This document describes the current stable version of Celery (5.2). For development docs, go here.

Source code for celery.backends.database

"""SQLAlchemy result store backend."""
import logging
from contextlib import contextmanager

from vine.utils import wraps

from celery import states
from celery.backends.base import BaseBackend
from celery.exceptions import ImproperlyConfigured
from celery.utils.time import maybe_timedelta

from .models import Task, TaskExtended, TaskSet
from .session import SessionManager

try:
    from sqlalchemy.exc import DatabaseError, InvalidRequestError
    from sqlalchemy.orm.exc import StaleDataError
except ImportError:  # pragma: no cover
    raise ImproperlyConfigured(
        'The database result backend requires SQLAlchemy to be installed.'
        'See https://pypi.org/project/SQLAlchemy/')

logger = logging.getLogger(__name__)

__all__ = ('DatabaseBackend',)


@contextmanager
def session_cleanup(session):
    try:
        yield
    except Exception:
        session.rollback()
        raise
    finally:
        session.close()


def retry(fun):

    @wraps(fun)
    def _inner(*args, **kwargs):
        max_retries = kwargs.pop('max_retries', 3)

        for retries in range(max_retries):
            try:
                return fun(*args, **kwargs)
            except (DatabaseError, InvalidRequestError, StaleDataError):
                logger.warning(
                    'Failed operation %s.  Retrying %s more times.',
                    fun.__name__, max_retries - retries - 1,
                    exc_info=True)
                if retries + 1 >= max_retries:
                    raise

    return _inner


class DatabaseBackend(BaseBackend):
    """The database result backend."""

    # ResultSet.iterate should sleep this much between each pool,
    # to not bombard the database with queries.
    subpolling_interval = 0.5

    task_cls = Task
    taskset_cls = TaskSet

    def __init__(self, dburi=None, engine_options=None, url=None, **kwargs):
        # The `url` argument was added later and is used by
        # the app to set backend by url (celery.app.backends.by_url)
        super().__init__(expires_type=maybe_timedelta,
                         url=url, **kwargs)
        conf = self.app.conf

        if self.extended_result:
            self.task_cls = TaskExtended

        self.url = url or dburi or conf.database_url
        self.engine_options = dict(
            engine_options or {},
            **conf.database_engine_options or {})
        self.short_lived_sessions = kwargs.get(
            'short_lived_sessions',
            conf.database_short_lived_sessions)

        schemas = conf.database_table_schemas or {}
        tablenames = conf.database_table_names or {}
        self.task_cls.configure(
            schema=schemas.get('task'),
            name=tablenames.get('task'))
        self.taskset_cls.configure(
            schema=schemas.get('group'),
            name=tablenames.get('group'))

        if not self.url:
            raise ImproperlyConfigured(
                'Missing connection string! Do you have the'
                ' database_url setting set to a real value?')

    @property
    def extended_result(self):
        return self.app.conf.find_value_for_key('extended', 'result')

    def ResultSession(self, session_manager=SessionManager()):
        return session_manager.session_factory(
            dburi=self.url,
            short_lived_sessions=self.short_lived_sessions,
            **self.engine_options)

    @retry
    def _store_result(self, task_id, result, state, traceback=None,
                      request=None, **kwargs):
        """Store return value and state of an executed task."""
        session = self.ResultSession()
        with session_cleanup(session):
            task = list(session.query(self.task_cls).filter(self.task_cls.task_id == task_id))
            task = task and task[0]
            if not task:
                task = self.task_cls(task_id)
                task.task_id = task_id
                session.add(task)
                session.flush()

            self._update_result(task, result, state, traceback=traceback, request=request)
            session.commit()

    def _update_result(self, task, result, state, traceback=None,
                       request=None):

        meta = self._get_result_meta(result=result, state=state,
                                     traceback=traceback, request=request,
                                     format_date=False, encode=True)

        # Exclude the primary key id and task_id columns
        # as we should not set it None
        columns = [column.name for column in self.task_cls.__table__.columns
                   if column.name not in {'id', 'task_id'}]

        # Iterate through the columns name of the table
        # to set the value from meta.
        # If the value is not present in meta, set None
        for column in columns:
            value = meta.get(column)
            setattr(task, column, value)

    @retry
    def _get_task_meta_for(self, task_id):
        """Get task meta-data for a task by id."""
        session = self.ResultSession()
        with session_cleanup(session):
            task = list(session.query(self.task_cls).filter(self.task_cls.task_id == task_id))
            task = task and task[0]
            if not task:
                task = self.task_cls(task_id)
                task.status = states.PENDING
                task.result = None
            data = task.to_dict()
            if data.get('args', None) is not None:
                data['args'] = self.decode(data['args'])
            if data.get('kwargs', None) is not None:
                data['kwargs'] = self.decode(data['kwargs'])
            return self.meta_from_decoded(data)

    @retry
    def _save_group(self, group_id, result):
        """Store the result of an executed group."""
        session = self.ResultSession()
        with session_cleanup(session):
            group = self.taskset_cls(group_id, result)
            session.add(group)
            session.flush()
            session.commit()
            return result

    @retry
    def _restore_group(self, group_id):
        """Get meta-data for group by id."""
        session = self.ResultSession()
        with session_cleanup(session):
            group = session.query(self.taskset_cls).filter(
                self.taskset_cls.taskset_id == group_id).first()
            if group:
                return group.to_dict()

    @retry
    def _delete_group(self, group_id):
        """Delete meta-data for group by id."""
        session = self.ResultSession()
        with session_cleanup(session):
            session.query(self.taskset_cls).filter(
                self.taskset_cls.taskset_id == group_id).delete()
            session.flush()
            session.commit()

    @retry
    def _forget(self, task_id):
        """Forget about result."""
        session = self.ResultSession()
        with session_cleanup(session):
            session.query(self.task_cls).filter(self.task_cls.task_id == task_id).delete()
            session.commit()

    def cleanup(self):
        """Delete expired meta-data."""
        session = self.ResultSession()
        expires = self.expires
        now = self.app.now()
        with session_cleanup(session):
            session.query(self.task_cls).filter(
                self.task_cls.date_done < (now - expires)).delete()
            session.query(self.taskset_cls).filter(
                self.taskset_cls.date_done < (now - expires)).delete()
            session.commit()

    def __reduce__(self, args=(), kwargs=None):
        kwargs = {} if not kwargs else kwargs
        kwargs.update(
            {'dburi': self.url,
             'expires': self.expires,
             'engine_options': self.engine_options})
        return super().__reduce__(args, kwargs)