Terminate with an error if started as in agent mode but configured with a remote JMX url

[fstab]

jmx_exporter can run in an agent mode (attaching directly to the JVM to be motitored) or in a standalone mode (connecting to a remote JVM to be monitored).

If you want to connect to a remote JVM you have to configure either the jmxUrl or the hostPort.

Here's an overview of the possible configurations:

1 Running in agent mode without configuring jmxUrl or hostPort

This is as expected, if you run jmx_exporter in agent mode you don't need to connect to a remote JVM.

2 Running in standalone mode and configuring jmxUrl or hostPort

This is as expected, if you run jmx_exporter in standalone mode you want to configure the remote connection to the JVM to be monitored.

3 Running in standalone mode without configuring jmxUrl or hostPort

This is now prevented and will result in a startup error.

jmx_exporter/collector/src/main/java/io/prometheus/jmx/JmxCollector.java

Lines 101 to 112 in 5c2c921

	private void exitIfJmxUrlMissing() {
	// If the jmxUrl configuration is missing, the JmxScraper implicitly monitors the JVM it runs in.
	// This is good if the JmxCollector is used in the Java agent, because the Java agent should monitor the JVM
	// it is attached to.
	// However, if the JmxCollector is used in the WebServer, the intention is that it monitors another process.
	// If the jmxUrl configuration is missing, it should not silently ignore this and start monitoring itself.
	// The WebServer sets jmxUrlRequired to true so that we can verify this and terminate with a proper error message.
	if (jmxUrlRequired && config.jmxUrl.isEmpty()) {
	LOGGER.severe("configuration error: one of jmxUrl or hostPort is required");
	System.exit(-1);
	}
	}

Previously, what happened was that jmx_exporter starts and monitors its own JVM, which is very likely not what you want.

4 Running in agent mode and configuring jmxUrl or hostPort

This is currently possible. What happens is that jmx_exporter attaches to a JVM, but instead of monitoring that JVM it tries to connect to a remote JVM to monitor.

Proposal

Modify the behavior in case 4 and make jmx_exporter terminate with an error. If you run jmx_exporter in agent mode attaching to a JVM you certainly don't want to use it to monitor a remote JVM.

[dhoard]

I agree... case 4 doesn't seem like something we would want to support.

If a user wants to run Tomcat/Jetty (or another JEE servlet container) to monitor an external application (as opposed to the built-in HTTPServer), they really should be using client_java and MetricsServlet directly.

[dhoard]

Closed as resolved.