diff --git a/src/client/opentelemetry_client_ocurl.ml b/src/client/opentelemetry_client_ocurl.ml
index ab8292d4..4800c2d8 100644
--- a/src/client/opentelemetry_client_ocurl.ml
+++ b/src/client/opentelemetry_client_ocurl.ml
@@ -55,10 +55,12 @@ module Backend() : Opentelemetry.Collector.BACKEND = struct
     | `Failure of string
   ]
 
+  (* TODO: use Curl multi *)
+
   (* send the content to the remote endpoint/path *)
   let send_ ~path ~decode (bod:string) : ('a, error) result =
     Curl.reset curl;
-    Curl.set_verbose curl true;
+    if !debug_ then Curl.set_verbose curl true;
     Curl.set_url curl (!url ^ path);
     Curl.set_httppost curl [];
     Curl.set_httpheader curl ["Content-Type: application/x-protobuf"];
@@ -106,29 +108,40 @@ module Backend() : Opentelemetry.Collector.BACKEND = struct
       Format.eprintf "@[<2>opentelemetry: export failed with@ http code=%d@ status %a@]@."
         code Status.pp_status status
 
-  let send_trace (tr:Trace_service.export_trace_service_request) : unit =
-    let@() = with_lock_ in
-    if !debug_ then Format.eprintf "send trace %a@." Trace_service.pp_export_trace_service_request tr;
-    Pbrt.Encoder.reset encoder;
-    Trace_service.encode_export_trace_service_request tr encoder;
-    match
-      send_ ~path:"/v1/traces" ~decode:(fun _ -> ())
-        (Pbrt.Encoder.to_string encoder)
-    with
-    | Ok () -> ()
-    | Error err -> report_err_ err
+  let send_trace : Trace_service.export_trace_service_request sender = {
+    send=fun tr ~over ~ret ->
+      let@() = with_lock_ in
+      if !debug_ then Format.eprintf "send trace %a@." Trace_service.pp_export_trace_service_request tr;
+      Pbrt.Encoder.reset encoder;
+      Trace_service.encode_export_trace_service_request tr encoder;
+      begin match
+          send_ ~path:"/v1/traces" ~decode:(fun _ -> ())
+            (Pbrt.Encoder.to_string encoder)
+        with
+        | Ok () -> ()
+        | Error err -> report_err_ err
+      end;
+      over();
+      ret()
+  }
 
-  let send_metrics (m:Metrics_service.export_metrics_service_request) : unit =
-    let@() = with_lock_ in
-    if !debug_ then Format.eprintf "send metrics %a@." Metrics_service.pp_export_metrics_service_request m;
-    Pbrt.Encoder.reset encoder;
-    Metrics_service.encode_export_metrics_service_request m encoder;
-    match
-      send_ ~path:"/v1/metrics" ~decode:(fun _ -> ())
-        (Pbrt.Encoder.to_string encoder);
-    with
-    | Ok () -> ()
-    | Error err -> report_err_ err
+  let send_metrics : Metrics_service.export_metrics_service_request sender = {
+    send=fun m ~over ~ret ->
+      let@() = with_lock_ in
+      if !debug_ then Format.eprintf "send metrics %a@." Metrics_service.pp_export_metrics_service_request m;
+      Pbrt.Encoder.reset encoder;
+      Metrics_service.encode_export_metrics_service_request m encoder;
+      begin
+        match
+          send_ ~path:"/v1/metrics" ~decode:(fun _ -> ())
+            (Pbrt.Encoder.to_string encoder);
+        with
+        | Ok () -> ()
+        | Error err -> report_err_ err
+      end;
+      over();
+      ret()
+  }
 
   let rand_bytes_8 () : bytes =
     let@() = with_lock_ in
diff --git a/src/opentelemetry.ml b/src/opentelemetry.ml
index 71701a7a..04765491 100644
--- a/src/opentelemetry.ml
+++ b/src/opentelemetry.ml
@@ -70,11 +70,18 @@ end
 module Collector = struct
   open Proto
 
+  (** Sender interface for a message of type [msg].
+      Inspired from Logs' reporter
+      (see {{:https://erratique.ch/software/logs/doc/Logs/index.html#sync} its doc}) *)
+  type 'msg sender = {
+    send: 'a. 'msg -> over:(unit -> unit) -> ret:(unit -> 'a) -> 'a;
+  }
+
   (** Collector client interface. *)
   module type BACKEND = sig
-    val send_trace : Trace_service.export_trace_service_request -> unit
+    val send_trace : Trace_service.export_trace_service_request sender
 
-    val send_metrics : Metrics_service.export_metrics_service_request -> unit
+    val send_metrics : Metrics_service.export_metrics_service_request sender
 
     val rand_bytes_16 : unit -> bytes
     (** Generate 16 bytes of random data *)
@@ -89,21 +96,21 @@ module Collector = struct
 
   let backend : backend option ref = ref None
 
-  let send_trace (l:Trace.resource_spans list) : unit =
+  let send_trace (l:Trace.resource_spans list) ~over ~ret =
     match !backend with
-    | None -> ()
+    | None -> over(); ret()
     | Some (module B) ->
       let ev = Trace_service.default_export_trace_service_request
           ~resource_spans:l () in
-      B.send_trace ev
+      B.send_trace.send ev ~over ~ret
 
-  let send_metrics (l:Metrics.resource_metrics list) : unit =
+  let send_metrics (l:Metrics.resource_metrics list) ~over ~ret =
     match !backend with
-    | None -> ()
+    | None -> over(); ret()
     | Some (module B) ->
       let ev = Metrics_service.default_export_metrics_service_request
           ~resource_metrics:l () in
-      B.send_metrics ev
+      B.send_metrics.send ev ~over ~ret
 
   let rand_bytes_16 () =
     match !backend with
@@ -272,12 +279,14 @@ module Trace = struct
 
   type span = Span.t
 
+  (** Sync emitter *)
   let emit (spans:span list) : unit =
     let ils =
       default_instrumentation_library_spans ~spans () in
     let rs = default_resource_spans ~instrumentation_library_spans:[ils] () in
-    Collector.send_trace [rs]
+    Collector.send_trace [rs] ~over:(fun () -> ()) ~ret:(fun () -> ())
 
+  (** Sync span guard *)
   let with_
       ?trace_state ?service_name ?attrs
       ?kind ?(trace_id=Trace_id.create()) ?parent ?links
@@ -351,17 +360,11 @@ module Metrics = struct
   (* TODO: summary *)
   (* TODO: exemplar *)
 
-  (** Emit a bunch of metrics to the collector. *)
+  (** Emit some metrics to the collector (sync). *)
   let emit (l:t list) : unit =
     let lm =
       default_instrumentation_library_metrics ~metrics:l () in
     let rm = default_resource_metrics
         ~instrumentation_library_metrics:[lm] () in
-    Collector.send_metrics [rm]
+    Collector.send_metrics [rm] ~over:ignore ~ret:ignore
 end
-
-(*
-module Span = Span
-module Timestamp = Timestamp
-   *)
-