diff --git a/src/main/learnocaml_main.ml b/src/main/learnocaml_main.ml
index 4bc49bad4..c09091920 100644
--- a/src/main/learnocaml_main.ml
+++ b/src/main/learnocaml_main.ml
@@ -292,9 +292,15 @@ let process_html_file orig_file dest_file base_url no_secret =
   Lwt_io.close ofile >>= fun () ->
   Lwt_io.close wfile
 
+let temp_app_dir o =
+  let open Filename in
+  concat
+    (dirname o.app_dir)
+    ((basename o.app_dir) ^ ".temp")
+
 let main o =
-  Printf.printf "Learnocaml v.%s running.\n" Learnocaml_api.version;
-  let grade () =
+  Printf.printf "Learnocaml v.%s running.\n%!" Learnocaml_api.version;
+  let grade o =
     if List.mem Grade o.commands then
       (if List.mem Build o.commands || List.mem Serve o.commands then
          failwith "The 'grade' command is incompatible with 'build' and \
@@ -322,9 +328,34 @@ let main o =
        >|= fun i -> Some i)
     else Lwt.return_none
   in
-  let generate () =
+  let generate o =
     if List.mem Build o.commands then
-      (Printf.printf "Updating app at %s\n%!" o.app_dir;
+      (let get_app_dir o =
+         if not (List.mem Serve o.commands) then
+           Lwt.return o.app_dir
+         else if o.server.Server.replace then
+           let app_dir = temp_app_dir o in
+           (if Sys.file_exists app_dir then
+             (Printf.eprintf "Warning: temporary directory %s already exists\n%!"
+                app_dir;
+              Lwt.return_unit)
+            else if Sys.file_exists o.app_dir then
+              Lwt_utils.copy_tree o.app_dir app_dir
+            else
+              Lwt.return_unit)
+           >>= fun () -> Lwt.return app_dir
+         else if Learnocaml_server.check_running () <> None then
+           (Printf.eprintf
+              "Error: another server is already running on port %d \
+               (consider using option `--replace`)\n%!"
+              !Learnocaml_server.port;
+            exit 10)
+         else Lwt.return o.app_dir
+       in
+       get_app_dir o >>= fun app_dir ->
+       let o = { o with app_dir } in
+       Learnocaml_store.static_dir := app_dir;
+       Printf.printf "Updating app at %s\n%!" o.app_dir;
        Lwt.catch
          (fun () -> Lwt_utils.copy_tree o.builder.Builder.contents_dir o.app_dir)
          (function
@@ -404,8 +435,44 @@ let main o =
     else
       Lwt.return true
   in
-  let run_server () =
+  let run_server o =
     if List.mem Serve o.commands then
+      let () =
+        if o.server.Server.replace then
+          let running = Learnocaml_server.check_running () in
+          Option.iter Learnocaml_server.kill_running running;
+          let temp = temp_app_dir o in
+          let app_dir =
+            if Filename.is_relative o.app_dir
+            then Filename.concat (Sys.getcwd ()) o.app_dir
+            else o.app_dir
+          in
+          let bak =
+            let f =
+              Filename.temp_file
+                ~temp_dir:(Filename.dirname app_dir)
+                (Filename.basename app_dir ^ ".bak.")
+                ""
+            in
+            Unix.unlink f; f
+          in
+          if Sys.file_exists app_dir then Sys.rename app_dir bak;
+          Sys.rename temp o.app_dir;
+          Learnocaml_store.static_dir := app_dir;
+          if Sys.file_exists bak then
+            Lwt.dont_wait (fun () ->
+                Lwt.pause () >>= fun () ->
+                Lwt_process.exec ("rm",[|"rm";"-rf";bak|]) >>= fun r ->
+                if r <> Unix.WEXITED 0 then
+                  Lwt.fail_with "Remove command failed"
+                else Lwt.return_unit
+              )
+              (fun ex ->
+                 Printf.eprintf
+                   "Warning: while cleaning up older application \
+                    directory %s:\n    %s\n%!"
+                   bak (Printexc.to_string ex))
+      in
       let native_server = Sys.executable_name ^ "-server" in
       if Sys.file_exists native_server then
         let server_args =
@@ -416,30 +483,39 @@ let main o =
           ("--port="^string_of_int o.server.port) ::
           (match o.server.cert with None -> [] | Some c -> ["--cert="^c])
         in
-        Unix.execv native_server (Array.of_list (native_server::server_args))
+        Lwt.return
+          (`Continuation
+             (fun () ->
+                Unix.execv native_server
+                  (Array.of_list (native_server::server_args))))
       else begin
           Printf.printf "Starting server on port %d\n%!"
             !Learnocaml_server.port;
           if o.builder.Builder.base_url <> "" then
             Printf.printf "Base URL: %s\n%!" o.builder.Builder.base_url;
-          Learnocaml_server.launch ()
+          Learnocaml_server.launch () >>= fun ret ->
+          Lwt.return (`Success ret)
         end
     else
-      Lwt.return true
+      Lwt.return (`Success true)
   in
   let ret =
     Lwt_main.run
-      (grade () >>= function
-        | Some i -> Lwt.return i
+      (grade o >>= function
+        | Some i -> Lwt.return (`Code i)
         | None ->
-            generate () >>= fun success ->
+            generate o >>= fun success ->
             if success then
-              run_server () >>= fun r ->
-              if r then Lwt.return 0 else Lwt.return 10
+              run_server o >>= function
+                | `Success true -> Lwt.return (`Code 0)
+                | `Success false -> Lwt.return (`Code 10)
+                | `Continuation f -> Lwt.return (`Continuation f)
             else
-              Lwt.return 1)
+              Lwt.return (`Code 1))
   in
-  exit ret
+  match ret with
+  | `Code n -> exit n
+  | `Continuation f -> f ()
 
