diff --git a/src/tef/common_.ml b/src/tef/common_.ml
index 114d81c..2148902 100644
--- a/src/tef/common_.ml
+++ b/src/tef/common_.ml
@@ -2,3 +2,5 @@ module Sub = Trace_subscriber
 module A = Trace_core.Internal_.Atomic_
 
 let ( let@ ) = ( @@ )
+
+type span_id = Sub.Span_sub.span_id
diff --git a/src/tef/subscriber.ml b/src/tef/subscriber.ml
index b90a6a3..d3d99c6 100644
--- a/src/tef/subscriber.ml
+++ b/src/tef/subscriber.ml
@@ -1,7 +1,7 @@
 open Common_
 open Trace_core
 open Trace_private_util
-module Span_tbl = Sub.Span_tbl
+module Span_sub = Sub.Span_sub
 
 module Buf_pool = struct
   type t = Buffer.t Rpool.t
@@ -15,16 +15,11 @@ 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;
@@ -35,18 +30,18 @@ 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;
   span_gen: Sub.Span_generator.t;
-  trace_id_gen: Sub.Trace_id_8B_generator.t;
 }
 (** Subscriber state *)
 
+(* TODO: this is nice to have, can we make it optional?
 open struct
   let print_non_closed_spans_warning spans =
     let module Str_set = Set.Make (String) in
@@ -67,12 +62,12 @@ open struct
       flush stderr
     )
 end
+*)
 
 let close (self : t) : unit =
-  if A.exchange self.active false then (
-    print_non_closed_spans_warning self.spans;
+  if A.exchange self.active false then
+    (* FIXME: 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 ()
@@ -83,108 +78,84 @@ let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
     exporter;
     buf_pool;
     pid;
-    spans = Span_tbl.create ();
     span_gen = Sub.Span_generator.create ();
-    trace_id_gen = Sub.Trace_id_8B_generator.create ();
   }
 
-module Callbacks = struct
+open struct
   type st = t
 
-  let new_span (self : st) = Sub.Span_generator.mk_span self.span_gen
-
-  let new_trace_id self =
-    Sub.Trace_id_8B_generator.mk_trace_id self.trace_id_gen
-
+  let new_span_id (self : st) = Sub.Span_generator.mk_span self.span_gen
   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", `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 on_enter_span (self : st) (span : Span_sub.t) : unit =
+    match span.flavor with
+    | `Async ->
+      let { Span_sub.time_ns; data; tid; name; _ } = span in
+      let time_us = time_us_of_time_ns time_ns in
+      let data = add_fun_name_ span.__FUNCTION__ data in
       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;
-
+      Writer.emit_begin buf ~pid:self.pid ~tid ~name ~id:42L ~ts:time_us
+        ~args:data ~flavor:span.flavor;
       self.exporter.on_json buf
+    | `Sync -> () (* done at exit *)
 
-  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_exit_span (self : st) ~time_ns:exit_time_ns ~tid:_ span : unit =
+    let { Span_sub.data; tid; flavor; name; _ } = span in
+    let exit_time_us = time_us_of_time_ns exit_time_ns in
 
-  let on_message (self : st) ~time_ns ~tid ~span:_ ~data msg : unit =
+    let@ buf = Rpool.with_ self.buf_pool in
+    (match flavor with
+    | `Sync ->
+      (* emit full event *)
+      let start_time_us = time_us_of_time_ns span.time_ns in
+      Writer.emit_duration_event buf ~pid:self.pid ~tid ~name
+        ~start:start_time_us ~end_:exit_time_us ~args:data
+    | `Async ->
+      Writer.emit_end buf ~pid:self.pid ~tid ~name ~id:42L ~ts:exit_time_us
+        ~flavor ~args:data);
+
+    self.exporter.on_json buf
+
+  let on_message (self : st) ~time_ns ~tid ~span:_ ~params:_ ~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 on_counter (self : st) ~time_ns ~tid ~params:_ ~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 on_name_thread_ (self : st) ~tid name : unit =
     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;
+    Writer.emit_name_thread buf ~pid:self.pid ~tid ~name;
     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 on_name_process_ (self : st) name : unit =
     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;
+    Writer.emit_name_process ~pid:self.pid ~name buf;
     self.exporter.on_json buf
 
