Cannot load Dropwizard metrics; is…

Error message

Cannot load Dropwizard metrics; is org.apache.flink:flink-metrics-dropwizard on the classpath?

What it means

Iceberg's Flink sink tries to construct Flink's DropwizardHistogramWrapper to expose codahale histograms as Flink metrics. This requires the optional org.apache.flink:flink-metrics-dropwizard artifact on the classpath. When the class or constructor cannot be loaded, the sink logs this warning and returns null, disabling Dropwizard-based writer metrics; writes continue normally without them.

Solutions

Add the flink-metrics-dropwizard JAR matching your Flink version to Flink's lib/ directory (or shade it into your job jar).
Verify the org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper class is loadable in the taskmanager classloader.
If metrics are not needed, ignore this warning; the sink falls back to null and runs without Dropwizard histograms.

Example fix

// before (pom.xml: optional metric dependency omitted)
<dependency>
  <groupId>org.apache.iceberg</groupId>
  <artifactId>iceberg-flink-runtime-1.20</artifactId>
</dependency>

// after
<dependency>
  <groupId>org.apache.iceberg</groupId>
  <artifactId>iceberg-flink-runtime-1.20</artifactId>
</dependency>
<dependency>
  <groupId>org.apache.flink</groupId>
  <artifactId>flink-metrics-dropwizard</artifactId>
  <version>${flink.version}</version>
</dependency>

Defensive patterns

Strategy: validation

Validate before calling

try {
  Class.forName("org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper");
} catch (ClassNotFoundException e) {
  LOG.info("flink-metrics-dropwizard not on classpath; writer histograms disabled");
}

Prevention

Ship flink-metrics-dropwizard with your Flink deployment (lib/) in every environment.
Keep optional metric dependency versions aligned with the Flink version.
Treat this warning as expected-and-harmless if Dropwizard metrics are intentionally disabled.

When it happens

Trigger: IcebergStreamWriterMetrics.loadDropwizardCtors runs DynConstructors.builder(Histogram.class).impl("org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper", codahaleHistogramClass).buildChecked() and catches ClassNotFoundException/NoSuchMethodException when flink-metrics-dropwizard is absent or its version is incompatible with the codahale Histogram class.

Common situations: Deploying the Iceberg Flink sink JAR without the metrics connector JAR in Flink's lib/ directory; using a shaded/uber job jar that excludes optional Flink metrics dependencies; Flink distribution upgrades where metrics-dropwizard was not carried over.

Understand the failure class

Background: "X is not installed. Please install it with pip install Y": missing optional dependency errors — ImportError/ValueError raised when a library's optional extra was never installed — this error's family across 22 libraries.

Related errors

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/50dc2ecae5b71f82. Report an issue: GitHub.

Appendix: source

Thrown at flink/v2.1/flink/src/main/java/org/apache/iceberg/flink/sink/IcebergStreamWriterMetrics.java:150

    try {
      Class<?> reservoirInterface =
          DynClasses.builder().impl("com.codahale.metrics.Reservoir").buildChecked();
      Class<?> codahaleHistogramClass =
          DynClasses.builder().impl("com.codahale.metrics.Histogram").buildChecked();
      return new DropwizardCtors(
          DynConstructors.builder()
              .impl("com.codahale.metrics.SlidingWindowReservoir", int.class)
              .buildChecked(),
          DynConstructors.builder()
              .impl("com.codahale.metrics.Histogram", reservoirInterface)
              .buildChecked(),
          DynConstructors.builder(Histogram.class)
              .impl(
                  "org.apache.flink.dropwizard.metrics.DropwizardHistogramWrapper",
                  codahaleHistogramClass)
              .buildChecked());
    } catch (ClassNotFoundException | NoSuchMethodException e) {
      LOG.warn(
          "Cannot load Dropwizard metrics; is org.apache.flink:flink-metrics-dropwizard on the classpath?",
          e);
      return null;
    }
  }

  private record DropwizardCtors(
      DynConstructors.Ctor<?> reservoirCtor,
      DynConstructors.Ctor<?> histogramCtor,
      DynConstructors.Ctor<Histogram> wrapperCtor) {}
}

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