This guide will help you instrument your Ruby on Rails application(s) with OpenTelemetry and send traces to Checkly. Although this guide is for Ruby on Rails, the steps are largely the same as instrumenting any Ruby application with OpenTelemetry. This guide assumes you have a basic understanding of Ruby on Rails and already have a working Rails application.

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Step 1: Install the OpenTelemetry packages

Go to the root of your Rails app and add the basic OpenTelemetry SDK, OTLP exporter and instrumentation gems to your Gemfile:
Terminal
bundle add opentelemetry-sdk \
opentelemetry-exporter-otlp \
opentelemetry-instrumentation-all
This should add the following lines:
Gemfile
gem "opentelemetry-sdk", "~> 1.4"
gem "opentelemetry-exporter-otlp", "~> 0.26.3"
gem "opentelemetry-instrumentation-all", "~> 0.60.0"

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Step 2: Initialize the instrumentation

As per the Ruby on Rails convention, we add an instrumentation.rb file to the config/initializers directory.
config/initializers/instrumentation.rb
require 'opentelemetry/sdk'
require 'opentelemetry/instrumentation/all'

class ChecklySampler

    def should_sample?(trace_id:, parent_context:, links:, name:, kind:, attributes:)
      tracestate = OpenTelemetry::Trace.current_span(parent_context).context.tracestate
      decision = tracestate.value('checkly') ? OpenTelemetry::SDK::Trace::Samplers::Decision::RECORD_AND_SAMPLE : 
        OpenTelemetry::SDK::Trace::Samplers::Decision::DROP
      puts(decision)  
      OpenTelemetry::SDK::Trace::Samplers::Result.new(decision: decision, attributes: {}, tracestate: tracestate)
    end
  
    def description
      'ChecklySampler'
    end
  end

OpenTelemetry::SDK.configure do |c|
  c.use_all()
end

OpenTelemetry.tracer_provider.sampler = ChecklySampler.new
Notice the ChecklySampler configuration. This is a custom, head-based sampler that will only sample spans that are generated by Checkly by inspecting the trace state. This way you only pay for the egress traffic generated by Checkly and not for any other traffic. Also note that the use_all() method will automatically install all available instrumentation libraries.

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Step 3: Start your app with the instrumentation

Toggle on Import Traces and grab your OTel API key in the OTel API keys section of the Traces page in the Checkly app and take a note of the endpoint for the region you want to use. Checkly OTEL API keys Now, export your API key in your shell by setting the OTEL_EXPORTER_OTLP_HEADERS environment variable.
Terminal
export OTEL_EXPORTER_OTLP_HEADERS="authorization=<your-api-key>"
Next, export the endpoint for the region you want to use and give your service a name.
Terminal
export OTEL_EXPORTER_OTLP_ENDPOINT="https://otel.eu-west-1.checklyhq.com"
export OTEL_SERVICE_NAME="your-service-name"
Then, explicitly set the protocol to use for the OTLP exporter.
Terminal
export OTEL_EXPORTER_OTLP_PROTOCOL="http/protobuf"
We are using the standard OpenTelemetry environment variables here to configure the OTLP exporter.
VariableDescription
OTEL_EXPORTER_OTLP_HEADERSThe Authorization HTTP header containing your Checkly OTel API key.
OTEL_EXPORTER_OTLP_ENDPOINTThe Checkly OTel API endpoint for the region you want to use.
OTEL_EXPORTER_OTLP_PROTOCOLThe protocol to use for the OTLP exporter.
OTEL_SERVICE_NAMEThe name of your service to identify it among the spans in the web UI.
You can now restart your Rails app with the instrumentation enabled.
Terminal
rails server

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Debugging and troubleshooting

If you run into any issues, you can also output any traces to the console as follows:
Terminal
env OTEL_TRACES_EXPORTER=console rails server
Similarly, you can set the OTEL_LOG_LEVEL environment variable to DEBUG to get more detailed logs.
Terminal
env OTEL_LOG_LEVEL=DEBUG rails server

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Further reading