diff --git a/src/tef/common_.ml b/src/tef/common_.ml
new file mode 100644
index 0000000..114d81c
--- /dev/null
+++ b/src/tef/common_.ml
@@ -0,0 +1,4 @@
+module Sub = Trace_subscriber
+module A = Trace_core.Internal_.Atomic_
+
+let ( let@ ) = ( @@ )
diff --git a/src/tef/emit_tef.ml b/src/tef/emit_tef.ml
new file mode 100644
index 0000000..e69de29
diff --git a/src/tef/event.ml b/src/tef/event.ml
deleted file mode 100644
index 7e5c0fc..0000000
--- a/src/tef/event.ml
+++ /dev/null
@@ -1,56 +0,0 @@
-open Trace_core
-module Sub = Trace_subscriber
-
-(** An event, specialized for TEF *)
-type t =
-  | E_tick
-  | E_message of {
-      tid: int;
-      msg: string;
-      time_us: float;
-      data: (string * Sub.user_data) list;
-    }
-  | E_define_span of {
-      tid: int;
-      name: string;
-      time_us: float;
-      id: span;
-      fun_name: string option;
-      data: (string * Sub.user_data) list;
-    }
-  | E_exit_span of {
-      id: span;
-      time_us: float;
-    }
-  | E_add_data of {
-      id: span;
-      data: (string * Sub.user_data) list;
-    }
-  | E_enter_manual_span of {
-      tid: int;
-      name: string;
-      time_us: float;
-      id: trace_id;
-      flavor: Sub.flavor option;
-      fun_name: string option;
-      data: (string * Sub.user_data) list;
-    }
-  | E_exit_manual_span of {
-      tid: int;
-      name: string;
-      time_us: float;
-      flavor: Sub.flavor option;
-      data: (string * Sub.user_data) list;
-      id: trace_id;
-    }
-  | E_counter of {
-      name: string;
-      tid: int;
-      time_us: float;
-      n: float;
-    }
-  | E_name_process of { name: string }
-  | E_name_thread of {
-      tid: int;
-      name: string;
-    }
diff --git a/src/tef/exporter.ml b/src/tef/exporter.ml
new file mode 100644
index 0000000..561d80d
--- /dev/null
+++ b/src/tef/exporter.ml
@@ -0,0 +1,85 @@
+(** An exporter, takes JSON objects and writes them somewhere *)
+
+open Common_
+
+type t = {
+  on_json: Buffer.t -> unit;
+      (** Takes a buffer and writes it somewhere. The buffer is only valid
+          during this call and must not be stored. *)
+  flush: unit -> unit;  (** Force write *)
+  close: unit -> unit;  (** Close underlying resources *)
+}
+(** An exporter, takes JSON objects and writes them somewhere.
+
+    This should be thread-safe if used in a threaded environment. *)
+
+open struct
+  let with_lock lock f =
+    Mutex.lock lock;
+    try
+      let res = f () in
+      Mutex.unlock lock;
+      res
+    with e ->
+      let bt = Printexc.get_raw_backtrace () in
+      Mutex.unlock lock;
+      Printexc.raise_with_backtrace e bt
+end
+
+(** Export to the channel
+    @param jsonl
+      if true, export as a JSON object per line, otherwise export as a single
+      big JSON array.
