Bug: NPE in MetaStrategy.java when collecting metrics
Status: Resolved
Priority: Normal
Resolution: Fixed
Created: 12/Jan/24 22:48
Resolved: 18/Jan/24 13:26

Now that the metrics exporter has been removed in CASSANDRA-18743, I thought I'd take a stab at setting up a metrics collector through DropWizard.  I have a POC of an agent that exposes a Prometheus endpoint using the Prometheus simpleclient here: https://github.com/rustyrazorblade/cassandra-prometheus-exporter

There's build instructions in the README - it's just a simple agent on a background thread.

Unfortunately when trying to view the metrics, Cassandra throws an NPE exception, stack trace is here:

{code:java}
WARN  [qtp104447770-28] 2024-01-12 14:26:50,102 ServletHandler.java:522 - /metrics
java.lang.NullPointerException: null
	at org.apache.cassandra.locator.MetaStrategy.getReplicationFactor(MetaStrategy.java:64)
	at org.apache.cassandra.metrics.KeyspaceMetrics.lambda$new$12(KeyspaceMetrics.java:224)
	at org.apache.cassandra.metrics.KeyspaceMetrics$1.getValue(KeyspaceMetrics.java:344)
	at org.apache.cassandra.metrics.KeyspaceMetrics$1.getValue(KeyspaceMetrics.java:338)
	at org.apache.cassandra.metrics.StorageMetrics.lambda$static$0(StorageMetrics.java:40)
	at org.apache.cassandra.metrics.StorageMetrics.lambda$createSummingGauge$2(StorageMetrics.java:55)
	at java.base/java.util.stream.ReferencePipeline$5$1.accept(ReferencePipeline.java:229)
	at com.google.common.collect.CollectSpliterators$1.lambda$forEachRemaining$1(CollectSpliterators.java:128)
	at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
	at com.google.common.collect.CollectSpliterators$1.forEachRemaining(CollectSpliterators.java:128)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.LongPipeline.reduce(LongPipeline.java:474)
	at java.base/java.util.stream.LongPipeline.sum(LongPipeline.java:432)
	at org.apache.cassandra.metrics.StorageMetrics.lambda$createSummingGauge$3(StorageMetrics.java:56)
	at io.prometheus.client.dropwizard.DropwizardExports.fromGauge(DropwizardExports.java:47)
	at io.prometheus.client.dropwizard.DropwizardExports.collect(DropwizardExports.java:133)
	at io.prometheus.client.Collector.collect(Collector.java:45)
	at io.prometheus.client.CollectorRegistry$MetricFamilySamplesEnumeration.findNextElement(CollectorRegistry.java:204)
	at io.prometheus.client.CollectorRegistry$MetricFamilySamplesEnumeration.<init>(CollectorRegistry.java:162)
	at io.prometheus.client.CollectorRegistry.filteredMetricFamilySamples(CollectorRegistry.java:140)
	at io.prometheus.client.exporter.MetricsServlet.doGet(MetricsServlet.java:43)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:735)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:648)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:455)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1072)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:382)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1006)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
	at org.eclipse.jetty.server.Server.handle(Server.java:365)
	at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:485)
	at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:926)
	at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:988)
	at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:635)
	at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
	at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:627)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:51)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
	at java.base/java.lang.Thread.run(Thread.java:829)
{code}

Related [discussion thread|https://lists.apache.org/thread/lppjlxrd91qn0r0dkz80r5y843jwl1qo].