 let man = 
   let open Manpage in
diff --git a/src/main/learnocaml_server_args.ml b/src/main/learnocaml_server_args.ml
index f089df0ae..04706025b 100644
--- a/src/main/learnocaml_server_args.ml
+++ b/src/main/learnocaml_server_args.ml
@@ -19,6 +19,7 @@ module type S = sig
     base_url: string;
     port: int;
     cert: string option;
+    replace: bool;
   }
 
   val term: string Cmdliner.Term.t -> string Cmdliner.Term.t -> t Cmdliner.Term.t
@@ -51,15 +52,21 @@ module Args (SN : Section_name) = struct
           HTTPS is enabled."
           default_http_port default_https_port)
 
+  let replace =
+    value & flag &
+    info ["replace"] ~doc:
+      "Replace a previously running instance of the server on the same port."
+
   type t = {
     sync_dir: string;
     base_url: string;
     port: int;
     cert: string option;
+    replace: bool;
   }
 
   let term app_dir base_url =
-    let apply app_dir sync_dir base_url port cert =
+    let apply app_dir sync_dir base_url port cert replace =
       Learnocaml_store.static_dir := app_dir;
       Learnocaml_store.sync_dir := sync_dir;
       let port = match port, cert with
@@ -73,10 +80,10 @@ module Args (SN : Section_name) = struct
         | None -> None);
       Learnocaml_server.port := port;
       Learnocaml_server.base_url := base_url;
-      { sync_dir; base_url; port; cert }
+      { sync_dir; base_url; port; cert; replace }
     in
   (* warning: if you add any options here, remember to pass them through when
      calling the native server from learn-ocaml main *)
-    Term.(const apply $ app_dir $ sync_dir $ base_url $ port $ cert)
+    Term.(const apply $ app_dir $ sync_dir $ base_url $ port $ cert $ replace)
 
 end
diff --git a/src/main/learnocaml_server_args.mli b/src/main/learnocaml_server_args.mli
index a4bfe27f2..c10e3bebd 100644
--- a/src/main/learnocaml_server_args.mli
+++ b/src/main/learnocaml_server_args.mli
@@ -16,9 +16,10 @@ module type S = sig
     base_url: string;
     port: int;
     cert: string option;
+    replace: bool;
   }
 
   val term: string Cmdliner.Term.t -> string Cmdliner.Term.t -> t Cmdliner.Term.t
 end
 
