Minimal Lwt multi-client echo server based on https://www.baturin.org/code/lwt-counter-server/

dune

(executable
  (name server)
  (libraries lwt.unix))

server.ml

open Lwt
open Lwt.Syntax

let rec request_response_loop ic oc =
  let* msg = Lwt_io.read_line_opt ic in
  match msg with
  | Some text ->
      let* () = Lwt_io.write_line oc text in
      request_response_loop ic oc
  | None -> return ()

let accept_conn conn_fd =
  let ic = Lwt_io.of_fd ~mode:Lwt_io.Input conn_fd in
  let oc = Lwt_io.of_fd ~mode:Lwt_io.Output conn_fd in
  request_response_loop ic oc

let rec server_loop sock =
  let* conn_fd, _ = Lwt_unix.accept sock in
  async (fun () -> accept_conn conn_fd);
  server_loop sock

let () =
  let open Lwt_unix in
  let sock = socket PF_INET SOCK_STREAM 0 in
  let sock_addr = ADDR_INET(Unix.inet_addr_loopback, 9000) in
  async (fun () -> bind sock sock_addr);
  listen sock 10;
  Lwt_main.run @@ server_loop sock