What causes "User (username={username}) already exists. Error: {e}" in mlflow/mlflow?

Use a different username, or query the existing user via get_user/auth APIs and update instead of create

mlflow/mlflow · error · MlflowException

RESOURCE_ALREADY_EXISTS

RESOURCE_ALREADY_EXISTS

Error message

User (username={username}) already exists. Error: {e}

What it means

create_user inserts a SqlUser and lets the database enforce username uniqueness via a unique constraint; on an IntegrityError (duplicate username), it re-raises as MlflowException with RESOURCE_ALREADY_EXISTS. This is the standard pre-check-free pattern relying on DB constraints.

Source

Thrown at mlflow/server/auth/sqlalchemy_store.py:157

        with self.ManagedSessionMaker() as session:
            try:
                user = self._get_user(session, username)
                return check_password_hash(user.password_hash, password)
            except MlflowException:
                return False

    def create_user(self, username: str, password: str, is_admin: bool = False) -> User:
        _validate_username(username)
        _validate_password(password)
        pwhash = generate_password_hash(password)
        with self.ManagedSessionMaker(read_only=False) as session:
            try:
                user = SqlUser(username=username, password_hash=pwhash, is_admin=is_admin)
                session.add(user)
                session.flush()
                return user.to_mlflow_entity()
            except IntegrityError as e:
                raise MlflowException(
                    f"User (username={username}) already exists. Error: {e}",
                    RESOURCE_ALREADY_EXISTS,
                ) from e

    @staticmethod
    def _get_user(session, username: str) -> SqlUser:
        try:
            return session.query(SqlUser).filter(SqlUser.username == username).one()
        except NoResultFound:
            raise MlflowException(
                f"User with username={username} not found",
                RESOURCE_DOES_NOT_EXIST,
            )
        except MultipleResultsFound:
            raise MlflowException(
                f"Found multiple users with username={username}",
                INVALID_STATE,
            )

View on GitHub (pinned to 6a27f2decc)

Solutions

Use a different username, or query the existing user via get_user/auth APIs and update instead of create
Catch MlflowException with code RESOURCE_ALREADY_EXISTS and treat it as idempotent in bootstrap scripts
Check for the user's existence before calling create_user

Example fix

// before
store.create_user("admin@example.com", password, True)
// after
try:
    store.create_user("admin@example.com", password, True)
except MlflowException as e:
    if e.get_http_status_code() != 409:
        raise

Defensive patterns

Strategy: try-catch

Validate before calling

existing = store.get_user(username)
if existing:
    user = existing
else:
    user = store.create_user(username, password, is_admin)

Try / catch

from mlflow.exceptions import MlflowException, RESOURCE_ALREADY_EXISTS
try:
    user = store.create_user(username, password, is_admin)
except MlflowException as e:
    if e.get_http_status_code() == 409 or RESOURCE_ALREADY_EXISTS in str(e.code):
        user = store.get_user(username)
    else:
        raise

Prevention

Make user-creation bootstrap scripts idempotent (check-then-create or catch 409)
Run bootstrap only once per database
Uniquely prefix usernames in test environments to avoid collisions

When it happens

Trigger: Calling create_user (or the REST /api/2.0/mlflow/users/create endpoint, or the initial admin-creation bootstrap) with a username that already exists in the users table.

Common situations: Re-running auth bootstrap scripts (e.g. MLFLOW_AUTH_CONFIG_PATH init creating the default admin) against an already-initialized DB; concurrent user creation; duplicate signup attempts.

Related errors

AI-assisted analysis of mlflow/mlflow@6a27f2decc (2026-08-29). Data as JSON: /api/errors/09e3ffce0a78c364. Report an issue: GitHub.