remove trace_id entirely, use meta-maps in more places

The direct use of span_id+trace_id in OTEL is a bad idea, and it makes
little sense in other collectors. In the end, `span` should be thought
of as an opaque key only the collector understands, and a meta-map
should be carried around for the collector to stash additional info such
as a (8B, 16B, ...) trace ID or other information.

src/core/collector.ml

@@ -7,13 +7,11 @@
 open Types
 
 let dummy_span : span = Int64.min_int
-let dummy_trace_id : trace_id = "<dummy>"
 
 let dummy_explicit_span : explicit_span =
-  { span = dummy_span; trace_id = dummy_trace_id; meta = Meta_map.empty }
+  { span = dummy_span; meta = Meta_map.empty }
 
-let dummy_explicit_span_ctx : explicit_span_ctx =
-  { span = dummy_span; trace_id = dummy_trace_id }
+let dummy_explicit_span_ctx : explicit_span_ctx = { meta = Meta_map.empty }
 
 (** Signature for a collector.
 

src/core/trace_core.ml

@@ -18,7 +18,7 @@ let current_level_ = A.make Level.Trace
 (* ## implementation ## *)
 
 let[@inline] ctx_of_span (sp : explicit_span) : explicit_span_ctx =
-  { span = sp.span; trace_id = sp.trace_id }
+  { meta = sp.meta }
 
 let data_empty_build_ () = []
 

src/core/types.ml

@@ -1,13 +1,13 @@
+(** Common types definitions.
+
+    The type [trace_id] was added in 0.10 and removed in NEXT_RELEASE, as it's
+    simply more flexible to use a meta-map ([Hmap.t] in the better case). *)
+
 type span = int64
 (** A span identifier.
 
     The meaning of the identifier depends on the collector. *)
 
-type trace_id = string
-(** A bytestring representing a (possibly distributed) trace made of async
-    spans. With opentelemetry this is 16 bytes.
-    @since 0.10 *)
-
 type user_data =
   [ `Int of int
   | `String of string
@@ -26,17 +26,24 @@ type span_flavor =
     @since NEXT_RELEASE *)
 
 type explicit_span_ctx = {
-  span: span;  (** The current span *)
-  trace_id: trace_id;  (** The trace this belongs to *)
+  meta: Meta_map.t;
+      (** Metadata for this span and its context. This can be used to store
+          trace IDs, attributes, etc. in a collector-specific way. *)
 }
-(** A context, passed around for async traces.
-    @since 0.10 *)
+[@@unboxed]
+(** A context, passed around for async traces. It might not correspond to any
+    span created via [Trace] at all, e.g. it might be built in a HTTP handler
+    from a {{:https://www.w3.org/TR/trace-context/} W3C trace contxt} header.
+
+    @since 0.10
+
+    The fields [span] and [trace_id] were removed in NEXT_RELEASE. Please use
+    the meta map instead. *)
 
 type explicit_span = {
   span: span;
       (** Identifier for this span. Several explicit spans might share the same
           identifier since we can differentiate between them via [meta]. *)
-  trace_id: trace_id;  (** The trace this belongs to *)
   mutable meta: Meta_map.t;
       (** Metadata for this span (and its context). This can be used by
           collectors to carry collector-specific information from the beginning
@@ -44,7 +51,10 @@ type explicit_span = {
 }
 (** Explicit span, with collector-specific metadata. This is richer than
     {!explicit_span_ctx} but not intended to be passed around (or sent across
-    the wire), unlike {!explicit_span_ctx}. *)
+    the wire), unlike {!explicit_span_ctx}.
+
+    The field [trace_id] was removed in NEXT_RELEASE, please use the meta map
+    instead. *)
 
 type extension_event = ..
 (** An extension event, used to add features that are backend specific or simply

src/event/event.ml

@@ -37,7 +37,6 @@ type t =
       tid: int;
       name: string;
       time_ns: int64;
-      id: trace_id;
       flavor: span_flavor option;
       fun_name: string option;
       data: (string * user_data) list;
@@ -48,7 +47,6 @@ type t =
       time_ns: int64;
       flavor: span_flavor option;
       data: (string * user_data) list;
-      id: trace_id;
     }
   | E_counter of {
       name: string;

