diff --git a/.gitmodules b/.gitmodules
index 6a111cdb..1554e2c4 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,6 @@
 [submodule "vendor/opentelemetry-proto"]
 	path = vendor/opentelemetry-proto
 	url = https://github.com/open-telemetry/opentelemetry-proto
+[submodule "vendor/atomic"]
+	path = vendor/atomic
+	url = https://github.com/c-cube/ocaml-atomic.git
diff --git a/dune-project b/dune-project
index d5a78af6..21e3f5a2 100644
--- a/dune-project
+++ b/dune-project
@@ -38,6 +38,8 @@
   (depends
    (ocaml (>= "4.08"))
    (dune (>= "2.3"))
+   (mtime (>= "1.4")) ; for spans
+   ; atomic ; vendored
    (opentelemetry (= :version))
    (ocaml-protoc (>= 2.1))
    (odoc :with-doc)
diff --git a/opentelemetry-client-ocurl.opam b/opentelemetry-client-ocurl.opam
index 7c4924a1..7133660b 100644
--- a/opentelemetry-client-ocurl.opam
+++ b/opentelemetry-client-ocurl.opam
@@ -10,6 +10,7 @@ bug-reports:
 depends: [
   "ocaml" {>= "4.08"}
   "dune" {>= "2.3"}
+  "mtime" {>= "1.4"}
   "opentelemetry" {= version}
   "ocaml-protoc" {>= "2.1"}
   "odoc" {with-doc}
diff --git a/src/client/dune b/src/client/dune
index c4f4685d..11696169 100644
--- a/src/client/dune
+++ b/src/client/dune
@@ -2,5 +2,6 @@
 (library
   (name opentelemetry_client_ocurl)
   (public_name opentelemetry-client-ocurl)
-  (libraries opentelemetry curl ocaml-protoc))
+  (libraries opentelemetry curl ocaml-protoc threads
+             atomic mtime mtime.clock.os))
 
diff --git a/src/client/opentelemetry_client_ocurl.ml b/src/client/opentelemetry_client_ocurl.ml
index 4800c2d8..d0ea0d31 100644
--- a/src/client/opentelemetry_client_ocurl.ml
+++ b/src/client/opentelemetry_client_ocurl.ml
@@ -6,6 +6,7 @@
 
 (* TODO *)
 
+module OT = Opentelemetry
 open Opentelemetry
 
 let[@inline] (let@) f x = f x
@@ -23,42 +24,90 @@ let set_mutex ~lock ~unlock : unit =
   lock_ := lock;
   unlock_ := unlock
 
+module Config = struct
+  type t = {
+    debug: bool;
+    url: string;
+    batch_traces: int option;
+    batch_metrics: int option;
+    batch_timeout_ms: int;
+    thread: bool;
+  }
+
+  let pp out self =
+    let ppiopt = Format.pp_print_option Format.pp_print_int in
+    let {debug; url; batch_traces; batch_metrics;
+         batch_timeout_ms; thread} = self in
+    Format.fprintf out "{@[ debug=%B;@ url=%S;@ \
+                        batch_traces=%a;@ batch_metrics=%a;@
+                        batch_timeout_ms=%d; thread=%B @]}"
+      debug url ppiopt batch_traces ppiopt batch_metrics
+      batch_timeout_ms thread
+
+  let make
+      ?(debug= !debug_)
+      ?(url= get_url())
+      ?(batch_traces=Some 400)
+      ?(batch_metrics=None)
+      ?(batch_timeout_ms=500)
+      ?(thread=true)
+      () : t =
+    { debug; url; batch_traces; batch_metrics; batch_timeout_ms;
+      thread; }
+end
+
+(* critical section for [f()] *)
 let[@inline] with_lock_ f =
   !lock_();
   Fun.protect ~finally:!unlock_ f
 
-let _init = lazy (
+let[@inline] with_mutex_ m f =
+  Mutex.lock m;
+  Fun.protect ~finally:(fun () -> Mutex.unlock m) f
+
+let _init_curl = lazy (
   Curl.global_init Curl.CURLINIT_GLOBALALL;
   at_exit Curl.global_cleanup;
 )
 
