diff --git a/src/fuchsia/buf_pool.ml b/src/fuchsia/buf_pool.ml
index 6ea615d..8a0748c 100644
--- a/src/fuchsia/buf_pool.ml
+++ b/src/fuchsia/buf_pool.ml
@@ -1,5 +1,5 @@
 open Common_
-open Trace_private_util
+open Trace_util
 
 type t = Buf.t Rpool.t
 
diff --git a/src/fuchsia/subscriber.ml b/src/fuchsia/collector_fuchsia.ml
similarity index 53%
rename from src/fuchsia/subscriber.ml
rename to src/fuchsia/collector_fuchsia.ml
index 9bf6ec1..157e938 100644
--- a/src/fuchsia/subscriber.ml
+++ b/src/fuchsia/collector_fuchsia.ml
@@ -1,4 +1,5 @@
 open Common_
+open Types
 open Trace_core
 
 let on_tracing_error = on_tracing_error
@@ -8,8 +9,7 @@ type t = {
   pid: int;
   buf_chain: Buf_chain.t;
   exporter: Exporter.t;
-  span_id_gen: Sub.Span_id_generator.t;
-  trace_id_gen: Sub.Trace_id_generator.t;
+  trace_id_gen: Types.Trace_id.Gen.t;
 }
 (** Subscriber state *)
 
@@ -61,15 +61,13 @@ let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
     buf_chain;
     exporter;
     pid;
-    span_id_gen = Sub.Span_id_generator.create ();
-    trace_id_gen = Sub.Trace_id_generator.create ();
+    trace_id_gen = Types.Trace_id.Gen.create ();
   }
 
 open struct
-  let new_span_id (self : t) = Sub.Span_id_generator.gen self.span_id_gen
-  let new_trace_id self = Sub.Trace_id_generator.gen self.trace_id_gen
+  let new_trace_id self = Types.Trace_id.Gen.gen self.trace_id_gen
 
-  let on_init (self : t) ~time_ns:_ =
+  let init (self : t) =
     Writer.Metadata.Magic_record.encode self.buf_chain;
     Writer.Metadata.Initialization_record.(
       encode self.buf_chain ~ticks_per_secs:default_ticks_per_sec ());
@@ -80,19 +78,7 @@ open struct
 
     write_ready_ self
 
-  let on_shutdown (self : t) ~time_ns:_ = close self
-
-  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 : 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) ]
-        ());
-    write_ready_ self
+  let shutdown (self : t) = close self
 
   (* add function name, if provided, to the metadata *)
   let add_fun_name_ fun_name data : _ list =
@@ -100,49 +86,84 @@ open struct
     | None -> data
     | Some f -> ("function", `String f) :: data
 
-  let on_enter_span (self : t) (span : Sub.Span_sub.t) : unit =
-    let { Sub.Span_sub.data; name; tid; time_ns; flavor; _ } = span in
+  let rec flavor_of_params = function
+    | [] -> `Sync
+    | Core_ext.Extension_span_flavor f :: _ -> f
+    | _ :: tl -> flavor_of_params tl
+
+  let enter_span (self : t) ~__FUNCTION__ ~__FILE__ ~__LINE__ ~params ~data
+      ~parent name : span =
+    let flavor = flavor_of_params params in
+    let time_ns = Trace_util.Mock_.now_ns () in
+    let tid = Trace_util.Mock_.get_tid () in
+
     match flavor with
-    | `Sync -> ()
+    | `Sync ->
+      Span_fuchsia_sync
+        {
+          __FUNCTION__;
+          name;
+          pid = self.pid;
+          tid;
+          args = data;
+          start_ns = time_ns;
+        }
     | `Async ->
-      let data = add_fun_name_ span.__FUNCTION__ data in
+      let data = add_fun_name_ __FUNCTION__ data in
+      let trace_id =
+        match parent with
+        | P_some (Span_fuchsia_async sp) -> sp.trace_id
+        | _ -> new_trace_id self
+      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:span.trace_id ());
-      write_ready_ self
+          ~time_ns ~async_id:trace_id ());
+      write_ready_ self;
 
-  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
+      Span_fuchsia_async { pid = self.pid; tid; trace_id; name; args = data }
 