src/event/subscriber.ml

@@ -10,14 +10,17 @@ open struct
   (* just use the same ones for everyone *)
 
   let span_gen = Sub.Span_generator.create ()
-  let trace_id_gen = Sub.Trace_id_8B_generator.create ()
 end
 
 module Callbacks : Sub.Callbacks.S with type st = event_consumer = struct
   type st = event_consumer
 
   let new_span (_self : st) = Sub.Span_generator.mk_span span_gen
-  let new_trace_id _self = Sub.Trace_id_8B_generator.mk_trace_id trace_id_gen
+
+  let new_explicit_span _self ~parent:_ =
+    let span = Sub.Span_generator.mk_span span_gen in
+    { span; meta = Meta_map.empty }
+
   let on_init (self : st) ~time_ns = self.on_event (E_init { time_ns })
   let on_shutdown (self : st) ~time_ns = self.on_event (E_shutdown { time_ns })
 
@@ -45,16 +48,13 @@ module Callbacks : Sub.Callbacks.S with type st = event_consumer = struct
     self.on_event @@ E_counter { name; n = f; time_ns; tid }
 
   let on_enter_manual_span (self : st) ~__FUNCTION__:fun_name ~__FILE__:_
-      ~__LINE__:_ ~time_ns ~tid ~parent:_ ~data ~name ~flavor ~trace_id _span :
-      unit =
+      ~__LINE__:_ ~time_ns ~tid ~parent:_ ~data ~name ~flavor _span : unit =
     self.on_event
-    @@ E_enter_manual_span
-         { id = trace_id; time_ns; tid; data; name; fun_name; flavor }
+    @@ E_enter_manual_span { time_ns; tid; data; name; fun_name; flavor }
 
   let on_exit_manual_span (self : st) ~time_ns ~tid ~name ~data ~flavor
-      ~trace_id (_ : span) : unit =
-    self.on_event
-    @@ E_exit_manual_span { tid; id = trace_id; name; time_ns; data; flavor }
+      (_ : explicit_span) : unit =
+    self.on_event @@ E_exit_manual_span { tid; name; time_ns; data; flavor }
 
   let on_extension_event (self : st) ~time_ns ~tid ext : unit =
     self.on_event @@ E_extension_event { tid; time_ns; ext }

src/fuchsia/subscriber.ml

@@ -22,7 +22,8 @@ type t = {
   buf_chain: Buf_chain.t;
   exporter: Exporter.t;
   span_gen: Sub.Span_generator.t;
-  trace_id_gen: Sub.Trace_id_8B_generator.t;
+  (* use the span generator, it's also 64 bits *)
+  trace_id_gen: Sub.Span_generator.t;
 }
 (** Subscriber state *)
 
@@ -49,6 +50,8 @@ open struct
         names;
       flush stderr
     )
+
+  let k_trace_id : int64 Meta_map.key = Meta_map.Key.create ()
 end
 
 let close (self : t) : unit =
@@ -76,7 +79,8 @@ let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
     pid;
     spans = Span_tbl.create ();
     span_gen = Sub.Span_generator.create ();
-    trace_id_gen = Sub.Trace_id_8B_generator.create ();
+    (* NOTE: we use a span generator because it's also making [int64] values *)
+    trace_id_gen = Sub.Span_generator.create ();
   }
 
 module Callbacks = struct
@@ -84,8 +88,21 @@ module Callbacks = struct
 
   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 get_trace_id_ meta : int64 =
+    try Meta_map.find_exn k_trace_id meta
+    with _ ->
+      failwith "fuchsia subscriber: could not find trace_id in meta-map"
+
+  let new_explicit_span (self : st) ~(parent : explicit_span_ctx option) :
+      explicit_span =
+    let span = Sub.Span_generator.mk_span self.span_gen in
+    let trace_id =
+      match parent with
+      | None -> Sub.Span_generator.mk_span self.trace_id_gen
+      | Some p -> get_trace_id_ p.meta
+    in
+    let meta = Meta_map.(empty |> add k_trace_id trace_id) in
+    { span; meta }
 
