docs/source/cpp/opentelemetry.rst

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.. _cpp_opentelemetry:

=============
OpenTelemetry
=============

Portions of Arrow C++ are instrumented with the `OpenTelemetry
<https://opentelemetry.io/>`_ C++ SDK which makes it possible to generate
detailed tracing information which can be analyzed in other tools.

Creating a Build with OpenTelemetry Tracing Enabled
---------------------------------------------------

OpenTelemetry tracing is not turned on by default so you must first create a
custom build of Arrow C++ with tracing turned on. See :ref:`Building Arrow C++
<building-arrow-cpp>` for general instructions on creating a custom build.

To enable tracing, specify ``ARROW_WITH_OPENTELEMETRY=ON`` when generating your
build. You may also want to specify ``CMAKE_BUILD_TYPE=RelWithDebInfo`` in order
to get representative timings while retaining debug information.

Exporting Tracing Information
-----------------------------

By default, no tracing information is exported until a tracing backend has been
specified. This is typically done by the executable using Arrow by installing a
``TracerProvider`` through the OpenTelemetry SDK API. Please refer to the
`OpenTelemetry SDK API C++ documentation
<https://opentelemetry-cpp.readthedocs.io/en/latest/sdk/sdk.html>`_.

In the rare cases you want Arrow to control the global resources of the
OpenTelemetry SDK, a tracing backend can be specified through the
:envvar:`ARROW_TRACING_BACKEND` environment variable. This overrides any
``TracerProvider`` previously set.

Possible values for :envvar:`ARROW_TRACING_BACKEND` are:
   - ``ostream``: emit textual log messages to stdout
   - ``otlp_http``: emit OTLP JSON encoded traces to a HTTP server (by default,
     the endpoint URL is "http://localhost:4318/v1/traces")
   - ``arrow_otlp_stdout``: emit JSON traces to stdout
   - ``arrow_otlp_stderr``: emit JSON traces to stderr

For example, to enable exporting JSON traces to stdout, set::

   export ARROW_TRACING_BACKEND=arrow_otlp_stdout

At this point, running the program you've linked to your custom build of
Arrow C++ should produce JSON traces on stdout.

Visualizing Traces with Jaeger UI
---------------------------------

Analyzing trace information exported to stdout/stderr may involve writing custom
processing scripts. As an alternative -- or even a complement -- to this
process, the "all-in-one" `Jaeger <https://jaegertracing.io>`_ `Docker
<https://www.docker.com/>`_ image is a relatively straightforward way of
visualizing trace data and is suitable for local development and testing.

Note: This assumes you have `Docker <https://www.docker.com/>`_ installed.

First, change your tracing backend to ``otlp_http``::

   export ARROW_TRACING_BACKEND=otlp_http

Then start the Jaeger all-in-one container::

   docker run \
     -e COLLECTOR_OTLP_ENABLED=true \
     -p 16686:16686 \
     -p 4317:4317 \
     -p 4318:4318 \
     jaegertracing/all-in-one:1.35

Now you should be able to run your program and view any traces in a web browser
at http://localhost:16686. Note that unlike with other methods of exporting
traces, no output will be made to stdout/stderr. However, if you tail your
Docker container logs, you should see output when traces are received by the
all-in-one container.