Factor batching logic out of the cohttp-lwt client

This will allow resuing the batching logic in the Eio client.
As a followup, we should refactor the ocurl client to use the same
batcher.

src/client-cohttp-lwt/opentelemetry_client_cohttp_lwt.ml

@@ -6,6 +6,7 @@
 module OT = Opentelemetry
 module Config = Config
 module Signal = Opentelemetry_client.Signal
+module Batch = Opentelemetry_client.Batch
 open Opentelemetry
 open Common_
 
@@ -164,88 +165,6 @@ end = struct
       )
 end
 
-(** Batch of resources to be pushed later.
-
-    This type is thread-safe. *)
-module Batch : sig
-  type 'a t
-
-  val push' : 'a t -> 'a -> unit
-
-  val pop_if_ready : ?force:bool -> now:Mtime.t -> 'a t -> 'a list option
-  (** Is the batch ready to be emitted? If batching is disabled, this is true as
-      soon as {!is_empty} is false. If a timeout is provided for this batch,
-      then it will be ready if an element has been in it for at least the
-      timeout.
-      @param now passed to implement timeout *)
-
-  val make : ?batch:int -> ?timeout:Mtime.span -> unit -> 'a t
-  (** Create a new batch *)
-end = struct
-  type 'a t = {
-    mutable size: int;
-    mutable q: 'a list;
-    batch: int option;
-    high_watermark: int;
-    timeout: Mtime.span option;
-    mutable start: Mtime.t;
-  }
-
-  let make ?batch ?timeout () : _ t =
-    Option.iter (fun b -> assert (b > 0)) batch;
-    let high_watermark = Option.fold ~none:100 ~some:(fun x -> x * 10) batch in
-    {
-      size = 0;
-      start = Mtime_clock.now ();
-      q = [];
-      batch;
-      timeout;
-      high_watermark;
-    }
-
-  let timeout_expired_ ~now self : bool =
-    match self.timeout with
-    | Some t ->
-      let elapsed = Mtime.span now self.start in
-      Mtime.Span.compare elapsed t >= 0
-    | None -> false
-
-  let is_full_ self : bool =
-    match self.batch with
-    | None -> self.size > 0
-    | Some b -> self.size >= b
-
-  let pop_if_ready ?(force = false) ~now (self : _ t) : _ list option =
-    if self.size > 0 && (force || is_full_ self || timeout_expired_ ~now self)
-    then (
-      let l = self.q in
-      self.q <- [];
-      self.size <- 0;
-      assert (l <> []);
-      Some l
-    ) else
-      None
-
-  let push (self : _ t) x : bool =
-    if self.size >= self.high_watermark then (
-      (* drop this to prevent queue from growing too fast *)
-      Atomic.incr n_dropped;
-      true
-    ) else (
-      if self.size = 0 && Option.is_some self.timeout then
-        (* current batch starts now *)
-        self.start <- Mtime_clock.now ();
-
-      (* add to queue *)
-      self.size <- 1 + self.size;
-      self.q <- x :: self.q;
-      let ready = is_full_ self in
-      ready
-    )
-
-  let push' self x = ignore (push self x : bool)
-end
-
 (** An emitter. This is used by {!Backend} below to forward traces/metrics/…
     from the program to whatever collector client we have. *)
 module type EMITTER = sig
@@ -280,13 +199,13 @@ let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
       else
         None
 
-    let batch_traces : Trace.resource_spans list Batch.t =
+    let batch_traces : Trace.resource_spans Batch.t =
       Batch.make ?batch:config.batch_traces ?timeout ()
 
-    let batch_metrics : Metrics.resource_metrics list Batch.t =
+    let batch_metrics : Metrics.resource_metrics Batch.t =
       Batch.make ?batch:config.batch_metrics ?timeout ()
 
-    let batch_logs : Logs.resource_logs list Batch.t =
+    let batch_logs : Logs.resource_logs Batch.t =
       Batch.make ?batch:config.batch_logs ?timeout ()
 
     let on_tick_cbs_ = Atomic.make (AList.make ())
@@ -317,13 +236,9 @@ let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
     let send_logs_http client (l : Logs.resource_logs list) =
       Conv.logs l |> send_http_ client ~url:config.url_logs
 
-    let maybe_pop ?force ~now batch =
-      Batch.pop_if_ready ?force ~now batch
-      |> Option.map (List.fold_left (fun acc l -> List.rev_append l acc) [])
-
     (* emit metrics, if the batch is full or timeout lapsed *)
     let emit_metrics_maybe ~now ?force httpc : bool Lwt.t =
-      match maybe_pop ?force ~now batch_metrics with
+      match Batch.pop_if_ready ?force ~now batch_metrics with
       | None -> Lwt.return false
       | Some l ->
         let batch = !gc_metrics @ l in
@@ -332,14 +247,14 @@ let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
         true
 
     let emit_traces_maybe ~now ?force httpc : bool Lwt.t =
-      match maybe_pop ?force ~now batch_traces with
+      match Batch.pop_if_ready ?force ~now batch_traces with
       | None -> Lwt.return false
       | Some l ->
         let+ () = send_traces_http httpc l in
         true
 
     let emit_logs_maybe ~now ?force httpc : bool Lwt.t =
-      match maybe_pop ?force ~now batch_logs with
+      match Batch.pop_if_ready ?force ~now batch_logs with
       | None -> Lwt.return false
       | Some l ->
         let+ () = send_logs_http httpc l in
@@ -381,9 +296,14 @@ let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
        can also be several user threads that produce spans and call
        the emit functions. *)
 
