Synnefo

# Copyright 2011-2012 GRNET S.A. All rights reserved.

#

# Redistribution and use in source and binary forms, with or without

# modification, are permitted provided that the following conditions

# are met:

#

#   1. Redistributions of source code must retain the above copyright

#      notice, this list of conditions and the following disclaimer.

#

#  2. Redistributions in binary form must reproduce the above copyright

#     notice, this list of conditions and the following disclaimer in the

#     documentation and/or other materials provided with the distribution.

#

# THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND

# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

# ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE

# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

# SUCH DAMAGE.

#

# The views and conclusions contained in the software and documentation are

# those of the authors and should not be interpreted as representing official

# policies, either expressed or implied, of GRNET S.A.

import datetime

from django.conf import settings

from django.db import models

from django.db import IntegrityError

import utils

from contextlib import contextmanager

from hashlib import sha1

from synnefo.api.faults import ServiceUnavailable

from synnefo import settings as snf_settings

from aes_encrypt import encrypt_db_charfield, decrypt_db_charfield

from synnefo.db.managers import ForUpdateManager, ProtectedDeleteManager

from synnefo.db import pools

from synnefo.logic.rapi_pool import (get_rapi_client,

                                     put_rapi_client)

import logging

log = logging.getLogger(__name__)

class Flavor(models.Model):

    cpu = models.IntegerField('Number of CPUs', default=0)

    ram = models.IntegerField('RAM size in MiB', default=0)

    disk = models.IntegerField('Disk size in GiB', default=0)

    disk_template = models.CharField('Disk template', max_length=32,

            default=settings.DEFAULT_GANETI_DISK_TEMPLATE)

    deleted = models.BooleanField('Deleted', default=False)

    class Meta:

        verbose_name = u'Virtual machine flavor'

        unique_together = ('cpu', 'ram', 'disk', 'disk_template')

    @property

    def name(self):

        """Returns flavor name (generated)"""

        return u'C%dR%dD%d' % (self.cpu, self.ram, self.disk)

    def __unicode__(self):

        return str(self.id)

class Backend(models.Model):

    clustername = models.CharField('Cluster Name', max_length=128, unique=True)

    port = models.PositiveIntegerField('Port', default=5080)

    username = models.CharField('Username', max_length=64, blank=True,

                                null=True)

    password_hash = models.CharField('Password', max_length=128, blank=True,

                                null=True)

    # Sha1 is up to 40 characters long

    hash = models.CharField('Hash', max_length=40, editable=False, null=False)

    # Unique index of the Backend, used for the mac-prefixes of the

    # BackendNetworks

    index = models.PositiveIntegerField('Index', null=False, unique=True,

                                        default=0)

    drained = models.BooleanField('Drained', default=False, null=False)

    offline = models.BooleanField('Offline', default=False, null=False)

    # Last refresh of backend resources

    updated = models.DateTimeField(auto_now_add=True)

    # Backend resources

    mfree = models.PositiveIntegerField('Free Memory', default=0, null=False)

    mtotal = models.PositiveIntegerField('Total Memory', default=0, null=False)

    dfree = models.PositiveIntegerField('Free Disk', default=0, null=False)

    dtotal = models.PositiveIntegerField('Total Disk', default=0, null=False)

    pinst_cnt = models.PositiveIntegerField('Primary Instances', default=0,

                                            null=False)

    ctotal = models.PositiveIntegerField('Total number of logical processors',

                                         default=0, null=False)

    # Custom object manager to protect from cascade delete

    objects = ProtectedDeleteManager()

    class Meta:

        verbose_name = u'Backend'

        ordering = ["clustername"]

    def __unicode__(self):

        return self.clustername + "(id=" + str(self.id) + ")"

    @property

    def backend_id(self):

        return self.id

    def get_client(self):

        """Get or create a client. """

        if self.offline:

            raise ServiceUnavailable

        return get_rapi_client(self.id, self.hash,

                               self.clustername,

                               self.port,

                               self.username,

                               self.password)

    @staticmethod

    def put_client(client):

            put_rapi_client(client)

    def create_hash(self):

        """Create a hash for this backend. """

        return sha1('%s%s%s%s' % \

                (self.clustername, self.port, self.username, self.password)) \

                .hexdigest()

    @property

    def password(self):

        return decrypt_db_charfield(self.password_hash)

    @password.setter

    def password(self, value):

        self.password_hash = encrypt_db_charfield(value)

    def save(self, *args, **kwargs):

        # Create a new hash each time a Backend is saved

        old_hash = self.hash

        self.hash = self.create_hash()

        super(Backend, self).save(*args, **kwargs)

        if self.hash != old_hash:

            # Populate the new hash to the new instances

            self.virtual_machines.filter(deleted=False).update(backend_hash=self.hash)

    def delete(self, *args, **kwargs):

        # Integrity Error if non-deleted VMs are associated with Backend

        if self.virtual_machines.filter(deleted=False).count():

            raise IntegrityError("Non-deleted virtual machines are associated "

                                 "with backend: %s" % self)

        else:

            # ON_DELETE = SET NULL

            self.virtual_machines.all().backend = None

            # Remove BackendNetworks of this Backend.

            # Do not use networks.all().delete(), since delete() method of

            # BackendNetwork will not be called!

            for net in self.networks.all():

                net.delete()

            super(Backend, self).delete(*args, **kwargs)

    def __init__(self, *args, **kwargs):

        super(Backend, self).__init__(*args, **kwargs)

        if not self.pk:

            # Generate a unique index for the Backend

            indexes = Backend.objects.all().values_list('index', flat=True)

            first_free = [x for x in xrange(0, 16) if x not in indexes][0]

            self.index = first_free

# A backend job may be in one of the following possible states

BACKEND_STATUSES = (

    ('queued', 'request queued'),

    ('waiting', 'request waiting for locks'),

    ('canceling', 'request being canceled'),

    ('running', 'request running'),

    ('canceled', 'request canceled'),

    ('success', 'request completed successfully'),

    ('error', 'request returned error')

)

class QuotaHolderSerial(models.Model):

    serial = models.BigIntegerField(null=False, primary_key=True, db_index=True)

    pending = models.BooleanField(default=True, db_index=True)

    accepted = models.BooleanField(default=False)

    rejected = models.BooleanField(default=False)

    class Meta:

        verbose_name = u'Quota Serial'

        ordering = ["serial"]

    def save(self, *args, **kwargs):

        self.pending = not (self.accepted or self.rejected)

        super(QuotaHolderSerial, self).save(*args, **kwargs)

class VirtualMachine(models.Model):

    # The list of possible actions for a VM

    ACTIONS = (

       ('CREATE', 'Create VM'),

       ('START', 'Start VM'),

       ('STOP', 'Shutdown VM'),

       ('SUSPEND', 'Admin Suspend VM'),

       ('REBOOT', 'Reboot VM'),

       ('DESTROY', 'Destroy VM')

    )

    # The internal operating state of a VM

    OPER_STATES = (

        ('BUILD', 'Queued for creation'),

        ('ERROR', 'Creation failed'),

        ('STOPPED', 'Stopped'),

        ('STARTED', 'Started'),

        ('DESTROYED', 'Destroyed')

    )

    # The list of possible operations on the backend

    BACKEND_OPCODES = (

        ('OP_INSTANCE_CREATE', 'Create Instance'),

        ('OP_INSTANCE_REMOVE', 'Remove Instance'),

        ('OP_INSTANCE_STARTUP', 'Startup Instance'),

        ('OP_INSTANCE_SHUTDOWN', 'Shutdown Instance'),

        ('OP_INSTANCE_REBOOT', 'Reboot Instance'),

        # These are listed here for completeness,

        # and are ignored for the time being

        ('OP_INSTANCE_SET_PARAMS', 'Set Instance Parameters'),

        ('OP_INSTANCE_QUERY_DATA', 'Query Instance Data'),

        ('OP_INSTANCE_REINSTALL', 'Reinstall Instance'),

        ('OP_INSTANCE_ACTIVATE_DISKS', 'Activate Disks'),

        ('OP_INSTANCE_DEACTIVATE_DISKS', 'Deactivate Disks'),

        ('OP_INSTANCE_REPLACE_DISKS', 'Replace Disks'),

        ('OP_INSTANCE_MIGRATE', 'Migrate Instance'),

        ('OP_INSTANCE_CONSOLE', 'Get Instance Console'),

        ('OP_INSTANCE_RECREATE_DISKS', 'Recreate Disks'),

        ('OP_INSTANCE_FAILOVER', 'Failover Instance')

    )