+    @param close_channel if true, closing the exporter will close the channel *)
+let of_out_channel ~close_channel ~jsonl oc : t =
+  let lock = Mutex.create () in
+  let first = ref true in
+  let closed = ref false in
+  let flush () =
+    let@ () = with_lock lock in
+    flush oc
+  in
+  let close () =
+    let@ () = with_lock lock in
+    if not !closed then (
+      closed := true;
+      if not jsonl then output_char oc ']';
+      if close_channel then close_out_noerr oc
+    )
+  in
+  let on_json buf =
+    let@ () = with_lock lock in
+    if not jsonl then
+      if !first then (
+        if not jsonl then output_char oc '[';
+        first := false
+      ) else
+        output_string oc ",\n";
+    Buffer.output_buffer oc buf;
+    if jsonl then output_char oc '\n'
+  in
+  { flush; close; on_json }
+
+let of_buffer ~jsonl (buf : Buffer.t) : t =
+  let lock = Mutex.create () in
+  let first = ref true in
+  let closed = ref false in
+  let close () =
+    let@ () = with_lock lock in
+    if not !closed then (
+      closed := true;
+      if not jsonl then Buffer.add_char buf ']'
+    )
+  in
+  let on_json json =
+    let@ () = with_lock lock in
+    if not jsonl then
+      if !first then (
+        if not jsonl then Buffer.add_char buf '[';
+        first := false
+      ) else
+        Buffer.add_string buf ",\n";
+    Buffer.add_buffer buf json;
+    if jsonl then Buffer.add_char buf '\n'
+  in
+  { flush = ignore; close; on_json }
diff --git a/src/tef/subscriber.ml b/src/tef/subscriber.ml
new file mode 100644
index 0000000..d3e4b54
--- /dev/null
+++ b/src/tef/subscriber.ml
@@ -0,0 +1,173 @@
+open Common_
+open Trace_core
+open Trace_private_util
+module Span_tbl = Sub.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 "trace-tef error: %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\n"
+           (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\n" 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 "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 "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) }
diff --git a/src/tef/subscriber.mli b/src/tef/subscriber.mli
new file mode 100644
index 0000000..9f2f235
--- /dev/null
+++ b/src/tef/subscriber.mli
@@ -0,0 +1,28 @@
+open Common_
+
+module Buf_pool : sig
+  type t
+
+  val create : ?max_size:int -> ?buf_size:int -> unit -> t
+end
+
+type t
+(** Main subscriber state. *)
+
+val create : ?buf_pool:Buf_pool.t -> pid:int -> exporter:Exporter.t -> unit -> t
+(** Create a subscriber state. *)
+
+val flush : t -> unit
+val close : t -> unit
+val active : t -> bool
+
+module Callbacks : Sub.Callbacks.S with type st = t
+
+val subscriber : t -> Sub.t
+(** Subscriber that writes json into this writer *)
+
+(**/**)
+
+val on_tracing_error : (string -> unit) ref
+
+(**/**)
diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
index d9dd4f8..e29488f 100644
--- a/src/tef/trace_tef.ml
+++ b/src/tef/trace_tef.ml
@@ -1,216 +1,7 @@
 open Trace_core
-open Trace_private_util
-open Event
-module Sub = Trace_subscriber
-module A = Trace_core.Internal_.Atomic_
-
-let on_tracing_error = ref (fun s -> Printf.eprintf "trace-tef error: %s\n%!" s)
-
-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
-
-module Mock_ = struct
-  let now = ref 0
-
-  (* used to mock timing *)
-  let get_now_ns () : int64 =