-module Backend() : Opentelemetry.Collector.BACKEND = struct
-  let() = Lazy.force _init
+type error = [
+  | `Status of int * Opentelemetry.Proto.Status.status
+  | `Failure of string
+]
 
-  (* TODO: use Curl.Multi, etc. *)
+let n_errors = Atomic.make 0
+let n_dropped = Atomic.make 0
+
+let report_err_ = function
+  | `Failure msg ->
+    Format.eprintf "@[<2>opentelemetry: export failed: %s@]@." msg
+  | `Status (code, status) ->
+    Format.eprintf "@[<2>opentelemetry: export failed with@ http code=%d@ status %a@]@."
+      code Proto.Status.pp_status status
+
+module type CURL = sig
+  val send : path:string -> decode:(Pbrt.Decoder.t -> 'a) -> string -> ('a, error) result
+  val cleanup : unit -> unit
+end
+
+(* create a curl client *)
+module Curl() : CURL = struct
+  open Opentelemetry.Proto
+  let() = Lazy.force _init_curl
 
-  let encoder = Pbrt.Encoder.create()
   let buf_res = Buffer.create 256
 
-  let rand_ = Random.State.make_self_init()
+  (* TODO: use Curl.Multi, etc. instead? *)
 
   (* http client *)
   let curl : Curl.t = Curl.init ()
 
   let cleanup () = Curl.cleanup curl
 
-  open Opentelemetry.Proto
-  open Opentelemetry.Collector
-
-  type error = [
-    | `Status of int * Status.status
-    | `Failure of string
-  ]
-
   (* TODO: use Curl multi *)
 
   (* send the content to the remote endpoint/path *)
-  let send_ ~path ~decode (bod:string) : ('a, error) result =
+  let send ~path ~decode (bod:string) : ('a, error) result =
     Curl.reset curl;
     if !debug_ then Curl.set_verbose curl true;
     Curl.set_url curl (!url ^ path);
@@ -100,48 +149,56 @@ module Backend() : Opentelemetry.Collector.BACKEND = struct
             ~code:(Int32.of_int code) ~message:(Bytes.unsafe_of_string msg) () in
         Error(`Status (code, status))
     with e -> Error (`Failure (Printexc.to_string e))
+end
 
-  let report_err_ = function
-    | `Failure msg ->
-      Format.eprintf "@[<2>opentelemetry: export failed: %s@]@." msg
-    | `Status (code, status) ->
-      Format.eprintf "@[<2>opentelemetry: export failed with@ http code=%d@ status %a@]@."
-        code Status.pp_status status
+module type PUSH = sig
+  type elt
+  val push : elt -> unit
+  val is_empty : unit -> bool
+  val is_big_enough : unit -> bool
+  val pop_iter_all : (elt -> unit) -> unit
+end
 
-  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()
+(* queue of fixed size *)
+module FQueue : sig
+  type 'a t
+  val create : dummy:'a -> int -> 'a t
+  val size : _ t -> int
+  val push : 'a t -> 'a -> bool (* true iff it could write element *)
+  val pop_iter_all : 'a t -> ('a -> unit) -> unit
+end = struct
+  type 'a t = {
+    arr: 'a array;
+    mutable i: int;
   }
 
-  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 create ~dummy n : _ t =
+    assert (n >= 1);
+    { arr=Array.make n dummy;
+      i=0;
+    }
+
+  let[@inline] size self = self.i
+  let[@inline] is_full self = self.i = Array.length self.arr
+
+  let push (self:_ t) x : bool =
+    if is_full self then false
+    else (
+      self.arr.(self.i) <- x;
+      self.i <- 1 + self.i;
+      true
+    )
+
+  let pop_iter_all (self: _ t) f =
+    for j=0 to self.i-1 do
+      f self.arr.(j)
+    done;
+    self.i <- 0
+end
+
+(* generate random IDs *)
+module Gen_ids() = struct
+  let rand_ = Random.State.make_self_init()
 
   let rand_bytes_8 () : bytes =
     let@() = with_lock_ in
@@ -171,15 +228,318 @@ module Backend() : Opentelemetry.Collector.BACKEND = struct
     b
 end
 
