diff --git a/src/trace/common_.ml b/src/trace/common_.ml
new file mode 100644
index 00000000..74f053cf
--- /dev/null
+++ b/src/trace/common_.ml
@@ -0,0 +1,7 @@
+module Otel = Opentelemetry
+module Otrace = Trace_core (* ocaml-trace *)
+module TSub = Trace_subscriber.Subscriber
+
+let ( let@ ) = ( @@ )
+
+let spf = Printf.sprintf
diff --git a/src/trace/conv.ml b/src/trace/conv.ml
new file mode 100644
index 00000000..eca660bb
--- /dev/null
+++ b/src/trace/conv.ml
@@ -0,0 +1,40 @@
+open Common_
+
+let[@inline] trace_id_of_otel (id : Otel.Trace_id.t) : Otrace.trace_id =
+  if id == Otel.Trace_id.dummy then
+    Otrace.Collector.dummy_trace_id
+  else
+    Bytes.unsafe_to_string (Otel.Trace_id.to_bytes id)
+
+let[@inline] trace_id_to_otel (id : Otrace.trace_id) : Otel.Trace_id.t =
+  if id == Otrace.Collector.dummy_trace_id then
+    Otel.Trace_id.dummy
+  else
+    Otel.Trace_id.of_bytes @@ Bytes.unsafe_of_string id
+
+let[@inline] span_id_of_otel (id : Otel.Span_id.t) : Otrace.span =
+  if id == Otel.Span_id.dummy then
+    Otrace.Collector.dummy_span
+  else
+    Bytes.get_int64_le (Otel.Span_id.to_bytes id) 0
+
+let[@inline] span_id_to_otel (id : Otrace.span) : Otel.Span_id.t =
+  if id == Otrace.Collector.dummy_span then
+    Otel.Span_id.dummy
+  else (
+    let b = Bytes.create 8 in
+    Bytes.set_int64_le b 0 id;
+    Otel.Span_id.of_bytes b
+  )
+
+let[@inline] ctx_to_otel (self : Otrace.explicit_span_ctx) : Otel.Span_ctx.t =
+  Otel.Span_ctx.make
+    ~trace_id:(trace_id_to_otel self.trace_id)
+    ~parent_id:(span_id_to_otel self.span)
+    ()
+
+let[@inline] ctx_of_otel (ctx : Otel.Span_ctx.t) : Otrace.explicit_span_ctx =
+  {
+    trace_id = trace_id_of_otel (Otel.Span_ctx.trace_id ctx);
+    span = span_id_of_otel (Otel.Span_ctx.parent_id ctx);
+  }
diff --git a/src/trace/opentelemetry_trace.ml b/src/trace/opentelemetry_trace.ml
index ba3c8462..4dda635f 100644
--- a/src/trace/opentelemetry_trace.ml
+++ b/src/trace/opentelemetry_trace.ml
@@ -1,53 +1,5 @@
-module Otel = Opentelemetry
-module Otrace = Trace_core (* ocaml-trace *)
+open Common_
 module TLS = Thread_local_storage
-module TSub = Trace_subscriber.Subscriber
-
-open struct
-  let spf = Printf.sprintf
-end
-
-module Conv = struct
-  let[@inline] trace_id_of_otel (id : Otel.Trace_id.t) : Otrace.trace_id =
-    if id == Otel.Trace_id.dummy then
-      Otrace.Collector.dummy_trace_id
-    else
-      Bytes.unsafe_to_string (Otel.Trace_id.to_bytes id)
-
-  let[@inline] trace_id_to_otel (id : Otrace.trace_id) : Otel.Trace_id.t =
-    if id == Otrace.Collector.dummy_trace_id then
-      Otel.Trace_id.dummy
-    else
-      Otel.Trace_id.of_bytes @@ Bytes.unsafe_of_string id
-
-  let[@inline] span_id_of_otel (id : Otel.Span_id.t) : Otrace.span =
-    if id == Otel.Span_id.dummy then
-      Otrace.Collector.dummy_span
-    else
-      Bytes.get_int64_le (Otel.Span_id.to_bytes id) 0
-
-  let[@inline] span_id_to_otel (id : Otrace.span) : Otel.Span_id.t =
-    if id == Otrace.Collector.dummy_span then
-      Otel.Span_id.dummy
-    else (
-      let b = Bytes.create 8 in
-      Bytes.set_int64_le b 0 id;
-      Otel.Span_id.of_bytes b
-    )
-
-  let[@inline] ctx_to_otel (self : Otrace.explicit_span_ctx) : Otel.Span_ctx.t =