-    let x = !now in
-    incr now;
-    Int64.(mul (of_int x) 1000L)
-
-  let get_tid_ () : int = 3
-end
-
-module Span_tbl = Hashtbl.Make (struct
-  include Int64
-
-  let hash : t -> int = Hashtbl.hash
-end)
-
-type span_info = {
-  tid: int;
-  name: string;
-  start_us: float;
-  mutable data: (string * Sub.user_data) list;
-}
-
-(** Writer: knows how to write entries to a file in TEF format *)
-module Writer = struct
-  type t = {
-    oc: out_channel;
-    jsonl: bool;  (** JSONL mode, one json event per line *)
-    mutable first: bool;  (** first event? useful in json mode *)
-    buf: Buffer.t;  (** Buffer to write into *)
-    must_close: bool;  (** Do we have to close the underlying channel [oc]? *)
-    pid: int;
-  }
-  (** A writer to a [out_channel]. It writes JSON entries in an array and closes
-      the array at the end. *)
-
-  let create ~(mode : [ `Single | `Jsonl ]) ~out () : t =
-    let jsonl = mode = `Jsonl in
-    let oc, must_close =
-      match out with
-      | `Stdout -> stdout, false
-      | `Stderr -> stderr, false
-      | `File path -> open_out path, true
-      | `File_append path ->
-        open_out_gen [ Open_creat; Open_wronly; Open_append ] 0o644 path, true
-      | `Output oc -> oc, false
-    in
-    let pid =
-      if !Sub.Private_.mock then
-        2
-      else
-        Unix.getpid ()
-    in
-    if not jsonl then output_char oc '[';
-    { oc; jsonl; first = true; pid; must_close; buf = Buffer.create 2_048 }
-
-  let close (self : t) : unit =
-    if self.jsonl then
-      output_char self.oc '\n'
-    else
-      output_char self.oc ']';
-    flush self.oc;
-    if self.must_close then close_out self.oc
-
-  let with_ ~mode ~out f =
-    let writer = create ~mode ~out () in
-    Fun.protect ~finally:(fun () -> close writer) (fun () -> f writer)
-
-  let[@inline] flush (self : t) : unit = flush self.oc
-
-  (** Emit "," if we need, and get the buffer ready *)
-  let emit_sep_and_start_ (self : t) =
-    Buffer.reset self.buf;
-    if self.jsonl then
-      Buffer.add_char self.buf '\n'
-    else if self.first then
-      self.first <- false
-    else
-      Buffer.add_string self.buf ",\n"
-
-  let char = Buffer.add_char
-  let raw_string = Buffer.add_string
-
-  let str_val (buf : Buffer.t) (s : string) =
-    char buf '"';
-    let encode_char c =
-      match c with
-      | '"' -> raw_string buf {|\"|}
-      | '\\' -> raw_string buf {|\\|}
-      | '\n' -> raw_string buf {|\n|}
-      | '\b' -> raw_string buf {|\b|}
-      | '\r' -> raw_string buf {|\r|}
-      | '\t' -> raw_string buf {|\t|}
-      | _ when Char.code c <= 0x1f ->
-        raw_string buf {|\u00|};
-        Printf.bprintf buf "%02x" (Char.code c)
-      | c -> char buf c
-    in
-    String.iter encode_char s;
-    char buf '"'
-
-  let pp_user_data_ (out : Buffer.t) : Sub.user_data -> unit = function
-    | U_none -> raw_string out "null"
-    | U_int i -> Printf.bprintf out "%d" i
-    | U_bool b -> Printf.bprintf out "%b" b
-    | U_string s -> str_val out s
-    | U_float f -> Printf.bprintf out "%g" f
-
-  (* emit args, if not empty. [ppv] is used to print values. *)
-  let emit_args_o_ ppv (out : Buffer.t) args : unit =
-    if args <> [] then (
-      Printf.bprintf out {json|,"args": {|json};
-      List.iteri
-        (fun i (n, value) ->