-let setup_ () =
-  let module B = Backend() in
+(** Callback for when an event is properly sent to the collector *)
+type over_cb = unit -> unit
+
+(** An emitter. This is used by {!Backend} below to forward traces/metrics/…
+    from the program to whatever collector client we have. *)
+module type EMITTER = sig
+  open Opentelemetry.Proto
+
+  val push_trace : Trace.resource_spans list -> over:over_cb -> unit
+  val push_metrics : Metrics.resource_metrics list -> over:over_cb -> unit
+
+  val cleanup : unit -> unit
+end
+
+type 'a push = (module PUSH with type elt = 'a)
+type on_full_cb = (unit -> unit)
+
+(* make a "push" object, along with a setter for a callback to call when
+   it's ready to emit a batch *)
+let mk_push (type a) ?batch () : (module PUSH with type elt = a) * (on_full_cb -> unit) =
+  let on_full: on_full_cb ref = ref ignore in
+  let push =
+    match batch with
+    | None ->
+      let r = ref None in
+      let module M = struct
+        type elt = a
+        let is_empty () = !r == None
+        let is_big_enough () = !r != None
+        let push x =
+          r := Some x; !on_full()
+        let pop_iter_all f = Option.iter f !r; r := None
+      end in
+      (module M : PUSH with type elt = a)
+
+    | Some n ->
+      let q = FQueue.create ~dummy:(Obj.magic 0) (3 * n) in
+      let module M = struct
+        type elt = a
+        let is_empty () = FQueue.size q = 0
+        let is_big_enough () = FQueue.size q >= n
+        let push x =
+          if not (FQueue.push q x) || FQueue.size q > n then (
+            !on_full();
+            if not (FQueue.push q x) then (
+              Atomic.incr n_dropped; (* drop item *)
+            )
+          )
+        let pop_iter_all f = FQueue.pop_iter_all q f
+      end in
+      (module M : PUSH with type elt = a)
+
+  in
+  push, ((:=) on_full)
+
+
+(* make an emitter.
+
+   exceptions inside should be caught, see
+   https://opentelemetry.io/docs/reference/specification/error-handling/ *)
+let mk_emitter ~(config:Config.t) () : (module EMITTER) =
+  let open Proto in
+
+  let continue = ref true in
+
+  let ((module E_trace) : (Trace.resource_spans list * over_cb) push), on_trace_full =
+    mk_push ?batch:config.batch_traces () in
+  let ((module E_metrics) : (Metrics.resource_metrics list * over_cb) push), on_metrics_full =
+    mk_push ?batch:config.batch_metrics () in
+
+  let encoder = Pbrt.Encoder.create() in
+
+  let ((module C) as curl) = (module Curl() : CURL) in
+
+  let emit_metrics (l:(Metrics.resource_metrics list*over_cb) list) =
+    Pbrt.Encoder.reset encoder;
+    let resource_metrics =
+      List.fold_left (fun acc (l,_) -> List.rev_append l acc) [] l in
+    Metrics_service.encode_export_metrics_service_request
+      (Metrics_service.default_export_metrics_service_request
+         ~resource_metrics ())
+      encoder;
+    begin match
+        C.send ~path:"/v1/metrics" ~decode:(fun _ -> ())
+          (Pbrt.Encoder.to_string encoder)
+      with
+      | Ok () -> ()
+      | Error err ->
+        (* TODO: log error _via_ otel? *)
+        Atomic.incr n_errors;
+        report_err_ err
+    end;
+    (* signal completion *)
+    List.iter (fun (_,over) -> over()) l;
+  in
+
+  let emit_traces (l:(Trace.resource_spans list * over_cb) list) =
+    Pbrt.Encoder.reset encoder;
+    let resource_spans =
+      List.fold_left (fun acc (l,_) -> List.rev_append l acc) [] l in
+    Trace_service.encode_export_trace_service_request
+      (Trace_service.default_export_trace_service_request ~resource_spans ())
+      encoder;
+    begin match
+        C.send ~path:"/v1/traces" ~decode:(fun _ -> ())
+          (Pbrt.Encoder.to_string encoder)
+      with
+      | Ok () -> ()
+      | Error err ->
+        (* TODO: log error _via_ otel? *)
+        Atomic.incr n_errors;
+        report_err_ err
+    end;
+    (* signal completion *)
+    List.iter (fun (_,over) -> over()) l;
+  in
+
+  let last_wakeup = Atomic.make (Mtime_clock.now()) in
+  let timeout = Mtime.Span.(config.batch_timeout_ms * ms) in
+  let batch_timeout() : bool =
+    let elapsed = Mtime.span (Mtime_clock.now()) (Atomic.get last_wakeup) in
+    Mtime.Span.compare elapsed timeout >= 0
+  in
+
+  let emit_metrics ?(force=false) () : bool =
+    if (force && not (E_metrics.is_empty())) ||
+        (not force && E_metrics.is_big_enough ()) then (
+      let batch = ref [] in
+      E_metrics.pop_iter_all (fun l -> batch := l :: !batch);
+      emit_metrics !batch;
+      Atomic.set last_wakeup (Mtime_clock.now());
+      true
+    ) else false
+  in
+  let emit_traces ?(force=false) () : bool =
+    if (force && not (E_trace.is_empty())) ||
+        (not force && E_trace.is_big_enough ()) then (
+      let batch = ref [] in
