General purpose OCaml library (development kit) Copyright (c) 2009 Ahrefs Released under the terms of LGPL-2.1 with OCaml linking exception.

opam install devkit

Devkit's Web module includes both simple, configurable, local tracing support, as well as distributed tracing support via OpenTelemetry.

To use local tracing, we use the mechanism provided by ocaml-trace: by default, the tracing calls in devkit are a no-op; but if the top-level application containing devkit installs and configures a "tracing backend" at runtime, then devkit's web-requests will each produce timed traces.

As an example, using trace-tef as a backend to produce a json file:

open Devkit

let run () =
  let resp = Web.http_request `GET "http://icanhazip.com" in
  (* ... *)

let () =
  Trace_tef.with_setup ~out:(`File "trace.json") () @@ fun () ->
  run ()

For distributed traces, you'll both need to configure an OpenTelemetry backend for ocaml-trace at runtime (to collect devkit's own traces); and you will most likely also want to ensure your own application's traces (using either ocaml-trace or the full ocaml-opentelemetry) are properly configured to correctly appear as the parent of devkit's traces.

Configuring an OpenTelemetry backend for the traces themselves is as simple wrapping your top-level application in a call to Opentelemetry_trace.setup_with_otel_backend or the like. That will configure both OpenTelemetry's collector and the ocaml-trace backend.

Then, to ensure that parentage is correctly propagated across your distributed architecture, devkit can produce a W3C Trace Context traceparent HTTP header. To ensure that it produces the correct traceparent, however, we depend upon 'ambient context'. For this to function properly, you'll need to follow the detailed instructions in the ocaml-ambient-context documentation.

Once thus configured, devkit will check Opentelemetry.Scope.get_ambient_scope before each HTTP request, and use the ambient tracing-span as the parent of the web-request's span; which will itself then be propagated to the remote service via the traceparent HTTP header.

As an example, if your top-level application is using Lwt, and using cohttp as the OpenTelemetry backend:

open Devkit

let run () =
  let* resp = Web.http_request_lwt `GET "http://icanhazip.com" in
  (* ... *)

let () =
  Ambient_context.set_storage_provider (Ambient_context_lwt.storage ()) ;
  Opentelemetry_trace.setup_with_otel_backend
    (Opentelemetry_client_cohttp_lwt.create_backend ())
  @@ fun () ->
  Lwt_main.run @@ fun () ->
  run ()

Install OCaml dependencies in your current / global switch:

opam install . --deps-only

Or to install them in a new, directory-local switch:

opam switch create . --deps-only --no-install
opam install . --deps-only --with-test

External dependencies:

opam list -s -e --resolve=devkit

To update ragel-generated code:

aptitude install ragel
make -B gen_ragel

To update metaocaml-generated code:

opam exec --switch=4.07.1+BER -- make gen_metaocaml