-          if i > 0 then raw_string out ",";
-          Printf.bprintf out {json|"%s":%a|json} n ppv value)
-        args;
-      char out '}'
-    )
-
-  let emit_duration_event ~tid ~name ~start ~end_ ~args (self : t) : unit =
-    let dur = end_ -. start in
-    let ts = start in
-
-    emit_sep_and_start_ self;
-
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"cat":"","tid": %d,"dur": %.2f,"ts": %.2f,"name":%a,"ph":"X"%a}|json}
-      self.pid tid dur ts str_val name
-      (emit_args_o_ pp_user_data_)
-      args;
-    Buffer.output_buffer self.oc self.buf
-
-  let emit_manual_begin ~tid ~name ~(id : trace_id) ~ts ~args
-      ~(flavor : Sub.flavor option) (self : t) : unit =
-    emit_sep_and_start_ self;
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"cat":"trace","id":%Ld,"tid": %d,"ts": %.2f,"name":%a,"ph":"%c"%a}|json}
-      self.pid (int64_of_trace_id_ id) tid ts str_val name
-      (match flavor with
-      | None | Some Async -> 'b'
-      | Some Sync -> 'B')
-      (emit_args_o_ pp_user_data_)
-      args;
-    Buffer.output_buffer self.oc self.buf
-
-  let emit_manual_end ~tid ~name ~(id : trace_id) ~ts
-      ~(flavor : Sub.flavor option) ~args (self : t) : unit =
-    emit_sep_and_start_ self;
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"cat":"trace","id":%Ld,"tid": %d,"ts": %.2f,"name":%a,"ph":"%c"%a}|json}
-      self.pid (int64_of_trace_id_ id) tid ts str_val name
-      (match flavor with
-      | None | Some Async -> 'e'
-      | Some Sync -> 'E')
-      (emit_args_o_ pp_user_data_)
-      args;
-    Buffer.output_buffer self.oc self.buf
-
-  let emit_instant_event ~tid ~name ~ts ~args (self : t) : unit =
-    emit_sep_and_start_ self;
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"cat":"","tid": %d,"ts": %.2f,"name":%a,"ph":"I"%a}|json}
-      self.pid tid ts str_val name
-      (emit_args_o_ pp_user_data_)
-      args;
-    Buffer.output_buffer self.oc self.buf
-
-  let emit_name_thread ~tid ~name (self : t) : unit =
-    emit_sep_and_start_ self;
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"tid": %d,"name":"thread_name","ph":"M"%a}|json} self.pid
-      tid
-      (emit_args_o_ pp_user_data_)
-      [ "name", U_string name ];
-    Buffer.output_buffer self.oc self.buf
-
-  let emit_name_process ~name (self : t) : unit =
-    emit_sep_and_start_ self;
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"name":"process_name","ph":"M"%a}|json} self.pid
-      (emit_args_o_ pp_user_data_)
-      [ "name", U_string name ];
-    Buffer.output_buffer self.oc self.buf
-
-  let emit_counter ~name ~tid ~ts (self : t) f : unit =
-    emit_sep_and_start_ self;
-    Printf.bprintf self.buf
-      {json|{"pid":%d,"tid":%d,"ts":%.2f,"name":"c","ph":"C"%a}|json} self.pid
-      tid ts
-      (emit_args_o_ pp_user_data_)
-      [ name, U_float f ];
-    Buffer.output_buffer self.oc self.buf
-end
+module Subscriber = Subscriber
+module Exporter = Exporter
+module Writer = Writer
 
 let block_signals () =
   try
@@ -228,97 +19,14 @@ let block_signals () =
         : _ list)
   with _ -> ()
 
-let print_non_closed_spans_warning spans =
-  let module Str_set = Set.Make (String) in
-  Printf.eprintf "trace-tef: warning: %d spans were not closed\n"
-    (Span_tbl.length spans);
-  let names = ref Str_set.empty in