+      E_trace.pop_iter_all (fun l -> batch := l :: !batch);
+      emit_traces !batch;
+      Atomic.set last_wakeup (Mtime_clock.now());
+      true
+    ) else false
+  in
+
+  let[@inline] guard f =
+    try f()
+    with e ->
+      Printf.eprintf "opentelemetry-curl: uncaught exception: %s\n%!"
+        (Printexc.to_string e)
+  in
+
+  let emit_all_force () =
+    let@ () = guard in
+    ignore (emit_traces ~force:true () : bool);
+    ignore (emit_metrics ~force:true () : bool);
+  in
+
+
+  if config.thread then (
+    begin
+      let m = Mutex.create() in
+      set_mutex ~lock:(fun () -> Mutex.lock m) ~unlock:(fun () -> Mutex.unlock m);
+    end;
+
+    let ((module C) as curl) = (module Curl() : CURL) in
+
+    let m = Mutex.create() in
+    let cond = Condition.create() in
+
+    let bg_thread () =
+      while !continue do
+        let@ () = guard in
+        let timeout = batch_timeout() in
+        if emit_metrics ~force:timeout () then ()
+        else if emit_traces ~force:timeout () then ()
+        else (
+          (* wait *)
+          let@ () = with_mutex_ m in
+          Condition.wait cond m;
+        )
+      done;
+      (* flush remaining events *)
+      begin
+        let@ () = guard in
+        ignore (emit_traces ~force:true () : bool);
+        ignore (emit_metrics ~force:true () : bool);
+        C.cleanup();
+      end
+    in
+
+    let _: Thread.t = Thread.create bg_thread () in
+
+    let wakeup () =
+      with_mutex_ m (fun () -> Condition.signal cond);
+      Thread.yield()
+    in
+
+    (* wake up if a batch is full *)
+    on_metrics_full wakeup;
+    on_trace_full wakeup;
+
+    let module M = struct
+      let push_trace e ~over =
+        E_trace.push (e,over);
+        if batch_timeout() then wakeup()
+      let push_metrics e ~over =
+        E_metrics.push (e,over);
+        if batch_timeout() then wakeup()
+      let cleanup () =
+        continue := false;
+        with_mutex_ m (fun () -> Condition.broadcast cond)
+    end in
+    (module M)
+  ) else (
+
+    on_metrics_full (fun () ->
+        ignore (emit_metrics () : bool));
+    on_trace_full (fun () ->
+        ignore (emit_traces () : bool));
+
+    let cleanup () =
+      emit_all_force();
+      C.cleanup();
+    in
+
+    let module M = struct
+      let push_trace e ~over =
+        let@() = guard in
+        E_trace.push (e,over);
+        if batch_timeout() then emit_all_force()
+
+      let push_metrics e ~over =
+        let@() = guard in
+        E_metrics.push (e,over);
+        if batch_timeout() then emit_all_force()
+
+      let cleanup = cleanup
+    end in
+    (module M)
+  )
+
+module Backend(Arg : sig val config : Config.t end)()
+  : Opentelemetry.Collector.BACKEND
+= struct
+  include Gen_ids()
+
+  include (val mk_emitter ~config:Arg.config ())
+
+  open Opentelemetry.Proto
+  open Opentelemetry.Collector
+
+  let send_trace : Trace.resource_spans list sender = {
+    send=fun l ~over ~ret ->
+      let@() = with_lock_ in
+      if !debug_ then Format.eprintf "send spans %a@." (Format.pp_print_list Trace.pp_resource_spans) l;
+      push_trace l ~over;
+      ret()
+  }
+
+  let last_sent_metrics = Atomic.make (Mtime_clock.now())
+  let timeout_sent_metrics = Mtime.Span.(5 * s) (* send metrics from time to time *)
+
+  let additional_metrics () : _ list =
+      (* add exporter metrics to the lot? *)
+      let last_emit = Atomic.get last_sent_metrics in
+      let now = Mtime_clock.now() in
+      let add_own_metrics =
+        let elapsed = Mtime.span last_emit now in
+        Mtime.Span.compare elapsed timeout_sent_metrics > 0
+      in
+
+      if add_own_metrics then (
+        let open OT.Metrics in
+        Atomic.set last_sent_metrics now;
+        [make_resource_metrics [
+            sum ~name:"otel-export.dropped" ~is_monotonic:true [
+              int ~start_time_unix_nano:(Mtime.to_uint64_ns last_emit)
+                ~now:(Mtime.to_uint64_ns now) (Atomic.get n_dropped);
+            ];
+            sum ~name:"otel-export.errors" ~is_monotonic:true [
+              int ~start_time_unix_nano:(Mtime.to_uint64_ns last_emit)
+                ~now:(Mtime.to_uint64_ns now) (Atomic.get n_errors);
+            ];
+          ]]
+      ) else []
+
+  let send_metrics : Metrics.resource_metrics list sender = {
+    send=fun m ~over ~ret ->
+      let@() = with_lock_ in
+      if !debug_ then Format.eprintf "send metrics %a@." (Format.pp_print_list Metrics.pp_resource_metrics) m;
+
+      let m = List.rev_append (additional_metrics()) m in
+      push_metrics m ~over;
+      ret()
+  }
+end
+
+let setup_ ~(config:Config.t) () =
+  debug_ := config.debug;
+  let module B = Backend(struct let config=config end)() in
   Opentelemetry.Collector.backend := Some (module B);
   B.cleanup
 
