apache/druid · error · ISE

Unknown type of emitter. Please set

Error message

Unknown type of emitter. Please set [%s], [%s] or provide registered subtype of org.apache.druid.java.util.emitter.core.factory.EmitterFactory via [%s]

What it means

Emitters.create() builds an emitter from configuration properties by detecting the emitter type. If none of the recognized properties (log emitter, http emitter, or custom EmitterFactory type) is present, it throws IllegalStateException telling the user which properties to set. Druid cannot determine what kind of emitter to instantiate.

Solutions

Set org.apache.druid.java.util.emitter.http.url for an HTTP emitter, e.g. http://recipient:8080/druid/metrics
Or set org.apache.druid.java.util.emitter.logging.logLevel (e.g. "info") for a logging emitter
Or configure org.apache.druid.java.util.emitter.type with a registered EmitterFactory subtype
Verify the properties file is actually on the classpath and loaded into the Properties object

Example fix

// before (runtime.properties)
# no emitter config
// after
org.apache.druid.java.util.emitter.http.url=http://metrics-host:8080/druid/metrics

Defensive patterns

Strategy: validation

Validate before calling

Properties props = ...;
boolean hasEmitter = props.getProperty("org.apache.druid.java.util.emitter.logging.logLevel") != null
  || props.getProperty("org.apache.druid.java.util.emitter.http.url") != null
  || props.getProperty("org.apache.druid.java.util.emitter.type") != null;
if (!hasEmitter) {
  throw new IllegalStateException("No emitter configured: set http url, logging level, or custom type");
}

Try / catch

try {
  Emitter e = Emitters.create(props);
} catch (IllegalStateException ex) {
  log.error(ex, "Emitter configuration missing");
  throw ex;
}

Prevention

Always set at least one emitter property (http url, logging logLevel, or custom type)
Copy property keys exactly; they are long and easy to mistype
Verify runtime.properties is loaded into the Properties object passed to create
Add a config smoke test that constructs the emitter at startup

When it happens

Trigger: Calling Emitters.create(Properties) when neither org.apache.druid.java.util.emitter.logging.logLevel, org.apache.druid.java.util.emitter.http.url, nor org.apache.druid.java.util.emitter.type (custom emitter type) is configured.

Common situations: Missing or misnamed emitter properties in runtime.properties; config file not loaded so properties are empty; upgrading Druid where the old type property name changed; typo in property key.

Understand the failure class

Background: "X is required", "must be set", "cannot be empty": the missing-required-config error family, from Vertex AI project/location to WeChat keys — this error's family across 18 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/8c472cfac791b33e. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/java/util/emitter/core/Emitters.java:63

  public static Emitter create(
      Properties props,
      AsyncHttpClient httpClient,
      ObjectMapper jsonMapper,
      Lifecycle lifecycle
  )
  {
    Map<String, Object> jsonified;
    if (props.getProperty(LOG_EMITTER_PROP) != null) {
      jsonified = makeLoggingMap(props);
      jsonified.put("type", "logging");
    } else if (props.getProperty(HTTP_EMITTER_PROP) != null) {
      jsonified = makeHttpMap(props);
      jsonified.put("type", "http");
    } else if (props.getProperty(CUSTOM_EMITTER_TYPE_PROP) != null) {
      jsonified = makeCustomFactoryMap(props);
    } else {
      throw new ISE(
          "Unknown type of emitter. Please set [%s], [%s] or provide registered subtype of org.apache.druid.java.util.emitter.core.factory.EmitterFactory via [%s]",
          LOG_EMITTER_PROP,
          HTTP_EMITTER_PROP,
          CUSTOM_EMITTER_TYPE_PROP
      );
    }
    return jsonMapper.convertValue(jsonified, EmitterFactory.class).makeEmitter(jsonMapper, httpClient, lifecycle);
  }

  // Package-visible for unit tests

  static Map<String, Object> makeHttpMap(Properties props)
  {
    Map<String, Object> httpMap = new HashMap<>();

    final String urlProperty = "org.apache.druid.java.util.emitter.http.url";

    final String baseUrl = props.getProperty(urlProperty);

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