-  Span_tbl.iter (fun _ span -> names := Str_set.add span.name !names) spans;
-  Str_set.iter
-    (fun name -> Printf.eprintf "  span %S was not closed\n" name)
-    !names;
-  flush stderr
-
-(** Background thread, takes events from the queue, puts them in context using
-    local state, and writes fully resolved TEF events to [out]. *)
-let bg_thread ~mode ~out (events : Event.t B_queue.t) : unit =
-  block_signals ();
-
-  (* open a writer to [out] *)
-  Writer.with_ ~mode ~out @@ fun writer ->
-  (* local state, to keep track of span information and implicit stack context *)
-  let spans : span_info Span_tbl.t = Span_tbl.create 32 in
-
-  (* 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
-  in
-
-  (* how to deal with an event *)
-  let handle_ev (ev : Event.t) : unit =
-    match ev with
-    | E_tick -> Writer.flush writer
-    | E_message { tid; msg; time_us; data } ->
-      Writer.emit_instant_event ~tid ~name:msg ~ts:time_us ~args:data writer
-    | E_define_span { tid; name; id; time_us; fun_name; data } ->
-      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 spans id info
-    | E_exit_span { id; time_us = stop_us } ->
-      (match Span_tbl.find_opt spans id with
-      | None -> !on_tracing_error (Printf.sprintf "cannot find span %Ld" id)
-      | Some { tid; name; start_us; data } ->
-        Span_tbl.remove spans id;
-        Writer.emit_duration_event ~tid ~name ~start:start_us ~end_:stop_us
-          ~args:data writer)
-    | E_add_data { id; data } ->
-      (match Span_tbl.find_opt spans id with
-      | None -> !on_tracing_error (Printf.sprintf "cannot find span %Ld" id)
-      | Some info -> info.data <- List.rev_append data info.data)
-    | E_enter_manual_span { tid; time_us; name; id; data; fun_name; flavor } ->
-      let data = add_fun_name_ fun_name data in
-      Writer.emit_manual_begin ~tid ~name ~id ~ts:time_us ~args:data ~flavor
-        writer
-    | E_exit_manual_span { tid; time_us; name; id; flavor; data } ->
-      Writer.emit_manual_end ~tid ~name ~id ~ts:time_us ~flavor ~args:data
-        writer
-    | E_counter { tid; name; time_us; n } ->
-      Writer.emit_counter ~name ~tid ~ts:time_us writer n
-    | E_name_process { name } -> Writer.emit_name_process ~name writer
-    | E_name_thread { tid; name } -> Writer.emit_name_thread ~tid ~name writer
-  in
-
-  try
-    while true do
-      (* get all the events in the incoming blocking queue, in
-         one single critical section. *)
-      let local = B_queue.pop_all events in
-      List.iter handle_ev local
-    done
-  with B_queue.Closed ->
-    (* write a message about us closing *)
-    Writer.emit_instant_event ~name:"tef-worker.exit"
-      ~tid:(Thread.id @@ Thread.self ())
-      ~ts:(time_us_of_time_ns @@ Sub.Private_.now_ns ())
-      ~args:[] writer;
-
-    (* warn if app didn't close all spans *)
-    if Span_tbl.length spans > 0 then print_non_closed_spans_warning spans;
-    ()
-
 (** Thread that simply regularly "ticks", sending events to the background
     thread so it has a chance to write to the file *)
-let tick_thread events : unit =
+let tick_thread (sub : Subscriber.t) : unit =
   block_signals ();
-  try
-    while true do
-      Thread.delay 0.5;
-      B_queue.push events E_tick
-    done
-  with B_queue.Closed -> ()
+  while Subscriber.active sub do
