diff --git a/dune-project b/dune-project
index 78001d0..75eb850 100644
--- a/dune-project
+++ b/dune-project
@@ -18,6 +18,18 @@
    (ocaml (>= 4.05))
    dune)
  (tags
-  (tracing observability profiling)))
+  (trace tracing observability profiling)))
+
+(package
+ (name trace-tef)
+ (synopsis "A simple backend for trace")
+ (depends
+   (ocaml (>= 4.05))
+   (trace (= :version))
+   mtime
+   base-unix
+   dune)
+ (tags
+  (trace tracing catapult)))
 
 ; See the complete stanza docs at https://dune.readthedocs.io/en/stable/dune-files.html#dune-project
diff --git a/src/tef/b_queue.ml b/src/tef/b_queue.ml
new file mode 100644
index 0000000..ce9add5
--- /dev/null
+++ b/src/tef/b_queue.ml
@@ -0,0 +1,53 @@
+type 'a t = {
+  mutex: Mutex.t;
+  cond: Condition.t;
+  q: 'a Queue.t;
+  mutable closed: bool;
+}
+
+exception Closed
+
+let create () : _ t =
+  {
+    mutex = Mutex.create ();
+    cond = Condition.create ();
+    q = Queue.create ();
+    closed = false;
+  }
+
+let close (self : _ t) =
+  Mutex.lock self.mutex;
+  if not self.closed then (
+    self.closed <- true;
+    Condition.broadcast self.cond (* awake waiters so they fail  *)
+  );
+  Mutex.unlock self.mutex
+
+let push (self : _ t) x : unit =
+  Mutex.lock self.mutex;
+  if self.closed then (
+    Mutex.unlock self.mutex;
+    raise Closed
+  ) else (
+    Queue.push x self.q;
+    Condition.signal self.cond;
+    Mutex.unlock self.mutex
+  )
+
+let pop (self : 'a t) : 'a =
+  Mutex.lock self.mutex;
+  let rec loop () =
+    if Queue.is_empty self.q then (
+      if self.closed then (
+        Mutex.unlock self.mutex;
+        raise Closed
+      );
+      Condition.wait self.cond self.mutex;
+      (loop [@tailcall]) ()
+    ) else (
+      let x = Queue.pop self.q in
+      Mutex.unlock self.mutex;
+      x
+    )
+  in
+  loop ()
diff --git a/src/tef/b_queue.mli b/src/tef/b_queue.mli
new file mode 100644
index 0000000..e833c92
--- /dev/null
+++ b/src/tef/b_queue.mli
@@ -0,0 +1,18 @@
+(** Basic Blocking Queue *)
+
+type 'a t
+
+val create : unit -> _ t
+
+exception Closed
+
+val push : 'a t -> 'a -> unit
+(** [push q x] pushes [x] into [q], and returns [()].
+    @raise Closed if [close q] was previously called.*)
+
+val pop : 'a t -> 'a
+(** [pop q] pops the next element in [q]. It might block until an element comes.
