# Kafka

Site [llms.txt](/pl/llms.txt)

---

This is used as a message queue service to connect the checkout service with the
accounting and fraud detection services.

[Kafka service source](https://github.com/open-telemetry/opentelemetry-demo/blob/main/src/kafka/)

## Configuration

| Variable      | Default  | Description                                                                                           |
| ------------- | -------- | ----------------------------------------------------------------------------------------------------- |
| `KAFKA_TOPIC` | `orders` | Topic that checkout produces order messages to, and that accounting and fraud detection consume from. |

All three services read the same variable, so change it in `.env` (or
`.env.override`) rather than on an individual service, otherwise the producer
and consumers end up on different topics.

## Auto-instrumentation

This service relies on the OpenTelemetry Java agent and the built in
[JMX Metric Insight Module](https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/instrumentation/jmx-metrics/javaagent)
to capture
[Kafka broker metrics](https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/instrumentation/jmx-metrics/library/kafka-broker.md)
and send them off to the collector via OTLP.

The agent is passed into the process using the `-javaagent` command line
argument. Command line arguments are added through the `KAFKA_OPTS` in the
`Dockerfile`.

```dockerfile
ENV KAFKA_OPTS="-javaagent:/tmp/opentelemetry-javaagent.jar -Dotel.jmx.target.system=kafka-broker"
```