+    Thread.delay 0.5;
+    Subscriber.flush sub
+  done
 
 type output =
   [ `Stdout
@@ -326,91 +34,45 @@ type output =
   | `File of string
   ]
 
-module Internal_st = struct
-  type t = {
-    active: bool A.t;
-    events: Event.t B_queue.t;
-    t_write: Thread.t;
-  }
-end
-
-let subscriber_ ~finally ~out ~(mode : [ `Single | `Jsonl ]) () : Sub.t =
-  let module M : Sub.Callbacks.S with type st = Internal_st.t = struct
-    type st = Internal_st.t
-
-    let on_init _ ~time_ns:_ = ()
-
-    let on_shutdown (self : st) ~time_ns:_ =
-      if A.exchange self.active false then (
-        B_queue.close self.events;
-        (* wait for writer thread to be done. The writer thread will exit
-           after processing remaining events because the queue is now closed *)
-        Thread.join self.t_write
-      )
-
-    let on_name_process (self : st) ~time_ns:_ ~tid:_ ~name : unit =
-      B_queue.push self.events @@ E_name_process { name }
-
-    let on_name_thread (self : st) ~time_ns:_ ~tid ~name : unit =
-      B_queue.push self.events @@ E_name_thread { tid; name }
-
-    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
-      B_queue.push self.events
-      @@ E_define_span { tid; name; time_us; id = span; fun_name; data }
-
-    let on_exit_span (self : st) ~time_ns ~tid:_ span : unit =
-      let time_us = time_us_of_time_ns @@ time_ns in
-      B_queue.push self.events @@ E_exit_span { id = span; time_us }
-
-    let on_add_data (self : st) ~data span =
-      if data <> [] then
-        B_queue.push self.events @@ E_add_data { id = span; data }
-
-    let on_message (self : st) ~time_ns ~tid ~span:_ ~data msg : unit =
-      let time_us = time_us_of_time_ns @@ time_ns in
-      B_queue.push self.events @@ E_message { tid; time_us; msg; data }
-
-    let on_counter (self : st) ~time_ns ~tid ~data:_ ~name f : unit =
-      let time_us = time_us_of_time_ns @@ time_ns in
-      B_queue.push self.events @@ E_counter { name; n = f; time_us; tid }
-
-    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
-      B_queue.push self.events
-      @@ E_enter_manual_span
-           { id = trace_id; time_us; tid; data; name; fun_name; flavor }
-
-    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
-      B_queue.push self.events
-      @@ E_exit_manual_span { tid; id = trace_id; name; time_us; data; flavor }
-
-    let on_extension_event _ ~time_ns:_ ~tid:_ _ev = ()
-  end in
-  let events = B_queue.create () in
-  let t_write =
-    Thread.create
-      (fun () -> Fun.protect ~finally @@ fun () -> bg_thread ~mode ~out events)
-      ()
+let subscriber_ ~finally ~out ~(mode : [ `Single | `Jsonl ]) () :
+    Trace_subscriber.t =
+  let jsonl = mode = `Jsonl in
+  let oc, must_close =
+    match out with
+    | `Stdout -> stdout, false
+    | `Stderr -> stderr, false
+    | `File path -> open_out path, true
+    | `File_append path ->
+      open_out_gen [ Open_creat; Open_wronly; Open_append ] 0o644 path, true
+    | `Output oc -> oc, false
+  in
+  let pid =
+    if !Trace_subscriber.Private_.mock then
+      2
+    else
+      Unix.getpid ()
   in
 
-  (* ticker thread, regularly sends a message to the writer thread.
-      no need to join it. *)
-  let _t_tick : Thread.t = Thread.create (fun () -> tick_thread events) () in
-  let st : Internal_st.t = { active = A.make true; events; t_write } in