-    Otel.Span_ctx.make
-      ~trace_id:(trace_id_to_otel self.trace_id)
-      ~parent_id:(span_id_to_otel self.span)
-      ()
-
-  let[@inline] ctx_of_otel (ctx : Otel.Span_ctx.t) : Otrace.explicit_span_ctx =
-    {
-      trace_id = trace_id_of_otel (Otel.Span_ctx.trace_id ctx);
-      span = span_id_of_otel (Otel.Span_ctx.parent_id ctx);
-    }
-end
-
 open Conv
 
 let on_internal_error =
diff --git a/src/trace/subscriber.ml b/src/trace/subscriber.ml
new file mode 100644
index 00000000..9b6e2c15
--- /dev/null
+++ b/src/trace/subscriber.ml
@@ -0,0 +1,176 @@
+open Common_
+open Trace_core
+module Span_tbl = Trace_subscriber.Span_tbl
+
+module Buf_pool = struct
+  type t = Buffer.t Rpool.t
+
+  let create ?(max_size = 32) ?(buf_size = 256) () : t =
+    Rpool.create ~max_size ~clear:Buffer.reset
+      ~create:(fun () -> Buffer.create buf_size)
+      ()
+end
+
+open struct
+  let[@inline] time_us_of_time_ns (t : int64) : float =
+    Int64.div t 1_000L |> Int64.to_float
+
+  let[@inline] int64_of_trace_id_ (id : Trace_core.trace_id) : int64 =
+    if id == Trace_core.Collector.dummy_trace_id then
+      0L
+    else
+      Bytes.get_int64_le (Bytes.unsafe_of_string id) 0
+end
+
+let on_tracing_error = ref (fun s -> Printf.eprintf "%s\n%!" s)
+
+type span_info = {
+  tid: int;
+  name: string;
+  start_us: float;
+  mutable data: (string * Sub.user_data) list;
+      (* NOTE: thread safety: this is supposed to only be modified by the thread
+that's running this (synchronous, stack-abiding) span. *)
+}
+(** Information we store about a span begin event, to emit a complete event when
+    we meet the corresponding span end event *)
+
+type t = {
+  active: bool A.t;
+  pid: int;
+  spans: span_info Span_tbl.t;
+  buf_pool: Buf_pool.t;
+  exporter: Exporter.t;
+}
+(** Subscriber state *)
+
+open struct
+  let print_non_closed_spans_warning spans =
+    let module Str_set = Set.Make (String) in
+    let spans = Span_tbl.to_list spans in
+    if spans <> [] then (
+      !on_tracing_error
+      @@ Printf.sprintf "trace-tef: warning: %d spans were not closed"
+           (List.length spans);
+      let names =
+        List.fold_left
+          (fun set (_, span) -> Str_set.add span.name set)
+          Str_set.empty spans
+      in
+      Str_set.iter
+        (fun name ->
+          !on_tracing_error @@ Printf.sprintf "  span %S was not closed" name)
+        names;
+      flush stderr
+    )
+end
+
+let close (self : t) : unit =
+  if A.exchange self.active false then (
+    print_non_closed_spans_warning self.spans;
+    self.exporter.close ()
+  )
+
+let[@inline] active self = A.get self.active
+
+let[@inline] flush (self : t) : unit = self.exporter.flush ()
+
+let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
+  { active = A.make true; exporter; buf_pool; pid; spans = Span_tbl.create () }
+
+module Callbacks = struct
+  type st = t
+
+  let on_init _ ~time_ns:_ = ()
+
+  let on_shutdown (self : st) ~time_ns:_ = close self
+
+  let on_name_process (self : st) ~time_ns:_ ~tid:_ ~name : unit =
+    let@ buf = Rpool.with_ self.buf_pool in
+    Writer.emit_name_process ~pid:self.pid ~name buf;
+    self.exporter.on_json buf
+
+  let on_name_thread (self : st) ~time_ns:_ ~tid ~name : unit =
