diff --git a/src/client/AList.ml b/src/client/AList.ml
new file mode 100644
index 00000000..0a8c54b0
--- /dev/null
+++ b/src/client/AList.ml
@@ -0,0 +1,13 @@
+module Atomic = Opentelemetry_atomic.Atomic
+
+type 'a t = 'a list Atomic.t
+let make () = Atomic.make []
+let add self x =
+  while
+    let old = Atomic.get self in
+    let l' = x :: old in
+    not (Atomic.compare_and_set self old l')
+  do () done
+let rec pop_all self =
+  let l = Atomic.get self in
+  if Atomic.compare_and_set self l [] then l else pop_all self
diff --git a/src/client/AList.mli b/src/client/AList.mli
new file mode 100644
index 00000000..f4db8292
--- /dev/null
+++ b/src/client/AList.mli
@@ -0,0 +1,6 @@
+(** Atomic list *)
+
+type 'a t
+val make : unit -> 'a t
+val add : 'a t -> 'a -> unit
+val pop_all : 'a t -> 'a list
diff --git a/src/client/FQueue.ml b/src/client/FQueue.ml
new file mode 100644
index 00000000..fc48def3
--- /dev/null
+++ b/src/client/FQueue.ml
@@ -0,0 +1,29 @@
+
+
+type 'a t = {
+  arr: 'a array;
+  mutable i: int;
+}
+
+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
diff --git a/src/client/FQueue.mli b/src/client/FQueue.mli
new file mode 100644
index 00000000..b80544d2
--- /dev/null
+++ b/src/client/FQueue.mli
@@ -0,0 +1,9 @@
+
+(** queue of fixed size *)
+
+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
+
diff --git a/src/client/common_.ml b/src/client/common_.ml
new file mode 100644
index 00000000..7646b327
--- /dev/null
+++ b/src/client/common_.ml
@@ -0,0 +1,26 @@
+module Atomic = Opentelemetry_atomic.Atomic
+
+let[@inline] (let@) f x = f x
+
+let debug_ = ref (try bool_of_string @@ Sys.getenv "DEBUG" with _ -> false)
+
+let lock_ : (unit -> unit) ref = ref ignore
+let unlock_ : (unit -> unit) ref = ref ignore
+
+let set_mutex ~lock ~unlock : unit =
+  lock_ := lock;
+  unlock_ := unlock
+
+(* critical section for [f()] *)
+let[@inline] with_lock_ f =
+  !lock_();
+  Fun.protect ~finally:!unlock_ f
+
+let[@inline] with_mutex_ m f =
+  Mutex.lock m;
+  Fun.protect ~finally:(fun () -> Mutex.unlock m) f
+
+let default_url = "http://localhost:4318"
+let url = ref (try Sys.getenv "OTEL_EXPORTER_OTLP_ENDPOINT" with _ -> default_url)
+let get_url () = !url
+let set_url s = url := s
diff --git a/src/client/config.ml b/src/client/config.ml
new file mode 100644
index 00000000..2ecb2b8d
--- /dev/null
+++ b/src/client/config.ml
@@ -0,0 +1,35 @@
+
+open Common_
+
+type t = {
+  debug: bool;
+  url: string;
+  batch_traces: int option;
+  batch_metrics: int option;
+  batch_timeout_ms: int;
+  thread: bool;
+  ticker_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; ticker_thread} = self in
+  Format.fprintf out
+    "{@[ debug=%B;@ url=%S;@ \
+     batch_traces=%a;@ batch_metrics=%a;@ \
+     batch_timeout_ms=%d; thread=%B;@ ticker_thread=%B @]}"
+    debug url ppiopt batch_traces ppiopt batch_metrics
+    batch_timeout_ms thread ticker_thread
+
+let make
+    ?(debug= !debug_)
+    ?(url= get_url())
+    ?(batch_traces=Some 400)
+    ?(batch_metrics=None)
+    ?(batch_timeout_ms=500)
+    ?(thread=true)
+    ?(ticker_thread=true)
+    () : t =
+  { debug; url; batch_traces; batch_metrics; batch_timeout_ms;
+    thread; ticker_thread; }
diff --git a/src/client/config.mli b/src/client/config.mli
new file mode 100644
index 00000000..892c91af
--- /dev/null
+++ b/src/client/config.mli
@@ -0,0 +1,51 @@
+
+type t = {
+  debug: bool;
+
+  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] *)
+
+  ticker_thread: bool;
+  (** Is there a ticker thread? Default [true].
+      This thread will regularly call [tick()] on the backend, to make
+      sure it makes progress, and regularly send events to the collector.
+      This option is ignored if [thread=false]. *)
+}
+
+val make :
+  ?debug:bool -> ?url:string ->
+  ?batch_traces:int option ->
+  ?batch_metrics:int option ->
+  ?batch_timeout_ms:int ->
+  ?thread:bool ->
+  ?ticker_thread:bool ->
+  unit -> t
+(** Make a configuration *)
+
+val pp : Format.formatter -> t -> unit
diff --git a/src/client/gen_ids.ml b/src/client/gen_ids.ml
new file mode 100644
index 00000000..049f6efc
--- /dev/null
+++ b/src/client/gen_ids.ml
@@ -0,0 +1,34 @@
+
+open Common_
+
+(* generate random IDs *)
+module Make() = struct
+  let rand_ = Random.State.make_self_init()
+
+  let rand_bytes_8 () : bytes =
+    let@() = with_lock_ in
+    let b = Bytes.create 8 in
+    for i=0 to 1 do
+      let r = Random.State.bits rand_ in (* 30 bits, of which we use 24 *)
+      Bytes.set b (i*3) (Char.chr (r land 0xff));
+      Bytes.set b (i*3+1) (Char.chr (r lsr 8 land 0xff));
+      Bytes.set b (i*3+2) (Char.chr (r lsr 16 land 0xff));
+    done;
+    let r = Random.State.bits rand_ in
+    Bytes.set b 6 (Char.chr (r land 0xff));
+    Bytes.set b 7 (Char.chr (r lsr 8 land 0xff));
+    b
+
+  let rand_bytes_16 () : bytes =
+    let@() = with_lock_ in
+    let b = Bytes.create 16 in
+    for i=0 to 4 do
+      let r = Random.State.bits rand_ in (* 30 bits, of which we use 24 *)
+      Bytes.set b (i*3) (Char.chr (r land 0xff));
+      Bytes.set b (i*3+1) (Char.chr (r lsr 8 land 0xff));
+      Bytes.set b (i*3+2) (Char.chr (r lsr 16 land 0xff));
+    done;
+    let r = Random.State.bits rand_ in
+    Bytes.set b 15 (Char.chr (r land 0xff)); (* last byte *)
+    b
+end
diff --git a/src/client/opentelemetry_client_ocurl.ml b/src/client/opentelemetry_client_ocurl.ml
index 0fca427d..780ef6c2 100644
--- a/src/client/opentelemetry_client_ocurl.ml
+++ b/src/client/opentelemetry_client_ocurl.ml
@@ -4,40 +4,9 @@
    https://github.com/open-telemetry/oteps/blob/main/text/0099-otlp-http.md
  *)
 