-    match flavor with
-    | `Sync ->
+  let exit_span (self : t) sp =
+    let end_time_ns = Trace_util.Mock_.now_ns () in
+
+    match sp with
+    | Span_fuchsia_sync { __FUNCTION__; name; tid; pid; args = data; start_ns }
+      ->
+      let data = add_fun_name_ __FUNCTION__ data in
       Writer.(
         Event.Duration_complete.encode self.buf_chain ~name
-          ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
+          ~t_ref:(Thread_ref.inline ~pid ~tid)
           ~time_ns:start_ns ~end_time_ns ~args:(args_of_user_data data) ());
       write_ready_ self
-    | `Async ->
+    | Span_fuchsia_async { name; tid; pid; trace_id; args = data } ->
       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:span.trace_id ());
+          ~t_ref:(Thread_ref.inline ~pid ~tid)
+          ~time_ns:end_time_ns ~async_id:trace_id ());
       write_ready_ self
+    | _ -> ()
 
-  let on_message (self : t) ~time_ns ~tid ~span:_ ~params:_ ~data msg : unit =
+  let add_data_to_span _st sp data =
+    match sp with
+    | Span_fuchsia_sync sp -> sp.args <- List.rev_append data sp.args
+    | Span_fuchsia_async sp -> sp.args <- List.rev_append data sp.args
+    | _ -> ()
+
+  let message (self : t) ~params:_ ~data ~span:_ msg : unit =
+    let time_ns = Trace_util.Mock_.now_ns () in
+    let tid = Trace_util.Mock_.get_tid () in
     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 : t) ~time_ns ~tid ~params:_ ~data ~name n : unit =
+  let counter_float (self : t) ~params:_ ~data name n : unit =
+    let tid = Trace_util.Mock_.get_tid () in
+    let time_ns = Trace_util.Mock_.now_ns () in
     Writer.(
       Event.Counter.encode self.buf_chain
         ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
@@ -151,16 +172,40 @@ open struct
         ());
     write_ready_ self
 
-  let on_extension_event (self : t) ~time_ns:_ ~tid ev =
+  let counter_int self ~params:_ ~data name n =
+    let tid = Trace_util.Mock_.get_tid () in
+    let time_ns = Trace_util.Mock_.now_ns () in
+    Writer.(
+      Event.Counter.encode self.buf_chain
+        ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)
+        ~name ~time_ns
+        ~args:((name, A_int n) :: args_of_user_data data)
+        ());
+    write_ready_ self
+
+  let 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 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) ]
+        ());
+    write_ready_ self
+
+  let extension (self : t) 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
+    | Core_ext.Extension_set_thread_name name ->
+      let tid = Trace_util.Mock_.get_tid () in
+      name_thread_ self ~tid name
+    | Core_ext.Extension_set_process_name name -> name_process_ self name
     | _ -> ()
 end
 
-let sub_callbacks : _ Sub.Callbacks.t =
-  Sub.Callbacks.make ~new_span_id ~new_trace_id ~on_init ~on_shutdown
-    ~on_enter_span ~on_exit_span ~on_message ~on_counter ~on_extension_event ()
+let callbacks : t Collector.Callbacks.t =
+  Collector.Callbacks.make ~init ~shutdown ~enter_span ~exit_span
+    ~add_data_to_span ~message ~counter_int ~counter_float ~extension ()
 
-let subscriber (self : t) : Sub.t =
-  Sub.Subscriber.Sub { st = self; callbacks = sub_callbacks }
+let collector (self : t) : Collector.t = Collector.C_some (self, callbacks)
diff --git a/src/fuchsia/subscriber.mli b/src/fuchsia/collector_fuchsia.mli
similarity index 78%
rename from src/fuchsia/subscriber.mli
rename to src/fuchsia/collector_fuchsia.mli
index 8eaf791..f771548 100644
--- a/src/fuchsia/subscriber.mli
+++ b/src/fuchsia/collector_fuchsia.mli
@@ -1,3 +1,5 @@
+open Trace_core
+
 type t
 (** Main subscriber state. *)
 
@@ -7,9 +9,9 @@ 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
-val sub_callbacks : t Trace_subscriber.Callbacks.t
+val callbacks : t Collector.Callbacks.t
 
-val subscriber : t -> Trace_subscriber.t
+val collector : t -> Collector.t
 (** Subscriber that writes json into this writer *)
 
