apache/druid · error · IllegalStateException

Cannot find generator for namespace

Error message

Cannot find generator for namespace [%s]

What it means

CacheScheduler.schedule() looks up a CacheGenerator by the namespace's concrete class in namespaceGeneratorMap. If no generator was registered for that ExtractionNamespace type, schedule() cannot create the entry and throws this ISE naming the namespace.

Solutions

1. Use a supported namespace type (JdbcExtractionNamespace, UriExtractionNamespace, etc.).
2. If custom, register your CacheGenerator for the namespace class when constructing CacheScheduler.
3. Load the extension module that provides the generator for the namespace type.

Example fix

// before
scheduler.schedule(new MyCustomNamespace(...)); // ISE: no generator
// after
// in scheduler construction:
new CacheScheduler(..., ImmutableMap.of(
    JdbcExtractionNamespace.class, new JdbcCacheGenerator(...),
    UriExtractionNamespace.class, new UriCacheGenerator(...),
    MyCustomNamespace.class, myGenerator));

Set<Class<?>> supported = Set.of(JdbcExtractionNamespace.class, UriExtractionNamespace.class);
if (!supported.contains(namespace.getClass())) throw new IllegalArgumentException("Unsupported namespace type: " + namespace.getClass());

boolean schedulable(ExtractionNamespace n) { return n instanceof JdbcExtractionNamespace || n instanceof UriExtractionNamespace; }

try { return scheduler.schedule(namespace); } catch (IllegalStateException e) { if (e.getMessage().startsWith("Cannot find generator for namespace")) { /* register generator or load extension */ } else { throw e; } }

When it happens

Trigger: Calling scheduler.schedule(namespace) with an ExtractionNamespace subclass that has no registered CacheGenerator (e.g. a custom namespace type not wired into the scheduler, or an extension namespace whose module wasn't loaded).

Common situations: Custom ExtractionNamespace implementation without registering its generator in the scheduler's constructor; using a lookup type provided by an extension that isn't loaded; reflection-created namespace objects with a type not supported in this runtime.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Already shut down, not starting again
• Already started, not starting again
• Cache reference is null
• can't start.
• can't stop.

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/2aed94b6dec99544. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/lookups-cached-global/src/main/java/org/apache/druid/server/lookup/namespace/cache/CacheScheduler.java:530

    finally {
      if (!success) {
        // ExecutionException's cause is logged in entry.close()
        entry.close();
        if (loadException != null) {
          log.error(loadException, "CacheScheduler[%s] - problem during start or waiting for the first run", entry);
        } else {
          log.error("CacheScheduler[%s] - problem during start or waiting for the first run", entry);
        }
      }
    }
  }

  public <T extends ExtractionNamespace> Entry schedule(final T namespace)
  {
    @SuppressWarnings("unchecked")
    final CacheGenerator<T> generator = (CacheGenerator<T>) namespaceGeneratorMap.get(namespace.getClass());
    if (generator == null) {
      throw new ISE("Cannot find generator for namespace [%s]", namespace);
    }
    return new Entry<>(namespace, generator);
  }
}

View on GitHub (pinned to 9b90983fd2)