    # The operating state of a VM,

    # upon the successful completion of a backend operation.

    # IMPORTANT: Make sure all keys have a corresponding

    # entry in BACKEND_OPCODES if you update this field, see #1035, #1111.

    OPER_STATE_FROM_OPCODE = {

        'OP_INSTANCE_CREATE': 'STARTED',

        'OP_INSTANCE_REMOVE': 'DESTROYED',

        'OP_INSTANCE_STARTUP': 'STARTED',

        'OP_INSTANCE_SHUTDOWN': 'STOPPED',

        'OP_INSTANCE_REBOOT': 'STARTED',

        'OP_INSTANCE_SET_PARAMS': None,

        'OP_INSTANCE_QUERY_DATA': None,

        'OP_INSTANCE_REINSTALL': None,

        'OP_INSTANCE_ACTIVATE_DISKS': None,

        'OP_INSTANCE_DEACTIVATE_DISKS': None,

        'OP_INSTANCE_REPLACE_DISKS': None,

        'OP_INSTANCE_MIGRATE': None,

        'OP_INSTANCE_CONSOLE': None,

        'OP_INSTANCE_RECREATE_DISKS': None,

        'OP_INSTANCE_FAILOVER': None

    }

    # This dictionary contains the correspondence between

    # internal operating states and Server States as defined

    # by the Rackspace API.

    RSAPI_STATE_FROM_OPER_STATE = {

        "BUILD": "BUILD",

        "ERROR": "ERROR",

        "STOPPED": "STOPPED",

        "STARTED": "ACTIVE",

        "DESTROYED": "DELETED"

    }

    name = models.CharField('Virtual Machine Name', max_length=255)

    userid = models.CharField('User ID of the owner', max_length=100,

                              db_index=True)

    backend = models.ForeignKey(Backend, null=True,

                                related_name="virtual_machines",)

    backend_hash = models.CharField(max_length=128, null=True, editable=False)

    created = models.DateTimeField(auto_now_add=True)

    updated = models.DateTimeField(auto_now=True)

    imageid = models.CharField(max_length=100, null=False)

    hostid = models.CharField(max_length=100)

    flavor = models.ForeignKey(Flavor)

    deleted = models.BooleanField('Deleted', default=False, db_index=True)

    suspended = models.BooleanField('Administratively Suspended',

                                    default=False)

    serial = models.ForeignKey(QuotaHolderSerial,

                              related_name='virtual_machine', null=True)

    # VM State

    # The following fields are volatile data, in the sense

    # that they need not be persistent in the DB, but rather

    # get generated at runtime by quering Ganeti and applying

    # updates received from Ganeti.

    # In the future they could be moved to a separate caching layer

    # and removed from the database.

    # [vkoukis] after discussion with [faidon].

    action = models.CharField(choices=ACTIONS, max_length=30, null=True)

    operstate = models.CharField(choices=OPER_STATES, max_length=30, null=True)

    backendjobid = models.PositiveIntegerField(null=True)

    backendopcode = models.CharField(choices=BACKEND_OPCODES, max_length=30,

                                     null=True)

    backendjobstatus = models.CharField(choices=BACKEND_STATUSES,

                                        max_length=30, null=True)

    backendlogmsg = models.TextField(null=True)

    buildpercentage = models.IntegerField(default=0)

    backendtime = models.DateTimeField(default=datetime.datetime.min)

    objects = ForUpdateManager()

    def get_client(self):

        if self.backend:

            return self.backend.get_client()

        else:

            raise ServiceUnavailable

    def get_last_diagnostic(self, **filters):

        try:

            return self.diagnostics.filter()[0]

        except IndexError:

            return None

    @staticmethod

    def put_client(client):

            put_rapi_client(client)

    def __init__(self, *args, **kw):

        """Initialize state for just created VM instances."""

        super(VirtualMachine, self).__init__(*args, **kw)

        # This gets called BEFORE an instance gets save()d for

        # the first time.

        if not self.pk:

            self.action = None

            self.backendjobid = None

            self.backendjobstatus = None

            self.backendopcode = None

            self.backendlogmsg = None

            self.operstate = 'BUILD'

    def save(self, *args, **kwargs):

        # Store hash for first time saved vm

        if (self.id is None or self.backend_hash == '') and self.backend:

            self.backend_hash = self.backend.hash

        super(VirtualMachine, self).save(*args, **kwargs)

    @property

    def backend_vm_id(self):

        """Returns the backend id for this VM by prepending backend-prefix."""

        if not self.id:

            raise VirtualMachine.InvalidBackendIdError("self.id is None")

        return "%s%s" % (settings.BACKEND_PREFIX_ID, str(self.id))

    class Meta:

        verbose_name = u'Virtual machine instance'

        get_latest_by = 'created'

    def __unicode__(self):

        return str(self.id)

    # Error classes

    class InvalidBackendIdError(Exception):

        def __init__(self, value):

            self.value = value

        def __str__(self):

            return repr(self.value)

    class InvalidBackendMsgError(Exception):

        def __init__(self, opcode, status):

            self.opcode = opcode

            self.status = status

        def __str__(self):

            return repr('<opcode: %s, status: %s>' % (self.opcode,

                        self.status))

    class InvalidActionError(Exception):

        def __init__(self, action):

            self._action = action

        def __str__(self):

            return repr(str(self._action))

    class DeletedError(Exception):

        pass

    class BuildingError(Exception):

        pass

class VirtualMachineMetadata(models.Model):

    meta_key = models.CharField(max_length=50)

    meta_value = models.CharField(max_length=500)

    vm = models.ForeignKey(VirtualMachine, related_name='metadata')

    class Meta:

        unique_together = (('meta_key', 'vm'),)

        verbose_name = u'Key-value pair of metadata for a VM.'

