Load Generator

The load generator is based on k6, a Go load testing tool that runs test scenarios written in JavaScript. By default it will simulate users requesting several different routes from the frontend. All of its OpenTelemetry instrumentation comes from the Go SDK, embedded in the k6 binary by the xk6-otel extension described below.

Load generator source

Traces

Initializing Tracing

Tracing in the load generator is provided by a custom xk6 extension (xk6-otel) that wraps the OpenTelemetry Go SDK and exposes it to k6 JavaScript scripts. The extension is compiled into the k6 binary at image build time.

The extension initializes a TracerProvider backed by an OTLP HTTP exporter on first use. The collector endpoint, protocol, resource attributes, and service name are read from the standard OpenTelemetry environment variables (OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_PROTOCOL, OTEL_RESOURCE_ATTRIBUTES, and OTEL_SERVICE_NAME).

Creating Spans

Scripts import the Tracer class from the k6/x/otel module and create spans manually around each simulated user action:

import { Tracer } from 'k6/x/otel';

const tracer = new Tracer();

function browseProduct() {
  const span = tracer.startSpan('user_browse_product', {
    'product.id': product,
  });
  http.get(`${BASE_URL}/api/products/${product}`, {
    headers: otelHeaders(span.traceParent()),
  });
  span.end();
}

The startSpan(name, attrs?) method starts a new client span and returns an object with three methods:

• traceParent() — returns the W3C traceparent header value for the span, used to propagate trace context to backend services.
• log(message) — emits a correlated OTel log record tied to the span’s trace and span IDs.
• end() — ends the span and flushes it to the exporter.

Metrics

The load generator emits two kinds of metrics:

• k6 built-in test metrics (request duration, error rate, throughput, etc.) are exported to the OpenTelemetry Collector via k6’s built-in opentelemetry output (--out opentelemetry). The output protocol and collector endpoint are configured via the K6_OTEL_EXPORTER_PROTOCOL and K6_OTEL_HTTP_EXPORTER_ENDPOINT environment variables.
• Go runtime metrics (memory, garbage collection, goroutines) are emitted by the xk6-otel extension using the OpenTelemetry runtime instrumentation.

Logs

Log records are emitted by calling span.log(message) on any active span. The xk6-otel extension injects the span’s trace and span IDs into each log record.

Baggage

OpenTelemetry Baggage is used by the load generator to indicate that traces are synthetically generated. Each outgoing HTTP request carries a baggage header and a traceparent header constructed by the otelHeaders helper:

function otelHeaders(traceParent, extra) {
  return Object.assign(
    {
      baggage: `synthetic_request=true,session.id=${sessionId}`,
      traceparent: traceParent,
    },
    extra,
  );
}

Baggage on its own doesn’t mark the telemetry. Each backend service reads the synthetic_request entry out of the incoming baggage and copies it onto its own spans and log records as an attribute, and it is that attribute which records whether the telemetry came from a synthetic flow. The frontend sets demo.synthetic_request, while the checkout and payment services set user_agent.synthetic.type to test. Because the marker ends up on the telemetry itself, you can filter load-generator traffic in or out of any query in your observability backend.