+    let@ buf = Rpool.with_ self.buf_pool in
+    Writer.emit_name_thread buf ~pid:self.pid ~tid ~name;
+    self.exporter.on_json buf
+
+  (* add function name, if provided, to the metadata *)
+  let add_fun_name_ fun_name data : _ list =
+    match fun_name with
+    | None -> data
+    | Some f -> ("function", Sub.U_string f) :: data
+
+  let[@inline] on_enter_span (self : st) ~__FUNCTION__:fun_name ~__FILE__:_
+      ~__LINE__:_ ~time_ns ~tid ~data ~name span : unit =
+    let time_us = time_us_of_time_ns @@ time_ns in
+    let data = add_fun_name_ fun_name data in
+    let info = { tid; name; start_us = time_us; data } in
+    (* save the span so we find it at exit *)
+    Span_tbl.add self.spans span info
+
+  let on_exit_span (self : st) ~time_ns ~tid:_ span : unit =
+    let time_us = time_us_of_time_ns @@ time_ns in
+
+    match Span_tbl.find_exn self.spans span with
+    | exception Not_found ->
+      !on_tracing_error
+        (Printf.sprintf "trace-tef: error: cannot find span %Ld" span)
+    | { tid; name; start_us; data } ->
+      Span_tbl.remove self.spans span;
+      let@ buf = Rpool.with_ self.buf_pool in
+      Writer.emit_duration_event buf ~pid:self.pid ~tid ~name ~start:start_us
+        ~end_:time_us ~args:data;
+
+      self.exporter.on_json buf
+
+  let on_add_data (self : st) ~data span =
+    if data <> [] then (
+      try
+        let info = Span_tbl.find_exn self.spans span in
+        info.data <- List.rev_append data info.data
+      with Not_found ->
+        !on_tracing_error
+          (Printf.sprintf "trace-tef: error: cannot find span %Ld" span)
+    )
+
+  let on_message (self : st) ~time_ns ~tid ~span:_ ~data msg : unit =
+    let time_us = time_us_of_time_ns @@ time_ns in
+    let@ buf = Rpool.with_ self.buf_pool in
+    Writer.emit_instant_event buf ~pid:self.pid ~tid ~name:msg ~ts:time_us
+      ~args:data;
+    self.exporter.on_json buf
+
+  let on_counter (self : st) ~time_ns ~tid ~data:_ ~name n : unit =
+    let time_us = time_us_of_time_ns @@ time_ns in
+    let@ buf = Rpool.with_ self.buf_pool in
+    Writer.emit_counter buf ~pid:self.pid ~name ~tid ~ts:time_us n;
+    self.exporter.on_json buf
+
+  let on_enter_manual_span (self : st) ~__FUNCTION__:fun_name ~__FILE__:_
+      ~__LINE__:_ ~time_ns ~tid ~parent:_ ~data ~name ~flavor ~trace_id _span :
+      unit =
+    let time_us = time_us_of_time_ns @@ time_ns in
+
+    let data = add_fun_name_ fun_name data in
+    let@ buf = Rpool.with_ self.buf_pool in
+    Writer.emit_manual_begin buf ~pid:self.pid ~tid ~name
+      ~id:(int64_of_trace_id_ trace_id)
+      ~ts:time_us ~args:data ~flavor;
+    self.exporter.on_json buf
+
+  let on_exit_manual_span (self : st) ~time_ns ~tid ~name ~data ~flavor
+      ~trace_id (_ : span) : unit =
+    let time_us = time_us_of_time_ns @@ time_ns in
+
+    let@ buf = Rpool.with_ self.buf_pool in
+    Writer.emit_manual_end buf ~pid:self.pid ~tid ~name
+      ~id:(int64_of_trace_id_ trace_id)
+      ~ts:time_us ~flavor ~args:data;
+    self.exporter.on_json buf
+
+  let on_extension_event _ ~time_ns:_ ~tid:_ _ev = ()
+end
+
+let subscriber (self : t) : Sub.t =
+  Sub.Subscriber.Sub { st = self; callbacks = (module Callbacks) }