diff --git a/src/plugins/server/main.ml b/src/plugins/server/main.ml
index 06d8471f772e6b7712291d9b7961f187290d5791..d825f29e92a4e384d772eb7adc8bf029c6baa27f 100644
--- a/src/plugins/server/main.ml
+++ b/src/plugins/server/main.ml
@@ -381,10 +381,19 @@ let in_range ~min:a ~max:b v = min (max a v) b
 
 let kill () = raise Killed
 
+let rooters = ref []
 let daemons = ref []
+let once callback = rooters := !rooters @ [ callback ]
 let on callback = daemons := !daemons @ [ callback ]
 let set_active activity =
-  List.iter (fun f -> try f activity with _ -> ()) !daemons
+  begin
+    if activity then
+      begin
+        List.iter (fun f -> try f () with _ -> ()) !rooters ;
+        rooters := [] ;
+      end ;
+    List.iter (fun f -> try f activity with _ -> ()) !daemons ;
+  end
 
 let create ~pretty ?(equal=(=)) ~fetch () =
   let polling = in_range ~min:1 ~max:200 (Senv.Polling.get ()) in
diff --git a/src/plugins/server/main.mli b/src/plugins/server/main.mli
index 4868d732093c60bcc6317b463158c7542b55ec76..8ca15e5190d7da690452d0208dd3f5efa3911a4c 100644
--- a/src/plugins/server/main.mli
+++ b/src/plugins/server/main.mli
@@ -141,6 +141,12 @@ val on_signal : signal -> (bool -> unit) -> unit
     Callbacks shall {i never} raise any exception. *)
 val on : (bool -> unit) -> unit
 
+(** Register a callback to listen for server initialization. All callbacks are
+    executed once, in their order of registration, and before activity
+    callbacks.
+    Callbacks shall {i never} raise any exception. *)
+val once : (unit -> unit) -> unit
+
 (** Register an asynchronous task on the server.
     When the server is not working in background, this is
     equivalent to [Task.call] ; otherwize,