-let setup() =
-  let cleanup = setup_() in
-  at_exit cleanup
+let setup ?(config=Config.make()) ?(enable=true) () =
+  if enable then (
+    let cleanup = setup_ ~config () in
+    at_exit cleanup
+  )
 
-let with_setup f =
-  let cleanup = setup_() in
-  Fun.protect ~finally:cleanup f
+let with_setup ?(config=Config.make()) ?(enable=true) () f =
+  if enable then (
+    let cleanup = setup_ ~config () in
+    Fun.protect ~finally:cleanup f
+  ) else f()
diff --git a/src/client/opentelemetry_client_ocurl.mli b/src/client/opentelemetry_client_ocurl.mli
index d933a982..e8ac12d9 100644
--- a/src/client/opentelemetry_client_ocurl.mli
+++ b/src/client/opentelemetry_client_ocurl.mli
@@ -11,8 +11,62 @@ val set_url : string -> unit
     or "OTEL_EXPORTER_OTLP_ENDPOINT" if set. *)
 
 val set_mutex : lock:(unit -> unit) -> unlock:(unit -> unit) -> unit
+(** Set a lock/unlock pair to protect the critical sections
+    of {!Opentelemetry.Collector.BACKEND} *)
 
-val setup : unit -> unit
-(** Setup endpoint. This modifies {!Opentelemetry.Collector.backend}. *)
+module Config : sig
+  type t = {
+    debug: bool;
 
