apache/hadoop · error · MetricsException

Tag {} already exists!

Error message

Tag {} already exists!

What it means

MetricsRegistry.checkTagName enforces per-registry uniqueness for tags: registering a tag name that already exists in tagsMap throws MetricsException('Tag <name> already exists!').

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/lib/MetricsRegistry.java:456

      char c = name.charAt(i);
      if (Character.isWhitespace(c)) {
        foundWhitespace = true;
        break;
      }
    }
    if (foundWhitespace) {
      throw new MetricsException("Metric name '"+ name +
          "' contains illegal whitespace character");
    }
    // Check if name has already been registered
    if (metricsMap.containsKey(name)) {
      throw new MetricsException("Metric name "+ name +" already exists!");
    }
  }

  private void checkTagName(String name) {
    if (tagsMap.containsKey(name)) {
      throw new MetricsException("Tag "+ name +" already exists!");
    }
  }

  /**
   * Sample all the mutable metrics and put the snapshot in the builder
   * @param builder to contain the metrics snapshot
   * @param all get all the metrics even if the values are not changed.
   */
  public synchronized void snapshot(MetricsRecordBuilder builder, boolean all) {
    for (MetricsTag tag : tags()) {
      builder.add(tag);
    }
    for (MutableMetric metric : metrics()) {
      metric.snapshot(builder, all);
    }
  }

  @Override

View on GitHub (pinned to 2add963021)

Solutions

1. Set each tag once per registry and reuse the existing value
2. Give the second tag a different name
3. Re-create the registry instead of re-declaring tags into it

Example fix

// before
registry.tag("Context", "Context", "mysource");
registry.tag("Context", "Context", "mysource");  // throws

// after
registry.tag("Context", "Context", "mysource");
registry.tag("Hostname", "Hostname", myHost);

Set<String> usedTags = ConcurrentHashMap.newKeySet();
if (!usedTags.add(tagName)) {
  throw new IllegalStateException("tag already registered: " + tagName);
}
registry.tag(tagName, desc, value);

When it happens

Trigger: Registering two tags with the same name in one registry — e.g., registry.tag("Context", ..., "mysource") followed by another tag("Context", ...) on the same registry.

Common situations: Re-initializing a component that re-declares its context/context-record tags into a shared registry; copy-pasted tag declarations; merging two metric classes into one registry.

Related errors

• Metric name {} already exists!
• Unexpected metrics type {} for {}
• Unsupported add(value) for metric {}
• Metric name '{}' contains illegal whitespace character
• Illegal tag pattern: {}

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/cf37ada4c65ecea1. Report an issue: GitHub.