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Running the server under init or daemontools

Next to VNC, another option for deploying servers as continuously running processes is to use the Unix started scripts. One option here is to use init, and the other is to use e.g., daemontools. At this moment we will not give a complete scenario here.

How to keep the server running?

When starting SWI-Prolog's multi-threaded HTTP server, the main thread starts a number of server threads and then presents the normal interactive Prolog toplevel. This is great for development and under the VNC solution, but of no use for a non-interactive server, because Prolog terminates if it reads an end-of-file at the toplevel. We need to give the toplevel something to do. The common solution is to use thread_get_message/1 for that because it provides a non-polling indefinite sleep that can be stopped by sending a message to the thread. For example:

run :-
        http_server(http_dispatch, []),

We can make thread_get_message/1 return by sending a message to the main thread, either from an HTTP handler or by catching a signal. For example:

:- on_signal(hup, _, hup).

hup(_Signal) :-
        thread_send_message(main, stop).