+   @raise Closed if the queue was closed before a new element was available. *)
+
+val close : _ t -> unit
+(** Close the queue, meaning there won't be any more [push] allowed. *)
diff --git a/src/tef/dune b/src/tef/dune
new file mode 100644
index 0000000..4dcd4df
--- /dev/null
+++ b/src/tef/dune
@@ -0,0 +1,6 @@
+
+(library
+  (name trace_tef)
+  (public_name trace-tef)
+  (synopsis "Simple and lightweight tracing using TEF/Catapult format, in-process")
+  (libraries trace mtime mtime.clock.os unix threads))
diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
new file mode 100644
index 0000000..495d772
--- /dev/null
+++ b/src/tef/trace_tef.ml
@@ -0,0 +1,272 @@
+open Trace
+module A = Trace.Internal_.Atomic_
+
+module Mock_mtime_ = struct
+  let enabled = ref false
+  let now = ref 0
+
+  let[@inline never] now_us () : float =
+    let x = !now in
+    incr now;
+    float_of_int x
+end
+
+let counter = Mtime_clock.counter ()
+
+(** Now, in microseconds *)
+let[@inline] now_us () : float =
+  if !Mock_mtime_.enabled then
+    Mock_mtime_.now_us ()
+  else (
+    let t = Mtime_clock.count counter in
+    Mtime.Span.to_float_ns t /. 1e3
+  )
+
+let protect ~finally f =
+  try
+    let x = f () in
+    finally ();
+    x
+  with exn ->
+    let bt = Printexc.get_raw_backtrace () in
+    finally ();
+    Printexc.raise_with_backtrace exn bt
+
+type event =
+  | E_message of {
+      (*
+      __FUNCTION__: string;
+      __FILE__: string;
+      __LINE__: int;
+      *)
+      tid: int;
+      msg: string;
+      time_us: float;
+    }
+  | E_define_span of {
+      (*
+      __FUNCTION__: string;
+      __FILE__: string;
+      __LINE__: int;
+      *)
+      tid: int;
+      name: string;
+      time_us: float;
+      id: span;
+    }
+  | E_exit_span of {
+      id: span;
+      time_us: float;
+    }
+
+module Span_tbl = Hashtbl.Make (struct
+  include Int64
+
+  let hash : t -> int = Hashtbl.hash
+end)
+
+type span_info = {
+  (*
+  __FUNCTION__: string;
+  __FILE__: string;
+  __LINE__: int;
+  *)
+  tid: int;
+  name: string;
+  start_us: float;
+}
+
+module Writer = struct
+  type t = {
+    oc: out_channel;
+    mutable first: bool;  (** first event? *)
+    must_close: bool;
+    pid: int;
+  }
+
+  let create ~out () : t =
+    let oc, must_close =
+      match out with
+      | `Stdout -> stdout, false
+      | `Stderr -> stderr, false
+      | `File path -> open_out path, true
+    in
+    let pid = Unix.getpid () in
+    output_char oc '[';
+    { oc; first = true; pid; must_close }
+
+  let close (self : t) : unit =
+    output_char self.oc ']';
+    flush self.oc;
+    if self.must_close then close_out self.oc
+
+  let emit_sep_ (self : t) =
+    if self.first then
+      self.first <- false
+    else
+      output_string self.oc ",\n"
+
+  let char = output_char
+  let raw_string = output_string
+
+  let str_val oc (s : string) =
+    char oc '"';
+    let encode_char c =
+      match c with
+      | '"' -> raw_string oc {|\"|}
+      | '\\' -> raw_string oc {|\\|}
+      | '\n' -> raw_string oc {|\n|}
+      | '\b' -> raw_string oc {|\b|}
+      | '\r' -> raw_string oc {|\r|}
+      | '\t' -> raw_string oc {|\t|}
+      | _ when Char.code c <= 0x1f ->
+        raw_string oc {|\u00|};
+        Printf.fprintf oc "%02x" (Char.code c)
+      | c -> char oc c
+    in
+    String.iter encode_char s;
+    char oc '"'
+
+  (* emit args, if not empty. [ppv] is used to print values. *)
+  let emit_args_o_ ppv oc args : unit =
+    if args <> [] then (
+      Printf.fprintf oc {json|,"args": {|json};
+      List.iteri
+        (fun i (n, value) ->
+          if i > 0 then Printf.fprintf oc ",";
+          Printf.fprintf oc {json|"%s":%a|json} n ppv value)
+        args;
+      char oc '}'
+    )
+
+  let emit_duration_event ~tid ~name ~start ~end_ ~args (self : t) : unit =
+    let dur = end_ -. start in
+    let ts = start in
+    emit_sep_ self;
+    Printf.fprintf self.oc
+      {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_ str_val) args;
+    ()
+
+  let emit_instant_event ~tid ~name ~ts ~args (self : t) : unit =
+    emit_sep_ self;
