session.py

# vim: tabstop=4 shiftwidth=4 softtabstop=4

# Copyright 2010 United States Government as represented by the
# Administrator of the National Aeronautics and Space Administration.
# 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.

"""Session Handling for SQLAlchemy backend."""

import time

import sqlalchemy.interfaces
import sqlalchemy.orm
from sqlalchemy.exc import DisconnectionError, OperationalError
from sqlalchemy.pool import NullPool, StaticPool

import cinder.exception
import cinder.flags as flags
from cinder.openstack.common import log as logging


FLAGS = flags.FLAGS
LOG = logging.getLogger(__name__)

_ENGINE = None
_MAKER = None


def get_session(autocommit=True, expire_on_commit=False):
    """Return a SQLAlchemy session."""
    global _MAKER

    if _MAKER is None:
        engine = get_engine()
        _MAKER = get_maker(engine, autocommit, expire_on_commit)

    session = _MAKER()
    session.query = cinder.exception.wrap_db_error(session.query)
    session.flush = cinder.exception.wrap_db_error(session.flush)
    return session


def synchronous_switch_listener(dbapi_conn, connection_rec):
    """Switch sqlite connections to non-synchronous mode"""
    dbapi_conn.execute("PRAGMA synchronous = OFF")


def ping_listener(dbapi_conn, connection_rec, connection_proxy):
    """
    Ensures that MySQL connections checked out of the
    pool are alive.

    Borrowed from:
    http://groups.google.com/group/sqlalchemy/msg/a4ce563d802c929f
    """
    try:
        dbapi_conn.cursor().execute('select 1')
    except dbapi_conn.OperationalError, ex:
        if ex.args[0] in (2006, 2013, 2014, 2045, 2055):
            LOG.warn('Got mysql server has gone away: %s', ex)
            raise DisconnectionError("Database server went away")
        else:
            raise


def is_db_connection_error(args):
    """Return True if error in connecting to db."""
    # NOTE(adam_g): This is currently MySQL specific and needs to be extended
    #               to support Postgres and others.
    conn_err_codes = ('2002', '2003', '2006')
    for err_code in conn_err_codes:
        if args.find(err_code) != -1:
            return True
    return False


def get_engine():
    """Return a SQLAlchemy engine."""
    global _ENGINE
    if _ENGINE is None:
        connection_dict = sqlalchemy.engine.url.make_url(FLAGS.sql_connection)

        engine_args = {
            "pool_recycle": FLAGS.sql_idle_timeout,
            "echo": False,
            'convert_unicode': True,
        }

        # Map our SQL debug level to SQLAlchemy's options
        if FLAGS.sql_connection_debug >= 100:
            engine_args['echo'] = 'debug'
        elif FLAGS.sql_connection_debug >= 50:
            engine_args['echo'] = True

        if "sqlite" in connection_dict.drivername:
            engine_args["poolclass"] = NullPool

            if FLAGS.sql_connection == "sqlite://":
                engine_args["poolclass"] = StaticPool
                engine_args["connect_args"] = {'check_same_thread': False}

        _ENGINE = sqlalchemy.create_engine(FLAGS.sql_connection, **engine_args)

        if 'mysql' in connection_dict.drivername:
            sqlalchemy.event.listen(_ENGINE, 'checkout', ping_listener)
        elif "sqlite" in connection_dict.drivername:
            if not FLAGS.sqlite_synchronous:
                sqlalchemy.event.listen(_ENGINE, 'connect',
                                        synchronous_switch_listener)

        try:
            _ENGINE.connect()
        except OperationalError, e:
            if not is_db_connection_error(e.args[0]):
                raise

            remaining = FLAGS.sql_max_retries
            if remaining == -1:
                remaining = 'infinite'
            while True:
                msg = _('SQL connection failed. %s attempts left.')
                LOG.warn(msg % remaining)
                if remaining != 'infinite':
                    remaining -= 1
                time.sleep(FLAGS.sql_retry_interval)
                try:
                    _ENGINE.connect()
                    break
                except OperationalError, e:
                    if (remaining != 'infinite' and remaining == 0) or \
                       not is_db_connection_error(e.args[0]):
                        raise
    return _ENGINE


def get_maker(engine, autocommit=True, expire_on_commit=False):
    """Return a SQLAlchemy sessionmaker using the given engine."""
    return sqlalchemy.orm.sessionmaker(bind=engine,
                                       autocommit=autocommit,
                                       expire_on_commit=expire_on_commit)