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datadog-porting-guide.md

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Porting Guide

Purpose

Aid developers who wish to port existing datadog (dd-trace-rb) instrumentation to opentelemetry.

Interface

  • There should be a subclass of OpenTelemetry::Instrumentation::Base that implements its DSL:
    • Implement install block, where all of the integration work happens
    • Implement present block
    • Optionally, implement compatible
    • See OpenTelemetry::Instrumentation::Base API documentation for more details

Basic structure

  • Add Gemfile, opentelemetry-instrumentation-#{name}.gemspec
  • Add runnable (docker) example (using its own Gemfile)
$ docker-compose run ex-instrumentation-myinstrumentation bundle install
$ docker-compose run ex-instrumentation-myinstrumentation
bash-5.0$ ruby trace_demonstration.rb

Examples and template instrumentation

  • instrumentation/faraday and instrumentation/sinatra were the earliest, simplest implementations

Implementation

  • It's ok to use require_relative for files that are internal to the project
  • Don't load integration implementation (require file) until install ('patch')-time
  • Most times, only want to run installation (via #install) once, but need to consider being able to reset somehow for, e.g., testing

otel: tracer

  • Tracer :name and :version should come from instrumentation/base, not from the instrumented library

otel: span

  • Wrap instrumented-library operations in span (via tracer.in_span (block), or tracer.start_span (returns Span))
    • Span.finish should be called if using tracer.start_span
  • Span :with_parent defaults to current_span

otel: span.name (dd-trace-rb: 'resource')

otel: span.attributes (dd-trace-rb: 'tags')

  • Span attribute keys should be strings, not symbols
  • Span attribute values should be strings, not symbols
  • Prefer to populate span attributes via method arguments (e.g., tracer.in_span(attributes: ...) instead of span.set_attribute
  • Some Span attribute naming is based on semantic conventions, for example HTTP semantic conventions
    • When opening a PR, it would be useful to note which attributes come from which semantic conventions, and which ones could not be found
  • Span :kind defaults to :internal (other available options include :client or :server)

Runnable example

  • Gemfile should include "opentelemetry-instrumentation-#{instrumentation_name}"

Runtime performance considerations

Watch for "low hanging fruit" performance improvements including:

  • reduce object allocations - move to a constant, or cache values that would be generated repeatedly
  • look for "easy wins" vs. "complete redesigns"

Testing

  • Add tests via minitest (not rspec)
  • Test against multiple versions of instrumented library via appraisal
    • GOTCHA: appraisal-2.2.0 + bundler-2.1.x (at least 2.1.4)
  • Configure .circleci/config.yml
  • Try to preserve all applicable tests from dd-trace-rb
    • Skip tests datadog-specific configurations
    • Skip tests for things that are not being ported

Context propagation

  • For client-only libraries, just inject
  • For server-side libraries, extract remote span context from request headers, then start a new span using parent context (e.g., tracer.in_span(with_parent_context: ...)
  • Use the plain http_text_format rather than rack_http_text_format

Dependencies

  • Allow instrumentation to function when underlying dependency (e.g., faraday gem) isn't present
    • Don't create a hard-dependency on instrumented libraries
  • opentelemetry-sdk is only needed for tests

Translation hints

  • span.set_tag => span.set_attribute
  • Datadog::Logger.log.debug => OpenTelemetry.logger.debug
  • span.resource => ?? possibly span.attributes[:url], etc., possibly ignored
  • span.service => span.name
  • span.span_type => span.attributes[:component]
  • options[:tracer] => OpenTelemetry.tracer_factory.tracer (then, e.g., Instrumentation.tracer)

Things to ignore (for now)

  • Pin ('patch info') -- probably deprecated in dd-trace-rb
  • Configuration option for :distributed_tracing (should always be enabled)
  • Deprecation paths
  • Passing spans/information around ENV
  • Span#set_error
  • Contrib::Analytics
  • Quantization

TBD (implementation differences)

  • set a tag (attribute) value only once (check if set already)?