tailbone/tailbone/tweens.py

# -*- coding: utf-8; -*-
################################################################################
#
#  Rattail -- Retail Software Framework
#  Copyright © 2010-2018 Lance Edgar
#
#  This file is part of Rattail.
#
#  Rattail is free software: you can redistribute it and/or modify it under the
#  terms of the GNU General Public License as published by the Free Software
#  Foundation, either version 3 of the License, or (at your option) any later
#  version.
#
#  Rattail is distributed in the hope that it will be useful, but WITHOUT ANY
#  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
#  FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
#  details.
#
#  You should have received a copy of the GNU General Public License along with
#  Rattail.  If not, see <http://www.gnu.org/licenses/>.
#
################################################################################
"""
Tween Factories
"""

from __future__ import unicode_literals, absolute_import

import six
from sqlalchemy.exc import OperationalError


def sqlerror_tween_factory(handler, registry):
    """
    Produces a tween which will convert ``sqlalchemy.exc.OperationalError``
    instances (caused by database server restart) into a retryable error
    instance, so that a second attempt may be made to connect to the database
    before really giving up.

    .. note::
       This tween alone is not enough to cause the transaction to be retried;
       it only marks the error as being *retryable*.  If you wish more than one
       attempt to be made, you must define the ``retry.attempts`` (or
       ``tm.attempts`` if running pyramid<1.9) setting within your Pyramid app
       configuration.  For more info see `Retrying`_.

       .. _Retrying: http://docs.pylonsproject.org/projects/pyramid_tm/en/latest/#retrying
    """

    def sqlerror_tween(request):
        try:
            from pyramid_retry import mark_error_retryable
        except ImportError:
            mark_error_retryable = None
            from transaction.interfaces import TransientError

        try:
            response = handler(request)
        except OperationalError as error:

            # if connection is invalid, allow retry
            if error.connection_invalidated:
                if mark_error_retryable:
                    mark_error_retryable(error)
                    raise error
                else:
                    raise TransientError(six.text_type(error))

            # if connection was *not* invalid, raise original error
            raise

        return response

    return sqlerror_tween