-module Args : functor (_ : Section_name) -> S
\ No newline at end of file
+module Args : functor (_ : Section_name) -> S
diff --git a/src/main/learnocaml_server_main.ml b/src/main/learnocaml_server_main.ml
index 00fccac88..3fd57d53b 100644
--- a/src/main/learnocaml_server_main.ml
+++ b/src/main/learnocaml_server_main.ml
@@ -31,6 +31,17 @@ let main o =
     Learnocaml_api.version o.port;
   if o.base_url <> "" then
     Printf.printf "Base URL: %s\n%!" o.base_url;
+  let () =
+    match Learnocaml_server.check_running (), o.replace with
+    | None, _ -> ()
+    | Some _, false ->
+        Printf.eprintf "Error: another server is already running on port %d \
+                        (consider using option `--replace`)\n%!"
+          !Learnocaml_server.port;
+        exit 10
+    | Some pid, true ->
+        Learnocaml_server.kill_running pid
+  in
   let rec run () =
     let minimum_duration = 15. in
     let t0 = Unix.time () in
diff --git a/src/repo/learnocaml_process_exercise_repository.ml b/src/repo/learnocaml_process_exercise_repository.ml
index 4b346f7a9..3bc70ec0a 100644
--- a/src/repo/learnocaml_process_exercise_repository.ml
+++ b/src/repo/learnocaml_process_exercise_repository.ml
@@ -47,6 +47,22 @@ let dump_dot exs =
 
 let n_processes = ref 1
 
+let grading_status, grading_status_add, grading_status_remove =
+  let in_progress = ref [] in
+  let tty = Unix.isatty Unix.stderr in
+  let show () =
+    match !in_progress with
+    | [] -> flush stderr
+    | prog ->
+        Printf.eprintf "Grading in progress: %s" (String.concat " " prog);
+        if tty then (flush stderr; prerr_string "\r\027[K") else prerr_newline ()
+  in
+  show,
+  (fun id -> in_progress := !in_progress @ [id]; show ()),
+  (fun id ->
+     in_progress := List.filter (fun x -> not (String.equal x id)) !in_progress;
+     show ())
+
 let print_grader_error exercise = function
   | Ok () -> ()
   | Error (-1) -> ()
@@ -59,25 +75,29 @@ let print_grader_error exercise = function
 
 let spawn_grader
     dump_outputs dump_reports
-    ?print_result ?dirname meta exercise output_json =
+    ?print_result ?dirname id meta ex_dir output_json =
   let rec sleep () =
     if !n_processes <= 0 then
-      Lwt_main.yield () >>= sleep
+      Lwt.pause () >>= sleep
     else (
       decr n_processes; Lwt.return_unit
     )
   in
   sleep () >>= fun () ->
   Lwt.catch (fun () ->
+      read_exercise ex_dir >>= fun exercise ->
+      grading_status_add id;
       Grader_cli.grade
         ~dump_outputs ~dump_reports ~display_callback:false
         ?print_result ?dirname meta exercise output_json
       >|= fun r ->
+      grading_status_remove id;
       print_grader_error exercise r;
       incr n_processes;
       r)
     (fun e ->
        incr n_processes;
+       grading_status_remove id;
        Printf.eprintf "Grader error: %s\n%!" (Printexc.to_string e);
        Lwt.return (Error 0))
 
