The OpenTelemetry Collector receives telemetry, processes it, and forwards it onward. Telemetry just means traces, metrics, and logs, the signals your software emits about what it's doing. Today, a fake client called telemetrygen sends traces to a Collector running locally, and we'll follow them through.

You need three things. Docker runs the Collector, so we pull image 0.164.0. Go installs telemetrygen, a fake client that generates telemetry. And GOBIN, set so installed Go binaries land on your PATH and you can run telemetrygen.

This docker run command starts the Collector. Each dash p flag publishes a container port on 127.0.0.1, so only your own machine can reach it. The last part pipes the output through tee, which shows the logs in your terminal and also saves them to collector-output.txt, so we can check them later.

The Collector has three separate doors. Port 4317 takes OTLP over gRPC, the default for most SDKs. Port 4318 takes OTLP over HTTP, for clients that don't support gRPC. Port 55679 is different: it's ZPages, a built-in debug page you open in a browser to look inside the Collector.

OTLP is the OpenTelemetry protocol, the shared language clients use to send telemetry. The data stays the same over gRPC or HTTP; only the transport differs.

In a second terminal, we run telemetrygen to act as the client. The traces subcommand with --traces 3 generates three traces. The --otlp-insecure flag skips TLS, which is fine for local use. The log line says traces generated, three. By default, telemetrygen sends them to localhost on port 4317.

Follow the three traces. telemetrygen sends them to the Receiver, which accepts OTLP over gRPC on port 4317. The Receiver passes the spans to the Processor, and the Processor forwards them to the Exporter. In this demo, the Exporter doesn't send to a real backend. It prints the spans to the terminal, so you can see them arrive.

This is proof the data arrived. The grep command pulls just the useful lines from the saved collector output. Each Span is one timed operation. You can see its Trace ID, its own ID, a Name like lets-go or okey-dokey, and a Kind, Client or Server.

Read the output as a tree. Both spans share trace ID f30faffb, so they belong to one trace. The span lets-go has no Parent ID, which makes it the root. The span okey-dokey lists lets-go's ID as its Parent ID, so lets-go is its parent.

Port 55679 serves ZPages, a built-in debug UI. While the Collector runs, you can open it in your browser, giving you a second way to inspect it.

In real setups, only the export end changes. telemetrygen sends OTLP into the Collector, and the Collector forwards that same telemetry onward. Here it lands in your terminal, but it could just as well go to Jaeger or Prometheus. Think of the Collector as a relay between sources and backends.

When you're done, press Control-C in the Collector terminal to stop it. Remember, this is a local demo, not production-ready. Configuration comes in the next lesson.

The Collector is a relay: receive on ports, process, export onward. To use it, send OTLP to port 4317 or 4318, then check the output.