-val with_setup : (unit -> 'a) -> 'a
+    url: string;
+    (** Url of the endpoint. Default is "http://localhost:4318",
+        or "OTEL_EXPORTER_OTLP_ENDPOINT" if set. *)
+
+    batch_traces: int option;
+    (** Batch traces? If [Some i], then this produces batches of (at most)
+        [i] items. If [None], there is no batching.
+
+        Note that traces and metrics are batched separately.
+        Default [Some 400].
+    *)
+
+    batch_metrics: int option;
+    (** Batch metrics? If [Some i], then this produces batches of (at most)
+        [i] items. If [None], there is no batching.
+
+        Note that traces and metrics are batched separately.
+        Default [None].
+    *)
+
+    batch_timeout_ms: int;
+    (** Number of milliseconds after which we will emit a batch, even
+        incomplete.
+        Note that the batch might take longer than that, because this is
+        only checked when a new event occurs. Default 500. *)
+
+    thread: bool;
+    (** Is there a background thread? Default [true] *)
+  }
+
+  val make :
+    ?debug:bool -> ?url:string ->
+    ?batch_traces:int option ->
+    ?batch_metrics:int option ->
+    ?batch_timeout_ms:int ->
+    ?thread:bool ->
+    unit -> t
+  (** Make a configuration *)
+
+  val pp : Format.formatter -> t -> unit
+end
+
+val setup : ?config:Config.t -> ?enable:bool -> unit -> unit
+(** Setup endpoint. This modifies {!Opentelemetry.Collector.backend}.
+    @param enable actually setup the backend (default true). This can
+      be used to enable/disable the setup depending on CLI arguments
+      or environment.
+    @param config configuration to use *)
+
+val with_setup : ?config:Config.t -> ?enable:bool -> unit -> (unit -> 'a) -> 'a
+(** [with_setup () f] is like [setup(); f()] but takes care of cleaning up
+    after [f()] returns. *)
diff --git a/src/opentelemetry.ml b/src/opentelemetry.ml
index 28cf5126..8ce4f77d 100644
--- a/src/opentelemetry.ml
+++ b/src/opentelemetry.ml
@@ -79,9 +79,9 @@ module Collector = struct
 
   (** Collector client interface. *)
   module type BACKEND = sig
-    val send_trace : Trace_service.export_trace_service_request sender
+    val send_trace : Trace.resource_spans list sender
 
-    val send_metrics : Metrics_service.export_metrics_service_request sender
+    val send_metrics : Metrics.resource_metrics list sender
 
     val rand_bytes_16 : unit -> bytes
     (** Generate 16 bytes of random data *)
@@ -102,18 +102,12 @@ module Collector = struct
   let send_trace (l:Trace.resource_spans list) ~over ~ret =
     match !backend with
     | None -> over(); ret()
-    | Some (module B) ->
-      let ev = Trace_service.default_export_trace_service_request
-          ~resource_spans:l () in
-      B.send_trace.send ev ~over ~ret
+    | Some (module B) -> B.send_trace.send l ~over ~ret
 
   let send_metrics (l:Metrics.resource_metrics list) ~over ~ret =
     match !backend with
     | None -> over(); ret()
-    | Some (module B) ->
-      let ev = Metrics_service.default_export_metrics_service_request
-          ~resource_metrics:l () in
-      B.send_metrics.send ev ~over ~ret
+    | Some (module B) -> B.send_metrics.send l ~over ~ret
 
   let rand_bytes_16 () =
     match !backend with
@@ -492,13 +486,17 @@ module Metrics = struct
   (* TODO: summary *)
   (* TODO: exemplar *)
 
-  (** Emit some metrics to the collector (sync). *)
-  let emit (l:t list) : unit =
+  (** Aggregate metrics into a {!Proto.Metrics.resource_metrics} *)
+  let make_resource_metrics (l:t list) : resource_metrics =
     let lm =
       default_instrumentation_library_metrics
         ~instrumentation_library:(Some Globals.instrumentation_library)
         ~metrics:l () in
-    let rm = default_resource_metrics
-        ~instrumentation_library_metrics:[lm] () in
+    default_resource_metrics
+      ~instrumentation_library_metrics:[lm] ()
+
+  (** Emit some metrics to the collector (sync). *)
+  let emit (l:t list) : unit =
+    let rm = make_resource_metrics l in
     Collector.send_metrics [rm] ~over:ignore ~ret:ignore
 end
diff --git a/tests/emit1.ml b/tests/emit1.ml
index cf63333d..38d06042 100644
--- a/tests/emit1.ml
+++ b/tests/emit1.ml
@@ -3,11 +3,30 @@ module T = Opentelemetry
 let spf = Printf.sprintf
 let (let@) f x = f x
 