 (**/**)
diff --git a/src/fuchsia/dune b/src/fuchsia/dune
index 4ef6048..d35d63a 100644
--- a/src/fuchsia/dune
+++ b/src/fuchsia/dune
@@ -5,8 +5,7 @@
   "A high-performance backend for trace, emitting a Fuchsia trace into a file")
  (libraries
   trace.core
-  trace.private.util
-  trace.subscriber
+  trace.util
   thread-local-storage
   bigarray
   mtime
diff --git a/src/fuchsia/trace_fuchsia.ml b/src/fuchsia/trace_fuchsia.ml
index 6529192..5cbebc6 100644
--- a/src/fuchsia/trace_fuchsia.ml
+++ b/src/fuchsia/trace_fuchsia.ml
@@ -3,7 +3,7 @@ module Buf = Buf
 module Buf_chain = Buf_chain
 module Buf_pool = Buf_pool
 module Exporter = Exporter
-module Subscriber = Subscriber
+module Collector_fuchsia = Collector_fuchsia
 module Writer = Writer
 
 type output =
@@ -18,25 +18,18 @@ let get_out_ (out : [< output ]) : Exporter.t =
     Exporter.of_out_channel ~close_channel:true oc
   | `Exporter e -> e
 
-let subscriber ~out () : Sub.t =
+let collector ~out () : Trace_core.Collector.t =
   let exporter = get_out_ out in
-  let pid =
-    if !Trace_subscriber.Private_.mock then
-      2
-    else
-      Unix.getpid ()
-  in
-  let sub = Subscriber.create ~pid ~exporter () in
-  Subscriber.subscriber sub
-
-let collector ~out () = Sub.collector @@ subscriber ~out ()
+  let pid = Trace_util.Mock_.get_pid () in
+  let coll_st = Collector_fuchsia.create ~pid ~exporter () in
+  Collector_fuchsia.collector coll_st
 
 let setup ?(out = `Env) () =
   match out with
   | `File path -> Trace_core.setup_collector @@ collector ~out:(`File path) ()
   | `Exporter _ as out ->
-    let sub = subscriber ~out () in
-    Trace_core.setup_collector @@ Sub.collector sub
+    let c = collector ~out () in
+    Trace_core.setup_collector c
   | `Env ->
     (match Sys.getenv_opt "TRACE" with
     | Some ("1" | "true") ->
@@ -52,23 +45,9 @@ 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 Internal_ = 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_util.Mock_.mock_all ();
     ()
 
   let on_tracing_error = on_tracing_error
diff --git a/src/fuchsia/trace_fuchsia.mli b/src/fuchsia/trace_fuchsia.mli
index 74905dc..b512439 100644
--- a/src/fuchsia/trace_fuchsia.mli
+++ b/src/fuchsia/trace_fuchsia.mli
@@ -10,7 +10,7 @@ module Buf = Buf
 module Buf_chain = Buf_chain
 module Buf_pool = Buf_pool
 module Exporter = Exporter
-module Subscriber = Subscriber
+module Collector_fuchsia = Collector_fuchsia
 module Writer = Writer
 
 type output =
@@ -18,8 +18,6 @@ type output =
   | `Exporter of Exporter.t
   ]
 
-val subscriber : out:[< output ] -> unit -> Trace_subscriber.t
-
 val collector : out:[< output ] -> unit -> Trace_core.collector
 (** Make a collector that writes into the given output. See {!setup} for more
     details. *)
diff --git a/src/fuchsia/types.ml b/src/fuchsia/types.ml
new file mode 100644
index 0000000..e2aaf0d
--- /dev/null
+++ b/src/fuchsia/types.ml
@@ -0,0 +1,21 @@
+open Trace_core
+module Trace_id = Trace_util.Trace_id64
+
+type trace_id = Trace_id.t
+
+type Trace_core.span +=
+  | Span_fuchsia_sync of {
+      __FUNCTION__: string option;
+      pid: int;
+      tid: int;
+      name: string;
+      start_ns: int64;
+      mutable args: (string * Trace_core.user_data) list;
+    }
+  | Span_fuchsia_async of {
+      pid: int;
+      tid: int;
+      name: string;
+      trace_id: trace_id;
+      mutable args: (string * Trace_core.user_data) list;
+    }