apache/iceberg · error · AlreadyExistsException

Cannot create namespace %s. Namespace already exists

Error message

Cannot create namespace %s. Namespace already exists

What it means

InMemoryCatalog.createNamespace throws AlreadyExistsException when the namespace is already registered. The in-memory catalog stores namespaces in a map keyed by Namespace; duplicate creation is rejected rather than overwritten.

Source

Thrown at core/src/main/java/org/apache/iceberg/inmemory/InMemoryCatalog.java:203

      if (views.containsKey(to)) {
        throw new AlreadyExistsException("Cannot rename %s to %s. View already exists", from, to);
      }

      tables.put(to, fromLocation);
      tables.remove(from);
    }
  }

  @Override
  public void createNamespace(Namespace namespace) {
    createNamespace(namespace, Collections.emptyMap());
  }

  @Override
  public void createNamespace(Namespace namespace, Map<String, String> metadata) {
    synchronized (this) {
      if (namespaceExists(namespace)) {
        throw new AlreadyExistsException(
            "Cannot create namespace %s. Namespace already exists", namespace);
      }

      namespaces.put(namespace, ImmutableMap.copyOf(metadata));
    }
  }

  @Override
  public boolean namespaceExists(Namespace namespace) {
    return namespaces.containsKey(namespace);
  }

  @Override
  public boolean dropNamespace(Namespace namespace) throws NamespaceNotEmptyException {
    synchronized (this) {
      if (!namespaceExists(namespace)) {
        return false;
      }

View on GitHub (pinned to 86d9c8fc54)

Solutions

1. Check namespaceExists(namespace) before calling createNamespace
2. Wrap in try-catch for AlreadyExistsException for idempotent setup
3. Use a unique namespace name per test/run
4. Clear or recreate the catalog between runs

Example fix

// before
catalog.createNamespace(Namespace.of("test_ns"));
// after
Namespace ns = Namespace.of("test_ns");
if (!catalog.namespaceExists(ns)) {
  catalog.createNamespace(ns);
}

Namespace ns = Namespace.of("test_ns");
if (!catalog.namespaceExists(ns)) { catalog.createNamespace(ns, metadata); }

try { catalog.createNamespace(ns, metadata); } catch (AlreadyExistsException e) { /* idempotent: ignore */ }

When it happens

Trigger: Calling catalog.createNamespace(ns) or createNamespace(ns, metadata) twice for the same namespace, including creating the root-level namespace that already exists. The nested createNamespace overload delegates here, so both entry points can throw.

Common situations: Test setup run twice against a shared catalog instance; idempotent bootstrap scripts that assume create-if-not-exists semantics; parallel test classes initializing the same namespace.

Understand the failure class

Background: "already exists" / EEXIST / FileAlreadyExistsException: what the 'file already exists' error means and how to fix it — this error's family across 37 libraries.

Related errors

• Cannot create namespace %s because it already exists in Glue
• Namespace already exists: %s
• Namespace already exists: %s
• Cannot list tables for namespace. Namespace does not exist:
• Cannot rename %s to %s. Namespace does not exist: %s

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/79ee7b1cd9ad407a. Report an issue: GitHub.