-  Sub.Subscriber.Sub { st; callbacks = (module M) }
+  let exporter = Exporter.of_out_channel oc ~jsonl ~close_channel:must_close in
+  let exporter =
+    {
+      exporter with
+      close =
+        (fun () ->
+          exporter.close ();
+          finally ());
+    }
+  in
+  let sub = Subscriber.create ~pid ~exporter () in
+  let _t_tick : Thread.t = Thread.create tick_thread sub in
+  Subscriber.subscriber sub
 
 let collector_ ~(finally : unit -> unit) ~(mode : [ `Single | `Jsonl ]) ~out ()
     : collector =
   let sub = subscriber_ ~finally ~mode ~out () in
-  Sub.collector sub
+  Trace_subscriber.collector sub
 
-let[@inline] subscriber ~out () : Sub.t =
+let[@inline] subscriber ~out () : Trace_subscriber.t =
   subscriber_ ~finally:ignore ~mode:`Single ~out ()
 
 let[@inline] collector ~out () : collector =
@@ -438,14 +100,26 @@ let with_setup ?out () f =
   setup ?out ();
   Fun.protect ~finally:Trace_core.shutdown f
 
+module Mock_ = struct
+  let now = ref 0
+
+  (* used to mock timing *)
+  let get_now_ns () : int64 =
+    let x = !now in
+    incr now;
+    Int64.(mul (of_int x) 1000L)
+
+  let get_tid_ () : int = 3
+end
+
 module Private_ = struct
   let mock_all_ () =
-    Sub.Private_.mock := true;
-    Sub.Private_.get_now_ns_ := Mock_.get_now_ns;
-    Sub.Private_.get_tid_ := Mock_.get_tid_;
+    Trace_subscriber.Private_.mock := true;
+    Trace_subscriber.Private_.get_now_ns_ := Mock_.get_now_ns;
+    Trace_subscriber.Private_.get_tid_ := Mock_.get_tid_;
     ()
 
-  let on_tracing_error = on_tracing_error
+  let on_tracing_error = Subscriber.on_tracing_error
 
   let subscriber_jsonl ~finally ~out () =
     subscriber_ ~finally ~mode:`Jsonl ~out ()
diff --git a/src/tef/trace_tef.mli b/src/tef/trace_tef.mli
index d5741fd..9cf8dd1 100644
--- a/src/tef/trace_tef.mli
+++ b/src/tef/trace_tef.mli
@@ -1,3 +1,7 @@
+module Subscriber = Subscriber
+module Exporter = Exporter
+module Writer = Writer
+
 type output =
   [ `Stdout
   | `Stderr
diff --git a/src/tef/writer.ml b/src/tef/writer.ml
new file mode 100644
index 0000000..9865988
--- /dev/null
+++ b/src/tef/writer.ml
@@ -0,0 +1,97 @@
+open Common_
+
+let char = Buffer.add_char
+let raw_string = Buffer.add_string
+
+let str_val (buf : Buffer.t) (s : string) =
+  char buf '"';
+  let encode_char c =
+    match c with
+    | '"' -> raw_string buf {|\"|}
+    | '\\' -> raw_string buf {|\\|}
+    | '\n' -> raw_string buf {|\n|}
+    | '\b' -> raw_string buf {|\b|}
+    | '\r' -> raw_string buf {|\r|}
+    | '\t' -> raw_string buf {|\t|}
+    | _ when Char.code c <= 0x1f ->
+      raw_string buf {|\u00|};
+      Printf.bprintf buf "%02x" (Char.code c)
+    | c -> char buf c
+  in
+  String.iter encode_char s;
+  char buf '"'
+
+let pp_user_data_ (out : Buffer.t) : Sub.user_data -> unit = function
+  | U_none -> raw_string out "null"
+  | U_int i -> Printf.bprintf out "%d" i
+  | U_bool b -> Printf.bprintf out "%b" b
+  | U_string s -> str_val out s
+  | U_float f -> Printf.bprintf out "%g" f
+
+(* emit args, if not empty. [ppv] is used to print values. *)
+let emit_args_o_ ppv (out : Buffer.t) args : unit =
+  if args <> [] then (
+    Printf.bprintf out {json|,"args": {|json};
+    List.iteri
+      (fun i (n, value) ->
+        if i > 0 then raw_string out ",";