+    let push_to_batch b e =
+      match Batch.push b e with
+      | `Ok -> ()
+      | `Dropped -> Atomic.incr n_errors
+
     let push_trace e =
       let@ () = guard_exn_ "push trace" in
-      Batch.push' batch_traces e;
+      push_to_batch batch_traces e;
       let now = Mtime_clock.now () in
       Lwt.async (fun () ->
           let+ (_ : bool) = emit_traces_maybe ~now httpc in
@@ -392,7 +312,7 @@ let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
     let push_metrics e =
       let@ () = guard_exn_ "push metrics" in
       sample_gc_metrics_if_needed ();
-      Batch.push' batch_metrics e;
+      push_to_batch batch_metrics e;
       let now = Mtime_clock.now () in
       Lwt.async (fun () ->
           let+ (_ : bool) = emit_metrics_maybe ~now httpc in
@@ -400,7 +320,7 @@ let mk_emitter ~stop ~(config : Config.t) () : (module EMITTER) =
 
     let push_logs e =
       let@ () = guard_exn_ "push logs" in
-      Batch.push' batch_logs e;
+      push_to_batch batch_logs e;
       let now = Mtime_clock.now () in
       Lwt.async (fun () ->
           let+ (_ : bool) = emit_logs_maybe ~now httpc in

src/client/batch.ml

@@ -0,0 +1,64 @@
+type 'a t = {
+  mutable size: int;
+  mutable q: 'a list list;
+  batch: int; (* Minimum size to batch before popping *)
+  high_watermark: int;
+  timeout: Mtime.span option;
+  mutable start: Mtime.t;
+}
+
+let make ?(batch = 1) ?timeout () : _ t =
+  assert (batch > 0);
+  let high_watermark =
+    if batch = 1 then
+      100
+    else
+      batch * 10
+  in
+  {
+    size = 0;
+    start = Mtime_clock.now ();
+    q = [];
+    batch;
+    timeout;
+    high_watermark;
+  }
+
+let timeout_expired_ ~now self : bool =
+  match self.timeout with
+  | Some t ->
+    let elapsed = Mtime.span now self.start in
+    Mtime.Span.compare elapsed t >= 0
+  | None -> false
+
+(* Big enough to send a batch *)
+let is_full_ self : bool = self.size >= self.batch
+
+let ready_to_pop ~force ~now self =
+  self.size > 0 && (force || is_full_ self || timeout_expired_ ~now self)
+
+let pop_if_ready ?(force = false) ~now (self : _ t) : _ list option =
+  if ready_to_pop ~force ~now self then (
+    let l = self.q in
+    self.q <- [];
+    self.size <- 0;
+    assert (l <> []);
+    let ls = List.fold_left (fun acc l -> List.rev_append l acc) [] l in
+    Some ls
+  ) else
+    None
+
+let push (self : _ t) x : [ `Dropped | `Ok ] =
+  if self.size >= self.high_watermark then
+    (* drop this to prevent queue from growing too fast *)
+    `Dropped
+  else (
+    if self.size = 0 && Option.is_some self.timeout then
+      (* current batch starts now *)
+      self.start <- Mtime_clock.now ();
+
+    (* add to queue *)
+    self.size <- 1 + self.size;
+    self.q <- x :: self.q;
+    `Ok
+  )

src/client/batch.mli

@@ -0,0 +1,38 @@
+(** A thread-safe batch of resources to be popper when ready . *)
+
+type 'a t
+
+val make : ?batch:int -> ?timeout:Mtime.span -> unit -> 'a t
+(** [make ()] is a new batch
+
+    @param batch
+      the number of elements after which the batch will be considered {b full},
+      and ready to pop. A "high water mark" is also derived form the batch as
+      [if batch = 1 then 100 else batch * 10]. This sets a limit after which new
+      elements will be [`Dropped] by {!push}. Set to [0] to disable batching.
+      Default [1].
+
+    @param timeout
+      the time span after which a batch is ready to pop, whether or not it is
+      {b full}. *)
+
+val pop_if_ready : ?force:bool -> now:Mtime.t -> 'a t -> 'a list option
+(** [pop_if_ready ~now b] is [Some xs], where is [xs] includes all the elements
+    {!push}ed since the last batch, if the batch ready to be emitted.
+
+    A batch is ready to pop if it contains some elements and
+
+    - batching is disabled, and any elements have been batched, or batching was
+      enabled and at least [batch] elements have been pushed, or
+    - a [timeout] was provided, and more than a [timeout] span has passed since
+      the last pop was ready, or
+    - the pop is [force]d,
+
+    @param now the current time
+
+    @param force override the other batch conditions *)
+
+val push : 'a t -> 'a list -> [ `Dropped | `Ok ]
+(** [push b xs] is [`Ok] if it succeeds in pushing the values in [xs] into the batch
+    [b], or [`Dropped] if the current size of the batch has exceeded the high water
+    mark determined by the [batch] argument to {!make}]. ) *)

src/client/dune

@@ -1,5 +1,5 @@
 (library
  (name opentelemetry_client)
  (public_name opentelemetry.client)
- (libraries opentelemetry pbrt)
+ (libraries opentelemetry pbrt mtime mtime.clock.os)
  (synopsis "Common types and logic shared between client implementations"))