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) ()

One important thing about Lwt is that when you start using it you must use only Lwt. You must not use any blocking function with Lwt. — Thomash, Sep 28 '13 at 22:16
So what is a blocking function from Lwt's perspective? And how does it know? — Olle Härstedt, Sep 29 '13 at 21:38
@Olle, Have you cosidered using postgresql? It has nice examples about OCaml and Lwt. I can point out some. — Kakadu, Sep 30 '13 at 18:05
Hi Kakadu, thanks for your tips. My codebase is using MySQL, so switching is not an option right now. At least it couldn't be done easily. Some examples could help still, perhaps? If you have any links, I'd be happy to read them. — Olle Härstedt, Sep 30 '13 at 22:30

score 3 · Accepted Answer · edited Mar 13 '15 at 16:41

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

Yes, its the ocaml-mysql one. I'll look into Lwt_preemptive! — Olle Härstedt, Oct 01 '13 at 00:05
Any suggestions on how to use `Lwt_preemptive`? My script just hangs if I use `run_in_main`. — Olle Härstedt, Oct 01 '13 at 10:47

Lwt and database access

1 Answers1

Edit