diff --git a/dune-project b/dune-project
index 32d47f5..c924572 100644
--- a/dune-project
+++ b/dune-project
@@ -29,6 +29,7 @@
    (trace (= :version))
    (mtime (>= 2.0))
    base-unix
+   thread-local-storage
    dune)
  (tags
   (trace tracing catapult)))
diff --git a/src/tef/dune b/src/tef/dune
index 7fd3a4a..3099988 100644
--- a/src/tef/dune
+++ b/src/tef/dune
@@ -3,4 +3,4 @@
   (name trace_tef)
   (public_name trace-tef)
   (synopsis "Simple and lightweight tracing using TEF/Catapult format, in-process")
-  (libraries trace.core mtime mtime.clock.os unix threads))
+  (libraries trace.core mtime mtime.clock.os unix threads thread-local-storage))
diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
index 159e3fb..999490a 100644
--- a/src/tef/trace_tef.ml
+++ b/src/tef/trace_tef.ml
@@ -1,5 +1,6 @@
 open Trace_core
 module A = Trace_core.Internal_.Atomic_
+module TLS = Thread_local_storage
 
 module Mock_ = struct
   let enabled = ref false
@@ -336,12 +337,16 @@ let bg_thread ~out (events : event B_queue.t) : unit =
         (Span_tbl.length spans);
     ()
 
+(** Current tick. *)
+let cur_tick_ : int A.t = A.make 0
+
 (** 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 =
   try
     while true do
       Thread.delay 0.5;
+      A.incr cur_tick_;
       B_queue.push events E_tick
     done
   with B_queue.Closed -> ()
@@ -352,6 +357,16 @@ type output =
   | `File of string
   ]
 
+type 'a batch = {
+  mutable last_tick: int;
+  batch: 'a Queue.t;
+}
+
+(** Key to access the batch for the current thread *)
+let k_tls_batch : event batch TLS.key =
+  TLS.new_key (fun () ->
+      { last_tick = A.get cur_tick_; batch = Queue.create () })
+
 let collector ~out () : collector =
   let module M = struct
     let active = A.make true
@@ -371,6 +386,8 @@ let collector ~out () : collector =
 
     let shutdown () =
       if A.exchange active false then (
+        (* FIXME: how do we make sure all batches are emptied
+           before we close [events]? *)
         B_queue.close events;
         (* wait for writer thread to be done. The writer thread will exit
            after processing remaining events because the queue is now closed *)
diff --git a/trace-tef.opam b/trace-tef.opam
index b7d427c..34c6fdb 100644
--- a/trace-tef.opam
+++ b/trace-tef.opam
@@ -14,6 +14,7 @@ depends: [
   "trace" {= version}
   "mtime" {>= "2.0"}
   "base-unix"
+  "thread-local-storage"
   "dune" {>= "2.9"}
   "odoc" {with-doc}
 ]