-(* TODO *)
-
 module OT = Opentelemetry
 open Opentelemetry
-module Atomic = Opentelemetry_atomic.Atomic
-
-(** Atomic list *)
-module AList : sig
-  type 'a t
-  val make : unit -> 'a t
-  val add : 'a t -> 'a -> unit
-  val pop_all : 'a t -> 'a list
-end = struct
-  type 'a t = 'a list Atomic.t
-  let make () = Atomic.make []
-  let add self x =
-    while
-      let old = Atomic.get self in
-      let l' = x :: old in
-      not (Atomic.compare_and_set self old l')
-    do () done
-  let rec pop_all self =
-    let l = Atomic.get self in
-    if Atomic.compare_and_set self l [] then l else pop_all self
-end
-
-let[@inline] (let@) f x = f x
-
-let debug_ = ref (try bool_of_string @@ Sys.getenv "DEBUG" with _ -> false)
-
-let default_url = "http://localhost:4318"
-let url = ref (try Sys.getenv "OTEL_EXPORTER_OTLP_ENDPOINT" with _ -> default_url)
-let get_url () = !url
-let set_url s = url := s
+include Common_
 
 let needs_gc_metrics = Atomic.make false
 
@@ -52,55 +21,7 @@ let sample_gc_metrics () =
           @@ Opentelemetry.GC_metrics.get_metrics() in
   AList.add gc_metrics l
 