+let sleep_inner = ref 0.1
+let sleep_outer = ref 2.0
+
 let run () =
   Printf.printf "collector is on %S\n%!" (Opentelemetry_client_ocurl.get_url());
+
+  (* regularly emit some metrics *)
+  let emit_gc() =
+    let gc = Gc.stat() in
+    T.Metrics.(
+      emit [
+        gauge ~name:"ocaml_opentracing.test.major_heap_words" [int gc.Gc.heap_words];
+        sum ~name:"ocaml_opentracing.test.minor_allocated" [float gc.Gc.minor_words];
+      ]);
+  in
+  let _al = Gc.create_alarm emit_gc in
+
+  let@ scope = T.Trace.with_ "run" in
+
   let i = ref 0 in
   while true do
-    let@ scope = T.Trace.with_ ~kind:T.Span.Span_kind_producer
+    let@ scope =
+      T.Trace.with_ ~trace_id:scope.trace_id ~parent:scope.span_id
+        ~kind:T.Span.Span_kind_producer
         "loop.outer" ~attrs:["i", `Int !i] in
 
     for j=0 to 4 do
@@ -16,15 +35,7 @@ let run () =
           ~trace_id:scope.trace_id ~parent:scope.span_id
           ~attrs:["j", `Int j]
           "loop.inner" in
-      Unix.sleepf 2.;
-
-      let gc = Gc.stat() in
-      T.Metrics.(
-        emit [
-          gauge ~name:"ocaml_opentracing.test.i" [int !i];
-          gauge ~name:"ocaml_opentracing.test.major_heap_words" [int gc.Gc.heap_words];
-          sum ~name:"ocaml_opentracing.test.minor_allocated" [float gc.Gc.minor_words];
-        ]);
+      Unix.sleepf !sleep_outer;
 
       incr i;
 
@@ -36,7 +47,7 @@ let run () =
         (* allocate some stuff *)
         let _arr = Sys.opaque_identity @@ Array.make (25 * 25551) 42.0 in
         ignore _arr;
-        Unix.sleepf 0.1;
+        Unix.sleepf !sleep_inner;
         if j=4 && !i mod 13 = 0 then failwith "oh no"; (* simulate a failure *)
 
         T.Trace.add_event scope (fun()->T.Event.make "done with alloc");
@@ -49,4 +60,26 @@ let () =
   Sys.catch_break true;
   T.Globals.service_name := "t1";
   T.Globals.service_namespace := Some "ocaml-otel.test";
-  Opentelemetry_client_ocurl.with_setup run
+
+  let thread = ref true in
+  let batch_traces = ref 400 in
+  let batch_metrics = ref 3 in
+  let opts = [
+    "--thread", Arg.Bool ((:=) thread), " use a background thread";
+    "--batch-traces", Arg.Int ((:=) batch_traces), " size of traces batch";
+    "--batch-metrics", Arg.Int ((:=) batch_metrics), " size of metrics batch";
+    "--sleep-inner", Arg.Set_float sleep_inner, " sleep (in s) in inner loop";
+    "--sleep-outer", Arg.Set_float sleep_outer, " sleep (in s) in outer loop";
+  ] |> Arg.align in
+
+  Arg.parse opts (fun _ -> ()) "emit1 [opt]*";
+
+  let some_if_nzero r = if !r > 0 then Some !r else None in
+  let config = Opentelemetry_client_ocurl.Config.make
+      ~batch_traces:(some_if_nzero batch_traces)
+      ~batch_metrics:(some_if_nzero batch_metrics)
+      ~thread:!thread () in
+  Format.printf "@[<2>sleep outer: %.3fs,@ sleep inner: %.3fs,@ config: %a@]@."
+    !sleep_outer !sleep_inner Opentelemetry_client_ocurl.Config.pp config;
+
+  Opentelemetry_client_ocurl.with_setup ~config () run
diff --git a/vendor/atomic b/vendor/atomic
new file mode 160000
index 00000000..12dc7c84
--- /dev/null
+++ b/vendor/atomic
@@ -0,0 +1 @@
+Subproject commit 12dc7c84f79606a8a0026ba1eb7856b3cdf6cab6
diff --git a/vendor/dune b/vendor/dune
new file mode 100644
index 00000000..b4ba0b2b
--- /dev/null
+++ b/vendor/dune
@@ -0,0 +1 @@
+(vendored_dirs atomic)