Written by Michael Nikitochkin
on June 13, 2022

How to begin with Traces in Crystal

TL;DR: Primitve two code examples to start working with Traces

Kirk Haines³ recently started working on making opentelemetry available for Crystal applications¹. I collected the primitive examples that help understand distributed traces.

First spans to stdout

OpenTelemetry supports a different kinds of exporters (in order to visualize and analyze your traces and metrics, you will need to export them to a backend). The basic and default one is to print traces directly to STDOUT. I find it very useful — exporter prints all details of collected information.

# stdout_exporter_sample.cr
require "opentelemetry-api"

STDOUT.sync = true

OpenTelemetry.configure do |config|
  config.service_name = "stdout_exporter_sample"
  config.service_version = "0.1.0"
  config.sampler = OpenTelemetry::Sampler::AlwaysOn.new
  config.exporter = OpenTelemetry::Exporter.new(variant: :stdout)
end
tracer = OpenTelemetry.tracer_provider.tracer

tracer.in_span("first_operation") do |root_span|
  root_span.consumer!
  root_span.set_attribute("foo", "BAR")
  root_span.set_attribute("url", "http://example.com/foo")

  root_span.add_event("dispatching logs")

  tracer.in_span("inner_operation") do |child_span|
    child_span.add_event("handling request")
    tracer.in_span("get_user_from_db") do |child_span|
      child_span.producer!
      child_span.add_event("querying database")
    end
  end

  tracer.in_span("process_second_stage") do |child_span|
    child_span.add_event("checked permissions")
    tracer.in_span("write_to_storage") do |child_span|
      child_span.producer!
      child_span.add_event("insert user")
    end
  end
end

# Make sure all exporter stdout finish in exporter fiber
sleep(1)

Execute the script and check output

$ crystal run stdout_exporter_sample.cr
{
 “type”:”trace”,
 “traceId”:”1479beb8000eda2fcff1fe91c777feb6",
…

First spans via OTLP

Of course, STDOUT is good only in local or test environments. The common protocol for OpenTelemetry is OTLP. For testing purposes I use Jaeger², but you can pick any other solutions: local or remote services. Jaeger² supports the OTLP protocol (besides its own solution). It simplifies our local environment to run trace collector and UI in a single process. Run Jaeger in the background with docker:

$ docker run -d — name jaeger \
 -e COLLECTOR_OTLP_ENABLED=true \
 -p 16686:16686 \
 -p 4317:4317 \
 -p 4318:4318 \
 jaegertracing/all-in-one:1.35

You can access UI with http://localhost:16686 . Modify the previous example a bit:

# otlp_http_exporter_sample.cr
require "opentelemetry-api"

# Another way to configure OTLP exporter in case different hostname
ENV["OTEL_EXPORTER_OTLP_ENDPOINT"] ||= "http://localhost:4318/v1/traces"

OpenTelemetry.configure do |config|
  config.service_name = "otlp_http_sample"
  config.service_version = "0.1.1"
  config.sampler = OpenTelemetry::Sampler::AlwaysOn.new
  config.exporter = OpenTelemetry::Exporter.new(variant: "http") do |c|
    # NOTICE: It allows to flush spans faster and not to wait for 100 spans or 5 seconds
    cc = c.as(OpenTelemetry::Exporter::Http)
    cc.batch_threshold = 5
    cc.batch_latency = 1
  end
end
tracer = OpenTelemetry.tracer_provider.tracer
puts "Check trace: http://localhost:16686/trace/#{tracer.trace_id.hexstring}"

tracer.in_span("first_operation") do |root_span|
  root_span.consumer!
  root_span.set_attribute("foo", "BAR")
  root_span.set_attribute("url", "http://example.com/foo")

  root_span.add_event("dispatching logs")

  tracer.in_span("inner_operation") do |child_span|
    child_span.add_event("handling request")
    tracer.in_span("get_user_from_db") do |child_span|
      child_span.producer!
      child_span.add_event("querying database")
    end
  end

  tracer.in_span("process_second_stage") do |child_span|
    child_span.add_event("checked permissions")
    tracer.in_span("write_to_storage") do |child_span|
      child_span.producer!
      child_span.add_event("insert user")
    end
  end
end

sleep(2)

Open Jaeger’s Web UI on http://localhost:16686/ and check traces. The single trace view would looks like:

Jeager: Trace view

That’s all Folks

Michael Nikitochkin is a Lead Software Engineer. Follow him on LinkedIn or GitHub.

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References

https://github.com/wyhaines/opentelemetry-api.cr/

https://www.jaegertracing.io/

https://github.com/wyhaines

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