Lwt and database access

Question

I can't get my database access work with lwt. Should I include it in a thread? How? Or make a new thread which returns a 'a lwt value? If so, what to do with that value?

The same goes for Printf.eprintf, which also seems to be blocked by lwt. So I use Lwt_io instead. But why would lwt block regular io?

What I have is a simple db request like Db.update session. It is within an Lwt_main.run main function. All this is within a CGI script (should not matter, database access works fine until I start with the lwt commands).

I can give you more code if needed.

Regards
Olle

Edit

let main sock env = 
  (* code omitted *)
  Gamesession.update_game_session env#db game_session_connected;
  (* code omitted *)

Lwt_main.run (main sock_listen env)

Edit 2

This was the solution:

Lwt_preemptive.detach (fun () -> Db.call) ()

Answer 1

Printf.eprintf is not "blocked", it's just that the buffering parameters are changed and often messages do not display before the end of the program. You should try eprintf "something\n%!" (%! means "flush"), but yes it's better to use Lwt_io.

For the database, I don't know, you did not say which library you're using (at least the one called ocaml-mysql is not Lwt-friendly, so it may require using Lwt_preemptive).

Edit

Your:

Lwt_preemptive.detach (fun () -> Db.call) ()

This call creates a thread that, once executed, returns immediately the function Db.call. So, basically in that case Lwt_preemptive.detach does nothing :)

I don't know ocaml-mysql but if:

Db.call: connection_params -> connection_handle

you would have

let lwt_db_call connection_params =
  Lwt_preemptive.detach Db.call connection_params