text: # Copyright (c) 2013 Rackspace Hosting
  # All Rights Reserved.
  #
  #    Licensed under the Apache License, Version 2.0 (the "License"); you may
  #    not use this file except in compliance with the License. You may obtain
  #    a copy of the License at
  #
  #         http://www.apache.org/licenses/LICENSE-2.0
  #
  #    Unless required by applicable law or agreed to in writing, software
  #    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
  #    License for the specific language governing permissions and limitations
  #    under the License.
  
  """Multiple DB API backend support.
  
  A DB backend module should implement a method named 'get_backend' which
  takes no arguments.  The method can return any object that implements DB
  API methods.
  """
  
  import functools
  import logging
  import threading
  import time
  
  from climate.openstack.common.db import exception
  from climate.openstack.common.gettextutils import _LE
  from climate.openstack.common import importutils
  
  
  LOG = logging.getLogger(__name__)
  
  
  def safe_for_db_retry(f):
      """Enable db-retry for decorated function, if config option enabled."""
      f.__dict__['enable_retry'] = True
      return f
  
  
  class wrap_db_retry(object):
      """Retry db.api methods, if DBConnectionError() raised
  
      Retry decorated db.api methods. If we enabled `use_db_reconnect`
      in config, this decorator will be applied to all db.api functions,
      marked with @safe_for_db_retry decorator.
      Decorator catchs DBConnectionError() and retries function in a
      loop until it succeeds, or until maximum retries count will be reached.
      """
  
      def __init__(self, retry_interval, max_retries, inc_retry_interval,
                   max_retry_interval):
          super(wrap_db_retry, self).__init__()
  
          self.retry_interval = retry_interval
          self.max_retries = max_retries
          self.inc_retry_interval = inc_retry_interval
          self.max_retry_interval = max_retry_interval
  
      def __call__(self, f):
          @functools.wraps(f)
          def wrapper(*args, **kwargs):
              next_interval = self.retry_interval
              remaining = self.max_retries
  
              while True:
                  try:
                      return f(*args, **kwargs)
                  except exception.DBConnectionError as e:
                      if remaining == 0:
                          LOG.exception(_LE('DB exceeded retry limit.'))
                          raise exception.DBError(e)
                      if remaining != -1:
                          remaining -= 1
                          LOG.exception(_LE('DB connection error.'))
                      # NOTE(vsergeyev): We are using patched time module, so
                      #                  this effectively yields the execution
                      #                  context to another green thread.
                      time.sleep(next_interval)
                      if self.inc_retry_interval:
                          next_interval = min(
                              next_interval * 2,
                              self.max_retry_interval
                          )
          return wrapper
  
  
  class DBAPI(object):
      def __init__(self, backend_name, backend_mapping=None, lazy=False,
                   **kwargs):
          """Initialize the chosen DB API backend.
  
          :param backend_name: name of the backend to load
          :type backend_name: str
  
          :param backend_mapping: backend name -> module/class to load mapping
          :type backend_mapping: dict
  
          :param lazy: load the DB backend lazily on the first DB API method call
          :type lazy: bool
  
          Keyword arguments:
  
          :keyword use_db_reconnect: retry DB transactions on disconnect or not
          :type use_db_reconnect: bool
  
          :keyword retry_interval: seconds between transaction retries
          :type retry_interval: int
  
          :keyword inc_retry_interval: increase retry interval or not
          :type inc_retry_interval: bool
  
          :keyword max_retry_interval: max interval value between retries
          :type max_retry_interval: int
  
          :keyword max_retries: max number of retries before an error is raised
          :type max_retries: int
  
          """
  
          self._backend = None
          self._backend_name = backend_name
          self._backend_mapping = backend_mapping or {}
          self._lock = threading.Lock()
  
          if not lazy:
              self._load_backend()
  
          self.use_db_reconnect = kwargs.get('use_db_reconnect', False)
          self.retry_interval = kwargs.get('retry_interval', 1)
          self.inc_retry_interval = kwargs.get('inc_retry_interval', True)
          self.max_retry_interval = kwargs.get('max_retry_interval', 10)
          self.max_retries = kwargs.get('max_retries', 20)
  
      def _load_backend(self):
          with self._lock:
              if not self._backend:
                  # Import the untranslated name if we don't have a mapping
                  backend_path = self._backend_mapping.get(self._backend_name,
                                                           self._backend_name)
                  backend_mod = importutils.import_module(backend_path)
                  self._backend = backend_mod.get_backend()
  
      def __getattr__(self, key):
          if not self._backend:
              self._load_backend()
  
          attr = getattr(self._backend, key)
          if not hasattr(attr, '__call__'):
              return attr
          # NOTE(vsergeyev): If `use_db_reconnect` option is set to True, retry
          #                  DB API methods, decorated with @safe_for_db_retry
          #                  on disconnect.
          if self.use_db_reconnect and hasattr(attr, 'enable_retry'):
              attr = wrap_db_retry(
                  retry_interval=self.retry_interval,
                  max_retries=self.max_retries,
                  inc_retry_interval=self.inc_retry_interval,
                  max_retry_interval=self.max_retry_interval)(attr)
  
          return attr
  