-let lock_ : (unit -> unit) ref = ref ignore
-let unlock_ : (unit -> unit) ref = ref ignore
-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;
-    ticker_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; ticker_thread} = self in
-    Format.fprintf out
-      "{@[ debug=%B;@ url=%S;@ \
-       batch_traces=%a;@ batch_metrics=%a;@ \
-       batch_timeout_ms=%d; thread=%B;@ ticker_thread=%B @]}"
-      debug url ppiopt batch_traces ppiopt batch_metrics
-      batch_timeout_ms thread ticker_thread
-
-  let make
-      ?(debug= !debug_)
-      ?(url= get_url())
-      ?(batch_traces=Some 400)
-      ?(batch_metrics=None)
-      ?(batch_timeout_ms=500)
-      ?(thread=true)
-      ?(ticker_thread=true)
-      () : t =
-    { debug; url; batch_traces; batch_metrics; batch_timeout_ms;
-      thread; ticker_thread; }
-end
-
-(* critical section for [f()] *)
-let[@inline] with_lock_ f =
-  !lock_();
-  Fun.protect ~finally:!unlock_ f
-
-let[@inline] with_mutex_ m f =
-  Mutex.lock m;
-  Fun.protect ~finally:(fun () -> Mutex.unlock m) f
+module Config = Config
 
 let _init_curl = lazy (
   Curl.global_init Curl.CURLINIT_GLOBALALL;
@@ -196,75 +117,6 @@ module type PUSH = sig
   val pop_iter_all : (elt -> unit) -> unit
 end
 
-(* 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 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
-    let b = Bytes.create 8 in
-    for i=0 to 1 do
-      let r = Random.State.bits rand_ in (* 30 bits, of which we use 24 *)
-      Bytes.set b (i*3) (Char.chr (r land 0xff));
-      Bytes.set b (i*3+1) (Char.chr (r lsr 8 land 0xff));
-      Bytes.set b (i*3+2) (Char.chr (r lsr 16 land 0xff));
-    done;
-    let r = Random.State.bits rand_ in
-    Bytes.set b 6 (Char.chr (r land 0xff));
-    Bytes.set b 7 (Char.chr (r lsr 8 land 0xff));
-    b
-
-  let rand_bytes_16 () : bytes =
-    let@() = with_lock_ in
-    let b = Bytes.create 16 in
-    for i=0 to 4 do
-      let r = Random.State.bits rand_ in (* 30 bits, of which we use 24 *)
-      Bytes.set b (i*3) (Char.chr (r land 0xff));
-      Bytes.set b (i*3+1) (Char.chr (r lsr 8 land 0xff));
-      Bytes.set b (i*3+2) (Char.chr (r lsr 16 land 0xff));
-    done;
-    let r = Random.State.bits rand_ in
-    Bytes.set b 15 (Char.chr (r land 0xff)); (* last byte *)
-    b
-end
-
 (** 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
@@ -544,7 +396,7 @@ let mk_emitter ~(config:Config.t) () : (module EMITTER) =
 module Backend(Arg : sig val config : Config.t end)()
   : Opentelemetry.Collector.BACKEND
 = struct
-  include Gen_ids()
+  include Gen_ids.Make()
 
   include (val mk_emitter ~config:Arg.config ())
 
diff --git a/src/client/opentelemetry_client_ocurl.mli b/src/client/opentelemetry_client_ocurl.mli
index 6774fedc..6147d5b1 100644
--- a/src/client/opentelemetry_client_ocurl.mli
+++ b/src/client/opentelemetry_client_ocurl.mli
@@ -14,58 +14,7 @@ val set_mutex : lock:(unit -> unit) -> unlock:(unit -> unit) -> unit
 (** Set a lock/unlock pair to protect the critical sections
     of {!Opentelemetry.Collector.BACKEND} *)
 
-module Config : sig
-  type t = {
-    debug: bool;
-
-    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] *)
-
-    ticker_thread: bool;
-    (** Is there a ticker thread? Default [true].
-        This thread will regularly call [tick()] on the backend, to make
-        sure it makes progress, and regularly send events to the collector.
-        This option is ignored if [thread=false]. *)
-  }
-
-  val make :
-    ?debug:bool -> ?url:string ->
-    ?batch_traces:int option ->
-    ?batch_metrics:int option ->
-    ?batch_timeout_ms:int ->
-    ?thread:bool ->
-    ?ticker_thread:bool ->
-    unit -> t
-  (** Make a configuration *)
-
-  val pp : Format.formatter -> t -> unit
-end
+module Config = Config
 
 val setup : ?config:Config.t -> ?enable:bool -> unit -> unit
 (** Setup endpoint. This modifies {!Opentelemetry.Collector.backend}.