connections with a pool of worker threads. The library library(http/thread_httpd) provides an example realising a mult-threaded HTTP server.

add_stream_to_pool(+Stream, :Goal)

Add Stream, which must be an input stream and ---on non-unix systems--- connected to a socket to the pool. If input is available on Stream, Goal is called.

delete_stream_from_pool(+Stream)

Delete the given stream from the pool. Succeeds, even if Stream is no member of the pool. If Stream is unbound the entire pool is emtied but unlike close_stream_pool/0 the streams are not closed.

close_stream_pool

Empty the pool, closing all streams that are part of it.

dispatch_stream_pool(+TimeOut)

Wait for maximum of TimeOut for input on any of the streams in the pool. If there is input, call the Goal associated with add_stream_to_pool/2. If Goal fails or raises an exception a message is printed. TimeOut is described with wait_for_input/3.

If Goal is called, there is some input on the associated stream. Goal must be careful not to block as this will block the entire pool.^{1This is hard to achieve at the moment as none of the Prolog read-commands provide for a timeout.}

stream_pool_main_loop

Calls dispatch_stream_pool/1 in a loop until the pool is empty.

Below is a very simple example that reads the first line of input and echos it back.

:- use_module(library(streampool)).

server(Port) :-
        tcp_socket(Socket),
        tcp_bind(Socket, Port),
        tcp_listen(Socket, 5),
        tcp_open_socket(Socket, In, _Out),
        add_stream_to_pool(In, accept(Socket)),
        stream_pool_main_loop.

accept(Socket) :-
        tcp_accept(Socket, Slave, Peer),
        tcp_open_socket(Slave, In, Out),
        add_stream_to_pool(In, client(In, Out, Peer)).

client(In, Out, _Peer) :-
        read_line_to_codes(In, Command),
        close(In),
        format(Out, 'Please to meet you: ~s~n', [Command]),
        close(Out),
        delete_stream_from_pool(In).