   let on_init (self : st) ~time_ns:_ =
     Writer.Metadata.Magic_record.encode self.buf_chain;
@@ -164,7 +181,9 @@ 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 =
+      ~time_ns ~tid ~parent:_ ~data ~name ~flavor:_ (span : explicit_span) :
+      unit =
+    let trace_id = get_trace_id_ span.meta in
     Writer.(
       Event.Async_begin.encode self.buf_chain ~name
         ~args:(args_of_user_data data)
@@ -173,7 +192,8 @@ module Callbacks = struct
     write_ready_ self
 
   let on_exit_manual_span (self : st) ~time_ns ~tid ~name ~data ~flavor:_
-      ~trace_id (_ : span) : unit =
+      (span : explicit_span) : unit =
+    let trace_id = get_trace_id_ span.meta in
     Writer.(
       Event.Async_end.encode self.buf_chain ~name ~args:(args_of_user_data data)
         ~t_ref:(Thread_ref.inline ~pid:self.pid ~tid)

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

src/subscriber/callbacks.ml

@@ -35,8 +35,9 @@ module type S = sig
   (** How to generate a new span?
       @since NEXT_RELEASE *)
 
-  val new_trace_id : st -> trace_id
-  (** How to generate a new trace ID?
+  val new_explicit_span : st -> parent:explicit_span_ctx option -> explicit_span
+  (** How to generate a new explicit span, with meta-map potentially containing
+      data such as a trace ID or request ID?
       @since NEXT_RELEASE *)
 
   val on_shutdown : st -> time_ns:int64 -> unit
@@ -96,12 +97,11 @@ module type S = sig
     __LINE__:int ->
     time_ns:int64 ->
     tid:int ->
-    parent:span option ->
+    parent:explicit_span_ctx option ->
     data:(string * Trace_core.user_data) list ->
     name:string ->
     flavor:Trace_core.span_flavor option ->
-    trace_id:trace_id ->
-    span ->
+    explicit_span ->
     unit
   (** Enter a manual (possibly async) span *)
 
@@ -112,8 +112,7 @@ module type S = sig
     name:string ->
     data:(string * Trace_core.user_data) list ->
     flavor:Trace_core.span_flavor option ->
-    trace_id:trace_id ->
-    span ->
+    explicit_span ->
     unit
   (** Exit a manual span *)
 
@@ -142,7 +141,7 @@ type 'st t = (module S with type st = 'st)
 module Dummy = struct
   let on_init _ ~time_ns:_ = ()
   let new_span _ = Collector.dummy_span
-  let new_trace_id _ = Collector.dummy_trace_id
+  let new_explicit_span _ ~parent:_ = Collector.dummy_explicit_span
   let on_shutdown _ ~time_ns:_ = ()
   let on_name_thread _ ~time_ns:_ ~tid:_ ~name:_ = ()
   let on_name_process _ ~time_ns:_ ~tid:_ ~name:_ = ()
@@ -157,13 +156,10 @@ module Dummy = struct
   let on_add_data _ ~data:_ _sp = ()
 
   let on_enter_manual_span _ ~__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_ ~time_ns:_
-      ~tid:_ ~parent:_ ~data:_ ~name:_ ~flavor:_ ~trace_id:_ _sp =
-    ()
-
-  let on_exit_manual_span _ ~time_ns:_ ~tid:_ ~name:_ ~data:_ ~flavor:_
-      ~trace_id:_ _ =
+      ~tid:_ ~parent:_ ~data:_ ~name:_ ~flavor:_ _sp =
     ()
 
+  let on_exit_manual_span _ ~time_ns:_ ~tid:_ ~name:_ ~data:_ ~flavor:_ _ = ()
   let on_extension_event _ ~time_ns:_ ~tid:_ _ = ()
 end
 

src/subscriber/subscriber.ml

@@ -24,9 +24,9 @@ open struct
       let (Sub { st = s; callbacks = (module CB) }) = Array.get st 0 in
       CB.new_span s
 
-    let new_trace_id st =
+    let new_explicit_span st =
       let (Sub { st = s; callbacks = (module CB) }) = Array.get st 0 in
-      CB.new_trace_id s
+      CB.new_explicit_span s
 
     let on_init st ~time_ns =
       for i = 0 to Array.length st - 1 do
@@ -85,19 +85,17 @@ open struct
       done
 
     let on_enter_manual_span st ~__FUNCTION__ ~__FILE__ ~__LINE__ ~time_ns ~tid
-        ~parent ~data ~name ~flavor ~trace_id span =
+        ~parent ~data ~name ~flavor span =
       for i = 0 to Array.length st - 1 do
         let (Sub { st = s; callbacks = (module CB) }) = Array.get st i in
         CB.on_enter_manual_span s ~__FUNCTION__ ~__FILE__ ~__LINE__ ~time_ns
-          ~tid ~parent ~data ~name ~flavor ~trace_id span
+          ~tid ~parent ~data ~name ~flavor span
       done
 