+        Printf.bprintf out {json|"%s":%a|json} n ppv value)
+      args;
+    char out '}'
+  )
+
+let emit_duration_event ~pid ~tid ~name ~start ~end_ ~args buf : unit =
+  let dur = end_ -. start in
+  let ts = start in
+
+  Printf.bprintf buf
+    {json|{"pid":%d,"cat":"","tid": %d,"dur": %.2f,"ts": %.2f,"name":%a,"ph":"X"%a}|json}
+    pid tid dur ts str_val name
+    (emit_args_o_ pp_user_data_)
+    args
+
+let emit_manual_begin ~pid ~tid ~name ~(id : int64) ~ts ~args
+    ~(flavor : Sub.flavor option) buf : unit =
+  Printf.bprintf buf
+    {json|{"pid":%d,"cat":"trace","id":%Ld,"tid": %d,"ts": %.2f,"name":%a,"ph":"%c"%a}|json}
+    pid id tid ts str_val name
+    (match flavor with
+    | None | Some Async -> 'b'
+    | Some Sync -> 'B')
+    (emit_args_o_ pp_user_data_)
+    args
+
+let emit_manual_end ~pid ~tid ~name ~(id : int64) ~ts
+    ~(flavor : Sub.flavor option) ~args buf : unit =
+  Printf.bprintf buf
+    {json|{"pid":%d,"cat":"trace","id":%Ld,"tid": %d,"ts": %.2f,"name":%a,"ph":"%c"%a}|json}
+    pid id tid ts str_val name
+    (match flavor with
+    | None | Some Async -> 'e'
+    | Some Sync -> 'E')
+    (emit_args_o_ pp_user_data_)
+    args
+
+let emit_instant_event ~pid ~tid ~name ~ts ~args buf : unit =
+  Printf.bprintf buf
+    {json|{"pid":%d,"cat":"","tid": %d,"ts": %.2f,"name":%a,"ph":"I"%a}|json}
+    pid tid ts str_val name
+    (emit_args_o_ pp_user_data_)
+    args
+
+let emit_name_thread ~pid ~tid ~name buf : unit =
+  Printf.bprintf buf
+    {json|{"pid":%d,"tid": %d,"name":"thread_name","ph":"M"%a}|json} pid tid
+    (emit_args_o_ pp_user_data_)
+    [ "name", U_string name ]
+
+let emit_name_process ~pid ~name buf : unit =
+  Printf.bprintf buf {json|{"pid":%d,"name":"process_name","ph":"M"%a}|json} pid
+    (emit_args_o_ pp_user_data_)
+    [ "name", U_string name ]
+
+let emit_counter ~pid ~tid ~name ~ts buf f : unit =
+  Printf.bprintf buf
+    {json|{"pid":%d,"tid":%d,"ts":%.2f,"name":"c","ph":"C"%a}|json} pid tid ts
+    (emit_args_o_ pp_user_data_)
+    [ name, U_float f ]
diff --git a/src/tef/writer.mli b/src/tef/writer.mli
new file mode 100644
index 0000000..d1563a7
--- /dev/null
+++ b/src/tef/writer.mli
@@ -0,0 +1,54 @@
+(** Write JSON events to a buffer.
+
+    This is the part of the code that knows how to emit TEF-compliant JSON from
+    raw event data. *)
+
+open Common_
+open Trace_core
+
+val emit_duration_event :
+  pid:int ->
+  tid:int ->
+  name:string ->
+  start:float ->
+  end_:float ->
+  args:(string * Sub.user_data) list ->
+  Buffer.t ->
+  unit
+
+val emit_manual_begin :
+  pid:int ->
+  tid:int ->
+  name:string ->
+  id:span ->
+  ts:float ->
+  args:(string * Sub.user_data) list ->
+  flavor:Sub.flavor option ->
+  Buffer.t ->
+  unit
+
+val emit_manual_end :
+  pid:int ->
+  tid:int ->
+  name:string ->
+  id:span ->
+  ts:float ->
+  flavor:Sub.flavor option ->
+  args:(string * Sub.user_data) list ->
+  Buffer.t ->
+  unit
+
+val emit_instant_event :
+  pid:int ->
+  tid:int ->
+  name:string ->
+  ts:float ->
+  args:(string * Sub.user_data) list ->
+  Buffer.t ->
+  unit
+
+val emit_name_thread : pid:int -> tid:int -> name:string -> Buffer.t -> unit
+val emit_name_process : pid:int -> name:string -> Buffer.t -> unit
+
+val emit_counter :
+  pid:int -> tid:int -> name:string -> ts:float -> Buffer.t -> float -> unit