Datadog and otel_ngx_module

Datadog wants to integrate more closely with otel_ngx_module, if for no other reason than ingress-nginx plans to drop support for nginx-opentracing in favor of otel_ngx_module.

This has been discussed previously in an ingress-nginx issue and in an opentelemetry-cpp issue.

The Datadog Agent (Datadog's equivalent to an OpenTelemetry collector) speaks OTLP, and so today Datadog customers can use otel_ngx_module as is.

However, Datadog tracing libraries support some features that are not currently expressible in the OpenTelemetry API. The most prominent example in the case of C++ is how sampling is configured.

Instead of restricting Datadog to the collector, we are considering (with the blessing of opentelemetry-cpp and ingress-nginx) writing a library that implements the opentelemetry-cpp API by defining a TracerProvider and related classes. Then, OpenTelemetry projects that wish to provide full-featured Datadog tracing can depend on this library and install its TracerProvider at runtime. Any code written in terms of the opentelemetry-cpp API would remain unchanged.

Let's call this hypothetical library "dd-opentelemetry-cpp".

I've opened up this issue to discuss with the maintainers of otel_ngx_module whether it would be acceptable to propose changes that would integrate dd-opentelemetry-cpp into otel_ngx_module.

At a glance, the configuration and runtime setup of the nginx module would have to be revised to allow for a "provider" degree of freedom, and whatever modifications to existing configuration that would require. The build would also be modified to include dd-opentelemetry-cpp.

dd-opentelemetry-cpp would be a C++17 library that depends only on opentelemetry-cpp, dd-trace-cpp, and optionally libcurl.

Finally, as background information, here is a survey of relevant Datadog libraries:

• dd-opentracing-cpp is an OpenTracing-based plugin that we seek to deprecate. It is what is currently used by ingress-nginx in conjunction with nginx-opentracing.
• dd-trace-cpp is a new dedicated Datadog tracing library that I wrote to replace dd-opentracing-cpp. It's a C++17 library with no dependencies aside from, optionally, libcurl.
• nginx-datadog is a Datadog-specific nginx module, based largely on nginx-opentracing, that uses dd-trace-cpp under the hood and allows for minimal-config tracing of nginx using Datadog.

What do you think about the idea of integrating the hypothetical dd-opentelemetry-cpp into otel_ngx_module?

The issue names no repository files or tests. Start by reviewing otel_ngx_module's configuration and runtime setup, then inspect its build definitions alongside the proposed dd-opentelemetry-cpp dependencies and TracerProvider integration. Done would require maintainer agreement on the design before implementation; no concrete acceptance criteria are stated.