@@ -213,7 +233,8 @@ let main dest_dir =
            if !n_processes = 1 then
              Lwt_list.map_s,
              fun dump_outputs dump_reports ?print_result ?dirname
-               meta exercise json_path ->
+               _id meta ex_dir json_path ->
+               read_exercise ex_dir >>= fun exercise ->
                Grader_cli.grade
                  ~dump_outputs ~dump_reports ~display_callback:true
                  ?print_result ?dirname
@@ -234,28 +255,31 @@ let main dest_dir =
                  else Lwt.return_unit)
                  (Lwt_unix.files_of_directory ex_dir) >>= fun () ->
                if not changed then begin
-                 Format.printf "%-24s (no changes)@." id ;
+                 Format.eprintf "%-24s (no changes)@." id ;
                  Lwt.return_true
                end else begin
                  Learnocaml_precompile_exercise.precompile ~exercise_dir:ex_dir
                  >>= fun () ->
-                 read_exercise ex_dir
-                 >>= fun exercise ->
                  grade dump_outputs dump_reports
-                   ~dirname:ex_dir (Index.find index id) exercise (Some json_path)
+                   ~dirname:ex_dir id (Index.find index id) ex_dir (Some json_path)
                  >>= function
                  | Ok () ->
-                     Format.printf "%-24s     [OK]@." id ;
+                     Format.eprintf "%-24s     [OK]@." id ;
                      Lwt.return true
                  | Error _ ->
-                     Format.printf "%-24s   [FAILED]@." id ;
+                     Format.eprintf "%-24s   [FAILED]@." id ;
                      Lwt.return false
-               end)
+               end
+                 >|= fun r -> grading_status (); r)
              processes_arguments
        end >>= fun results ->
        Lwt.return (List.for_all ((=) true) results))
     (fun exn ->
        let print_unknown ppf = function
+         | Unix.Unix_error (Unix.EMFILE, _, _) ->
+             Format.fprintf ppf
+               "Too many open files. Try reducing the number of concurrent jobs \
+                (with the `-j` flag) or use `ulimit -n` with a higher value"
          | Failure msg -> Format.fprintf ppf "Cannot process exercises: %s" msg
          | exn -> Format.fprintf ppf "Cannot process exercises: %s"  (Printexc.to_string exn) in
        Json_encoding.print_error ~print_unknown Format.err_formatter exn ;
diff --git a/src/server/learnocaml_server.ml b/src/server/learnocaml_server.ml
index af1fdd830..1a4c7664f 100644
--- a/src/server/learnocaml_server.ml
+++ b/src/server/learnocaml_server.ml
@@ -715,3 +715,35 @@ let launch () =
   | e ->
       Printf.eprintf "Server error: %s\n%!" (Printexc.to_string e);
       Lwt.return false
+
+let check_running () =
+  try
+    let ic = Printf.ksprintf Unix.open_process_in "lsof -Qti tcp:%d -s tcp:LISTEN" !port in
+    let pid = match input_line ic with
+      | "" -> None
+      | s -> int_of_string_opt s
+      | exception End_of_file -> None
+    in
+    close_in ic;
+    pid
+  with Unix.Unix_error _ ->
+    Printf.eprintf "Warning: could not check for previously running instance";
+    None
+
+let kill_running pid =
+  let timeout = 15 in
+  Unix.kill pid Sys.sigint;
+  Printf.eprintf "Waiting for process %d to terminate... %2d%!" pid timeout;
+  let rec aux tout =
+    Printf.eprintf "\027[2D%2d" tout;
+    if Printf.ksprintf Sys.command "lsof -ti tcp:%d -p %d >/dev/null" !port pid
+       = 0
+    then
+      if tout <= 0 then
+        (prerr_endline "Error: process didn't terminate in time"; exit 10)
+      else
+        (Unix.sleep 1;
+         aux (tout - 1))
+  in
+  aux timeout;
+  prerr_endline "\027[2Dok"
diff --git a/src/server/learnocaml_server.mli b/src/server/learnocaml_server.mli
index e8153306c..a1606a433 100644
--- a/src/server/learnocaml_server.mli
+++ b/src/server/learnocaml_server.mli
@@ -16,5 +16,12 @@ val args: (Arg.key * Arg.spec * Arg.doc) list
 
 (** Main *)
 
-(* Returns [false] if interrupted prematurely due to an error *)
+val check_running: unit -> int option
+(** Returns the pid or an existing process listening on the tcp port *)
+
+val kill_running: int -> unit
+(** Kills the given process and waits for termination (fails upon
+    reaching a timeout) *)
+
 val launch: unit -> bool Lwt.t
+(** Returns [false] if interrupted prematurely due to an error *)