diff --git a/src/fuchsia/subscriber.ml b/src/fuchsia/subscriber.ml
index 768ae46..94b64e9 100644
--- a/src/fuchsia/subscriber.ml
+++ b/src/fuchsia/subscriber.ml
@@ -1,28 +1,14 @@
 open Common_
 open Trace_core
-module Span_tbl = Trace_subscriber.Span_tbl
 
 let on_tracing_error = on_tracing_error
 
-type span_info = {
-  tid: int;
-  name: string;
-  start_ns: int64;
-  mutable data: (string * 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_chain: Buf_chain.t;
   exporter: Exporter.t;
   span_gen: Sub.Span_generator.t;
-  trace_id_gen: Sub.Trace_id_8B_generator.t;
 }
 (** Subscriber state *)
 
@@ -32,6 +18,7 @@ open struct
     if Buf_chain.has_ready self.buf_chain then
       Buf_chain.pop_ready self.buf_chain ~f:self.exporter.write_bufs
 
+  (* TODO: nice to have, can we make it optional?
   let print_non_closed_spans_warning spans =
     let module Str_set = Set.Make (String) in
     let spans = Span_tbl.to_list spans in
@@ -49,15 +36,14 @@ open struct
         names;
       flush stderr
     )
+    *)
 end
 
 let close (self : t) : unit =
   if A.exchange self.active false then (
     Buf_chain.ready_all_non_empty self.buf_chain;
     write_ready_ self;
-    self.exporter.close ();
-
-    print_non_closed_spans_warning self.spans
+    self.exporter.close () (* TODO: print_non_closed_spans_warning self.spans *)
   )
 
 let[@inline] active self = A.get self.active
@@ -74,20 +60,13 @@ let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
     buf_chain;
     exporter;
     pid;
-    spans = Span_tbl.create ();
     span_gen = Sub.Span_generator.create ();
-    trace_id_gen = Sub.Trace_id_8B_generator.create ();
   }
 
-module Callbacks = struct
-  type st = t
+open struct
+  let new_span_id (self : t) = Sub.Span_generator.mk_span self.span_gen
 
-  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 on_init (self : st) ~time_ns:_ =
+  let on_init (self : t) ~time_ns:_ =
     Writer.Metadata.Magic_record.encode self.buf_chain;
     Writer.Metadata.Initialization_record.(
       encode self.buf_chain ~ticks_per_secs:default_ticks_per_sec ());
@@ -98,14 +77,14 @@ module Callbacks = struct
 
     write_ready_ self
 
-  let on_shutdown (self : st) ~time_ns:_ = close self
+  let on_shutdown (self : t) ~time_ns:_ = close self
 
-  let on_name_process (self : st) ~time_ns:_ ~tid:_ ~name : unit =
+  let on_name_process_ (self : t) name : unit =
     Writer.Kernel_object.(
       encode self.buf_chain ~name ~ty:ty_process ~kid:self.pid ~args:[] ());
     write_ready_ self
 
-  let on_name_thread (self : st) ~time_ns:_ ~tid ~name : unit =
+  let on_name_thread_ (self : t) ~tid name : unit =
     Writer.Kernel_object.(
       encode self.buf_chain ~name ~ty:ty_thread ~kid:tid
         ~args:[ "process", A_kid (Int64.of_int self.pid) ]
@@ -118,43 +97,49 @@ module Callbacks = struct
     | 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 data = add_fun_name_ fun_name data in
-    let info = { tid; name; start_ns = time_ns; data } in
-    (* save the span so we find it at exit *)
-    Span_tbl.add self.spans span info
+  let on_enter_span (self : t) (span : Sub.Span_sub.t) : unit =
+    let { Sub.Span_sub.data; name; tid; time_ns; flavor; _ } = span in
+    match flavor with
+    | `Sync -> ()
+    | `Async ->
+      let data = add_fun_name_ span.__FUNCTION__ data in
+      Writer.(
+        Event.Async_begin.encode self.buf_chain ~name
+          ~args:(args_of_user_data data)
+          ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
+          ~time_ns ~async_id:0L ());
+      write_ready_ self
 
-  let on_exit_span (self : st) ~time_ns ~tid:_ span : unit =
-    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_ns; data } ->
-      Span_tbl.remove self.spans span;
+  let on_exit_span (self : t) ~time_ns:end_time_ns ~tid:_
+      (span : Sub.Span_sub.t) : unit =
+    let { Sub.Span_sub.tid; name; flavor; time_ns = start_ns; data; _ } =
+      span
+    in
+    let data = add_fun_name_ span.__FUNCTION__ data in
+
+    match flavor with
+    | `Sync ->
       Writer.(
         Event.Duration_complete.encode self.buf_chain ~name
           ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
-          ~time_ns:start_ns ~end_time_ns:time_ns ~args:(args_of_user_data data)
-          ());
+          ~time_ns:start_ns ~end_time_ns ~args:(args_of_user_data data) ());
+      write_ready_ self
+    | `Async ->
+      Writer.(
+        Event.Async_end.encode self.buf_chain ~name
+          ~args:(args_of_user_data data)
+          ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
+          ~time_ns:end_time_ns ~async_id:0L ());
       write_ready_ self
 