+    Printf.fprintf self.oc
+      {json|{"pid": %d,"cat":"","tid": %d,"ts": %.2f,"name":%a,"ph":"I"%a}|json}
+      self.pid tid ts str_val name (emit_args_o_ str_val) args;
+    ()
+end
+
+let bg_thread ~out (events : event B_queue.t) : unit =
+  let writer = Writer.create ~out () in
+  protect ~finally:(fun () -> Writer.close writer) @@ fun () ->
+  let spans : span_info Span_tbl.t = Span_tbl.create 32 in
+
+  (* how to deal with an event *)
+  let handle_ev (ev : event) : unit =
+    match ev with
+    | E_message { (* __FUNCTION__; __FILE__; __LINE__; *) tid; msg; time_us } ->
+      Writer.emit_instant_event ~tid ~name:msg ~ts:time_us ~args:[] writer
+    | E_define_span
+        { (* __FUNCTION__; __FILE__; __LINE__; *) tid; name; id; time_us } ->
+      (* save the span so we find it at exit *)
+      Span_tbl.add spans id
+        {
+          (* __FUNCTION__; __FILE__; __LINE__; *) tid;
+          name;
+          start_us = time_us;
+        }
+    | E_exit_span { id; time_us = stop_us } ->
+      (match Span_tbl.find_opt spans id with
+      | None -> (* bug! *) ()
+      | Some { (* __FUNCTION__; __FILE__; __LINE__; *) tid; name; start_us } ->
+        Writer.emit_duration_event ~tid ~name ~start:start_us ~end_:stop_us
+          ~args:[] writer)
+  in
+
+  try
+    while true do
+      let ev = B_queue.pop events in
+      handle_ev ev
+    done
+  with B_queue.Closed -> ()
+
+type output =
+  [ `Stdout
+  | `Stderr
+  | `File of string
+  ]
+
+let collector ~out () : collector =
+  let module M = struct
+    let active = A.make true
+
+    (** generator for span ids *)
+    let span_id_gen_ = A.make 0
+
+    let enabled () = true
+
+    (* queue of messages to write *)
+    let events : event B_queue.t = B_queue.create ()
+
+    (* writer thread. It receives events and writes them to [oc]. *)
+    let t_write : Thread.t = Thread.create (fun () -> bg_thread ~out events) ()
+
+    let shutdown () =
+      if A.exchange active false then (
+        Printf.eprintf "shutdown\n%!";
+        B_queue.close events;
+        Printf.eprintf "wait\n%!";
+        Thread.join t_write
+      )
+
+    let[@inline] get_tid_ () : int = Thread.id (Thread.self ())
+
+    let create_span ?__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_ name : span =
+      let span = Int64.of_int (A.fetch_and_add span_id_gen_ 1) in
+      let tid = get_tid_ () in
+      let time_us = now_us () in
+      B_queue.push events
+        (E_define_span
+           {
+             (* __FUNCTION__; __FILE__; __LINE__; *) tid;
+             name;
+             time_us;
+             id = span;
+           });
+      span
+
+    let exit_span span : unit =
+      let time_us = now_us () in
+      B_queue.push events (E_exit_span { id = span; time_us })
+
+    let message ?__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_ msg : unit =
+      let time_us = now_us () in
+      let tid = get_tid_ () in
+      B_queue.push events
+        (E_message { (* __FUNCTION__; __FILE__; __LINE__; *) tid; time_us; msg })
+  end in
+  (module M)
+
+let setup ?(out = `Env) () =
+  match out with
+  | `Stderr -> Trace.setup_collector @@ collector ~out:`Stderr ()
+  | `Stdout -> Trace.setup_collector @@ collector ~out:`Stdout ()
+  | `File path -> Trace.setup_collector @@ collector ~out:(`File path) ()
+  | `Env ->
+    (match Sys.getenv_opt "TRACE" with
+    | Some "1" ->
+      let path = "trace.json" in
+      let c = collector ~out:(`File path) () in
+      Trace.setup_collector c
+    | Some path ->
+      let c = collector ~out:(`File path) () in
+      Trace.setup_collector c
+    | None -> ())
+
+let with_setup ?out f =
+  setup ?out ();
+  protect ~finally:Trace.shutdown f
+
+module Internal_ = struct
+  let use_mock_mtime_ () = Mock_mtime_.enabled := true
+end
diff --git a/src/tef/trace_tef.mli b/src/tef/trace_tef.mli
new file mode 100644
index 0000000..f9d3de1
--- /dev/null
+++ b/src/tef/trace_tef.mli
@@ -0,0 +1,44 @@
+open Trace
+
+val collector :
+  out:[ `File of string | `Stderr | `Stdout ] -> unit -> Trace.collector
+(** Make a collector that writes into the given output.