-  let on_extension_event _ ~time_ns:_ ~tid:_ _ev = ()
+  let on_extension_event (self : st) ~time_ns:_ ~tid ev =
+    match ev with
+    | Core_ext.Extension_set_thread_name name -> on_name_thread_ self ~tid name
+    | Core_ext.Extension_set_process_name name -> on_name_process_ self name
+    | _ -> ()
 end
 
+let sub_callbacks : _ Sub.Callbacks.t =
+  Sub.Callbacks.make ~new_span_id ~on_init ~on_shutdown ~on_enter_span
+    ~on_exit_span ~on_message ~on_counter ~on_extension_event ()
+
 let subscriber (self : t) : Sub.t =
-  Sub.Subscriber.Sub { st = self; callbacks = (module Callbacks) }
+  Sub.Subscriber.Sub { st = self; callbacks = sub_callbacks }
diff --git a/src/tef/subscriber.mli b/src/tef/subscriber.mli
index 9f2f235..1cc1af3 100644
--- a/src/tef/subscriber.mli
+++ b/src/tef/subscriber.mli
@@ -16,7 +16,8 @@ val flush : t -> unit
 val close : t -> unit
 val active : t -> bool
 
-module Callbacks : Sub.Callbacks.S with type st = t
+val sub_callbacks : t Sub.Callbacks.t
+(** Callbacks used for the subscriber *)
 
 val subscriber : t -> Sub.t
 (** Subscriber that writes json into this writer *)
diff --git a/src/tef/writer.ml b/src/tef/writer.ml
index 26286c3..e84b4d8 100644
--- a/src/tef/writer.ml
+++ b/src/tef/writer.ml
@@ -50,25 +50,23 @@ let emit_duration_event ~pid ~tid ~name ~start ~end_ ~args buf : unit =
     (emit_args_o_ pp_user_data_)
     args
 
-let emit_manual_begin ~pid ~tid ~name ~(id : int64) ~ts ~args
-    ~(flavor : Trace_core.span_flavor option) buf : unit =
+let emit_begin ~pid ~tid ~name ~(id : int64) ~ts ~args ~flavor 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')
+    | `Async -> 'b'
+    | `Sync -> 'B')
     (emit_args_o_ pp_user_data_)
     args
 
-let emit_manual_end ~pid ~tid ~name ~(id : int64) ~ts
-    ~(flavor : Trace_core.span_flavor option) ~args buf : unit =
+let emit_end ~pid ~tid ~name ~(id : int64) ~ts ~args ~flavor 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')
+    | `Async -> 'e'
+    | `Sync -> 'E')
     (emit_args_o_ pp_user_data_)
     args
 
diff --git a/src/tef/writer.mli b/src/tef/writer.mli
index 536b602..465545c 100644
--- a/src/tef/writer.mli
+++ b/src/tef/writer.mli
@@ -4,7 +4,6 @@
     raw event data. *)
 
 open Common_
-open Trace_core
 
 val emit_duration_event :
   pid:int ->
@@ -16,25 +15,25 @@ val emit_duration_event :
   Buffer.t ->
   unit
 
-val emit_manual_begin :
+val emit_begin :
   pid:int ->
   tid:int ->
   name:string ->
-  id:span ->
+  id:span_id ->
   ts:float ->
   args:(string * Trace_core.user_data) list ->
-  flavor:Trace_core.span_flavor option ->
+  flavor:[ `Sync | `Async ] ->
   Buffer.t ->
   unit
 
-val emit_manual_end :
+val emit_end :
   pid:int ->
   tid:int ->
   name:string ->
-  id:span ->
+  id:span_id ->
   ts:float ->
-  flavor:Trace_core.span_flavor option ->
   args:(string * Trace_core.user_data) list ->
+  flavor:[ `Sync | `Async ] ->
   Buffer.t ->
   unit