-  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 on_message (self : t) ~time_ns ~tid ~span:_ ~params:_ ~data msg : unit =
     Writer.(
       Event.Instant.encode self.buf_chain
         ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
         ~name:msg ~time_ns ~args:(args_of_user_data data) ());
     write_ready_ self
 
-  let on_counter (self : st) ~time_ns ~tid ~data ~name n : unit =
+  let on_counter (self : t) ~time_ns ~tid ~params:_ ~data ~name n : unit =
     Writer.(
       Event.Counter.encode self.buf_chain
         ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
@@ -163,25 +148,16 @@ module Callbacks = struct
         ());
     write_ready_ self
 
-  let on_enter_manual_span (self : st) ~__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_
-      ~time_ns ~tid ~parent:_ ~data ~name ~flavor:_ ~trace_id _span : unit =
-    Writer.(
-      Event.Async_begin.encode self.buf_chain ~name
-        ~args:(args_of_user_data data)
-        ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
-        ~time_ns ~async_id:trace_id ());
-    write_ready_ self
-
-  let on_exit_manual_span (self : st) ~time_ns ~tid ~name ~data ~flavor:_
-      ~trace_id (_ : span) : unit =
-    Writer.(
-      Event.Async_end.encode self.buf_chain ~name ~args:(args_of_user_data data)
-        ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
-        ~time_ns ~async_id:trace_id ());
-    write_ready_ self
-
-  let on_extension_event _ ~time_ns:_ ~tid:_ _ev = ()
+  let on_extension_event (self : t) ~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/fuchsia/subscriber.mli b/src/fuchsia/subscriber.mli
index 66318f5..8eaf791 100644
--- a/src/fuchsia/subscriber.mli
+++ b/src/fuchsia/subscriber.mli
@@ -7,8 +7,7 @@ val create : ?buf_pool:Buf_pool.t -> pid:int -> exporter:Exporter.t -> unit -> t
 val flush : t -> unit
 val close : t -> unit
 val active : t -> bool
-
-module Callbacks : Trace_subscriber.Callbacks.S with type st = t
+val sub_callbacks : t Trace_subscriber.Callbacks.t
 
 val subscriber : t -> Trace_subscriber.t
 (** Subscriber that writes json into this writer *)
diff --git a/src/fuchsia/writer.ml b/src/fuchsia/writer.ml
index 972d9eb..ffc363c 100644
--- a/src/fuchsia/writer.ml
+++ b/src/fuchsia/writer.ml
@@ -4,15 +4,6 @@
 
 open Common_
 module Util = Util
-
-open struct
-  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
-
 open Util
 
 type user_data = Trace_core.user_data
@@ -491,7 +482,7 @@ module Event = struct
       + Arguments.size_word args + 1 (* async id *)
 
     let encode (bufs : Buf_chain.t) ~name ~(t_ref : Thread_ref.t) ~time_ns
-        ~(async_id : Trace_core.trace_id) ~args () : unit =
+        ~(async_id : int64) ~args () : unit =
       let name = truncate_string name in
       let size = size_word ~name ~t_ref ~args () in
       let@ buf = Buf_chain.with_buf bufs ~available_word:size in
@@ -516,7 +507,7 @@ module Event = struct
 
       Buf.add_string buf name;
       Arguments.encode buf args;
-      Buf.add_i64 buf (int64_of_trace_id_ async_id);
+      Buf.add_i64 buf async_id;
       ()
   end
 
@@ -527,7 +518,7 @@ module Event = struct
       + Arguments.size_word args + 1 (* async id *)
 
     let encode (bufs : Buf_chain.t) ~name ~(t_ref : Thread_ref.t) ~time_ns
-        ~(async_id : Trace_core.trace_id) ~args () : unit =
+        ~(async_id : int64) ~args () : unit =
       let name = truncate_string name in
       let size = size_word ~name ~t_ref ~args () in
       let@ buf = Buf_chain.with_buf bufs ~available_word:size in
@@ -552,7 +543,7 @@ module Event = struct
 
       Buf.add_string buf name;
       Arguments.encode buf args;
-      Buf.add_i64 buf (int64_of_trace_id_ async_id);
+      Buf.add_i64 buf async_id;
       ()
   end
 end