+    See {!setup} for more details. *)
+
+type output =
+  [ `Stdout
+  | `Stderr
+  | `File of string
+  ]
+(** Output for tracing.
+
+    - [`Stdout] will enable tracing and print events on stdout
+    - [`Stderr] will enable tracing and print events on stderr
+    - [`File "foo"] will enable tracing and print events into file
+        named "foo"
+*)
+
+val setup : ?out:[ output | `Env ] -> unit -> unit
+(** [setup ()] installs the collector depending on [out].
+
+    @param out can take different values:
+    - regular {!output} value to specify where events go
+    - [`Env] will enable tracing if the environment
+      variable "TRACE" is set.
+
+      If it's set to anything but "1", the value is taken
+      to be the file path into which to write.
+      If it's set to "1", then the file is "trace.json".
+*)
+
+val with_setup : ?out:[ output | `Env ] -> (unit -> 'a) -> 'a
+(** Setup, and make sure to shutdown before exiting *)
+
+(**/**)
+
+module Internal_ : sig
+  val use_mock_mtime_ : unit -> unit
+  (* use fake, deterministic timestamps *)
+end
+
+(**/**)
diff --git a/src/trace.ml b/src/trace.ml
index 809fc30..08d9548 100644
--- a/src/trace.ml
+++ b/src/trace.ml
@@ -64,3 +64,7 @@ let shutdown () =
   match A.exchange collector None with
   | None -> ()
   | Some (module C) -> C.shutdown ()
+
+module Internal_ = struct
+  module Atomic_ = Atomic_
+end
diff --git a/src/trace.mli b/src/trace.mli
index f7d714f..e437db6 100644
--- a/src/trace.mli
+++ b/src/trace.mli
@@ -23,6 +23,8 @@ val with_ :
 val message :
   ?__FUNCTION__:string -> __FILE__:string -> __LINE__:int -> string -> unit
 
+(* TODO: counter/plot/metric *)
+
 val messagef :
   ?__FUNCTION__:string ->
   __FILE__:string ->
@@ -40,3 +42,12 @@ val setup_collector : collector -> unit
     collector. *)
 
 val shutdown : unit -> unit
+
+(**/**)
+
+(* no guarantee of stability *)
+module Internal_ : sig
+  module Atomic_ = Atomic_
+end
+
+(**/**)
diff --git a/trace-tef.opam b/trace-tef.opam
new file mode 100644
index 0000000..026ee06
--- /dev/null
+++ b/trace-tef.opam
@@ -0,0 +1,34 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+synopsis: "A simple backend for trace"
+maintainer: ["Simon Cruanes"]
+authors: ["Simon Cruanes"]
+license: "MIT"
+tags: ["trace" "tracing" "catapult"]
+homepage: "https://github.com/c-cube/trace"
+bug-reports: "https://github.com/c-cube/trace/issues"
+depends: [
+  "ocaml" {>= "4.05"}
+  "trace" {= version}
+  "mtime"
+  "base-unix"
+  "dune" {>= "2.9"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "--promote-install-files=false"
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+  ["dune" "install" "-p" name "--create-install-files" name]
+]
+dev-repo: "git+https://github.com/c-cube/trace.git"
diff --git a/trace.opam b/trace.opam
index 162e697..ea1e948 100644
--- a/trace.opam
+++ b/trace.opam
@@ -4,7 +4,7 @@ synopsis: "A stub for tracing/observability"
 maintainer: ["Simon Cruanes"]
 authors: ["Simon Cruanes"]
 license: "MIT"
-tags: ["tracing" "observability" "profiling"]
+tags: ["trace" "tracing" "observability" "profiling"]
 homepage: "https://github.com/c-cube/trace"
 bug-reports: "https://github.com/c-cube/trace/issues"
 depends: [