-    let on_exit_manual_span st ~time_ns ~tid ~name ~data ~flavor ~trace_id span
-        =
+    let on_exit_manual_span st ~time_ns ~tid ~name ~data ~flavor span =
       for i = 0 to Array.length st - 1 do
         let (Sub { st = s; callbacks = (module CB) }) = Array.get st i in
-        CB.on_exit_manual_span s ~time_ns ~tid ~name ~data ~flavor ~trace_id
-          span
+        CB.on_exit_manual_span s ~time_ns ~tid ~name ~data ~flavor span
       done
 
     let on_extension_event st ~time_ns ~tid ev : unit =

src/subscriber/trace_subscriber.ml

@@ -71,36 +71,27 @@ let collector (Sub { st; callbacks = (module CB) } : Subscriber.t) : collector =
 
     let enter_manual_span ~(parent : explicit_span_ctx option) ~flavor
         ~__FUNCTION__ ~__FILE__ ~__LINE__ ~data name : explicit_span =
-      let span = CB.new_span st in
       let tid = tid_ () in
       let time_ns = now_ns () in
 
-      (* get the common trace id, or make a new one *)
-      let trace_id, parent =
-        match parent with
-        | Some m -> m.trace_id, Some m.span
-        | None -> CB.new_trace_id st, None
-      in
+      let espan : explicit_span = CB.new_explicit_span st ~parent in
+      espan.meta <-
+        Meta_map.add key_manual_info { name; flavor; data = [] } espan.meta;
 
       CB.on_enter_manual_span st ~__FUNCTION__ ~__FILE__ ~__LINE__ ~parent ~data
-        ~time_ns ~tid ~name ~flavor ~trace_id span;
-      let meta =
-        Meta_map.empty
-        |> Meta_map.add key_manual_info { name; flavor; data = [] }
-      in
-      { span; trace_id; meta }
+        ~time_ns ~tid ~name ~flavor espan;
+      espan
 
     let exit_manual_span (es : explicit_span) : unit =
       let time_ns = now_ns () in
       let tid = tid_ () in
-      let trace_id = es.trace_id in
       let minfo =
         match Meta_map.find key_manual_info es.meta with
         | None -> assert false
         | Some m -> m
       in
       CB.on_exit_manual_span st ~tid ~time_ns ~data:minfo.data ~name:minfo.name
-        ~flavor:minfo.flavor ~trace_id es.span
+        ~flavor:minfo.flavor es
 
     let add_data_to_manual_span (es : explicit_span) data =
       if data <> [] then (
@@ -154,15 +145,3 @@ module Span_generator = struct
   let create () = A.make 0
   let[@inline] mk_span self = A.fetch_and_add self 1 |> Int64.of_int
 end
-
-module Trace_id_8B_generator = struct
-  type t = int A.t
-
-  let create () = A.make 0
-
-  let[@inline] mk_trace_id (self : t) : trace_id =
-    let n = A.fetch_and_add self 1 in
-    let b = Bytes.create 8 in
-    Bytes.set_int64_le b 0 (Int64.of_int n);
-    Bytes.unsafe_to_string b
-end

src/subscriber/trace_subscriber.mli

@@ -37,15 +37,6 @@ module Span_generator : sig
   val mk_span : t -> Trace_core.span
 end
 
-(** A counter-based trace ID generator, producing 8-byte trace IDs.
-    @since NEXT_RELEASE *)
-module Trace_id_8B_generator : sig
-  type t
-
-  val create : unit -> t
-  val mk_trace_id : t -> Trace_core.trace_id
-end
-
 (**/**)
 
 module Private_ : sig
@@ -53,7 +44,7 @@ module Private_ : sig
   (** Global mock flag. If enable, all timestamps, tid, etc should be faked. *)
 
   val get_now_ns_ : (unit -> int64) ref
-  (** The callback used to get the current timestamp *)
+  (** The callback used to get the current timestamp, in nanoseconds *)
 
   val get_tid_ : (unit -> int) ref
   (** The callback used to get the current thread's id *)

src/tef/subscriber.ml

@@ -16,11 +16,11 @@ open struct
   let[@inline] time_us_of_time_ns (t : int64) : float =
     Int64.div t 1_000L |> Int64.to_float
 
-  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
+  (** Used to store async spans' trace ID *)
+  let k_trace_id : int64 Meta_map.key = Meta_map.Key.create ()
+
+  (* 8B in both cases *)
+  module Trace_id_8B_generator = Sub.Span_generator
 end
 
 let on_tracing_error = ref (fun s -> Printf.eprintf "%s\n%!" s)
@@ -43,7 +43,7 @@ type t = {
   buf_pool: Buf_pool.t;
   exporter: Exporter.t;
   span_gen: Sub.Span_generator.t;
-  trace_id_gen: Sub.Trace_id_8B_generator.t;
+  trace_id_gen: Trace_id_8B_generator.t;
 }
 (** Subscriber state *)
 
@@ -85,7 +85,7 @@ let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
     pid;
     spans = Span_tbl.create ();
     span_gen = Sub.Span_generator.create ();
-    trace_id_gen = Sub.Trace_id_8B_generator.create ();
+    trace_id_gen = Trace_id_8B_generator.create ();
   }
 
 module Callbacks = struct
@@ -93,8 +93,19 @@ module Callbacks = struct
 
   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 new_explicit_span self ~(parent : explicit_span_ctx option) :
+      explicit_span =
+    let trace_id =
+      match parent with
+      | None -> Trace_id_8B_generator.mk_span self.trace_id_gen
+      | Some p ->
+        (match Meta_map.find k_trace_id p.meta with
+        | Some t -> t
+        | None -> Trace_id_8B_generator.mk_span self.trace_id_gen)
+    in
+    let span = new_span self in
+    let meta = Meta_map.(empty |> add k_trace_id trace_id) in
+    { span; meta }
 
   let on_init _ ~time_ns:_ = ()
   let on_shutdown (self : st) ~time_ns:_ = close self
@@ -162,25 +173,30 @@ module Callbacks = struct
     self.exporter.on_json buf
 
   let on_enter_manual_span (self : st) ~__FUNCTION__:fun_name ~__FILE__:_
-      ~__LINE__:_ ~time_ns ~tid ~parent:_ ~data ~name ~flavor ~trace_id _span :
-      unit =
+      ~__LINE__:_ ~time_ns ~tid ~parent:_ ~data ~name ~flavor
+      (span : explicit_span) : unit =
     let time_us = time_us_of_time_ns @@ time_ns in
 
+    let trace_id =
+      try Meta_map.find_exn k_trace_id span.meta with _ -> assert false
+    in
+
     let data = add_fun_name_ fun_name data in
     let@ buf = Rpool.with_ self.buf_pool in
-    Writer.emit_manual_begin buf ~pid:self.pid ~tid ~name
-      ~id:(int64_of_trace_id_ trace_id)
+    Writer.emit_manual_begin buf ~pid:self.pid ~tid ~name ~id:trace_id
       ~ts:time_us ~args:data ~flavor;
     self.exporter.on_json buf
 
   let on_exit_manual_span (self : st) ~time_ns ~tid ~name ~data ~flavor
-      ~trace_id (_ : span) : unit =
+      (span : explicit_span) : unit =
     let time_us = time_us_of_time_ns @@ time_ns in
+    let trace_id =
+      try Meta_map.find_exn k_trace_id span.meta with _ -> assert false
+    in
 
     let@ buf = Rpool.with_ self.buf_pool in
-    Writer.emit_manual_end buf ~pid:self.pid ~tid ~name
-      ~id:(int64_of_trace_id_ trace_id)
-      ~ts:time_us ~flavor ~args:data;
+    Writer.emit_manual_end buf ~pid:self.pid ~tid ~name ~id:trace_id ~ts:time_us
+      ~flavor ~args:data;
     self.exporter.on_json buf
 
   let on_extension_event _ ~time_ns:_ ~tid:_ _ev = ()