    def __unicode__(self):

        return u'%s: %s' % (self.meta_key, self.meta_value)

class Network(models.Model):

    OPER_STATES = (

        ('PENDING', 'Pending'),

        ('ACTIVE', 'Active'),

        ('DELETED', 'Deleted'),

        ('ERROR', 'Error')

    )

    ACTIONS = (

       ('CREATE', 'Create Network'),

       ('DESTROY', 'Destroy Network'),

    )

    RSAPI_STATE_FROM_OPER_STATE = {

        'PENDING': 'PENDING',

        'ACTIVE': 'ACTIVE',

        'DELETED': 'DELETED',

        'ERROR': 'ERROR'

    }

    FLAVORS = {

        'CUSTOM': {

             'mode': 'bridged',

             'link': settings.DEFAULT_BRIDGE,

             'mac_prefix': settings.DEFAULT_MAC_PREFIX,

             'tags': None,

             'desc': "Basic flavor used for a bridged network",

        },

        'IP_LESS_ROUTED': {

             'mode': 'routed',

             'link': settings.DEFAULT_ROUTING_TABLE,

             'mac_prefix': settings.DEFAULT_MAC_PREFIX,

             'tags': 'ip-less-routed',

             'desc': "Flavor used for an IP-less routed network using"

                     " Proxy ARP",

        },

        'MAC_FILTERED': {

             'mode': 'bridged',

             'link': settings.DEFAULT_MAC_FILTERED_BRIDGE,

             'mac_prefix': 'pool',

             'tags': 'private-filtered',

             'desc': "Flavor used for bridged networks that offer isolation"

                     " via filtering packets based on their src "

                     " MAC (ebtables)",

        },

        'PHYSICAL_VLAN': {

             'mode': 'bridged',

             'link': 'pool',

             'mac_prefix': settings.DEFAULT_MAC_PREFIX,

             'tags': 'physical-vlan',

             'desc': "Flavor used for bridged network that offer isolation"

                     " via dedicated physical vlan",

        },

    }

    name = models.CharField('Network Name', max_length=128)

    userid = models.CharField('User ID of the owner', max_length=128,

                              null=True, db_index=True)

    subnet = models.CharField('Subnet', max_length=32, default='10.0.0.0/24')

    subnet6 = models.CharField('IPv6 Subnet', max_length=64, null=True)

    gateway = models.CharField('Gateway', max_length=32, null=True)

    gateway6 = models.CharField('IPv6 Gateway', max_length=64, null=True)

    dhcp = models.BooleanField('DHCP', default=True)

    flavor = models.CharField('Flavor', max_length=32, null=False)

    mode = models.CharField('Network Mode', max_length=16, null=True)

    link = models.CharField('Network Link', max_length=32, null=True)

    mac_prefix = models.CharField('MAC Prefix', max_length=32, null=False)

    tags = models.CharField('Network Tags', max_length=128, null=True)

    public = models.BooleanField(default=False, db_index=True)

    created = models.DateTimeField(auto_now_add=True)

    updated = models.DateTimeField(auto_now=True)

    deleted = models.BooleanField('Deleted', default=False, db_index=True)

    state = models.CharField(choices=OPER_STATES, max_length=32,

                             default='PENDING')

    machines = models.ManyToManyField(VirtualMachine,

                                      through='NetworkInterface')

    action = models.CharField(choices=ACTIONS, max_length=32, null=True,

                              default=None)

    pool = models.OneToOneField('IPPoolTable', related_name='network',

                default=lambda: IPPoolTable.objects.create(available_map='',

                                                           reserved_map='',

                                                           size=0),

                null=True)

    serial = models.ForeignKey(QuotaHolderSerial, related_name='network',

                               null=True)

    objects = ForUpdateManager()

    def __unicode__(self):

        return str(self.id)

    @property

    def backend_id(self):

        """Return the backend id by prepending backend-prefix."""

        if not self.id:

            raise Network.InvalidBackendIdError("self.id is None")

        return "%snet-%s" % (settings.BACKEND_PREFIX_ID, str(self.id))

    @property

    def backend_tag(self):

        """Return the network tag to be used in backend

        """

        if self.tags:

            return self.tags.split(',')

        else:

            return []

    def create_backend_network(self, backend=None):

        """Create corresponding BackendNetwork entries."""

        backends = [backend] if backend else Backend.objects.all()

        for backend in backends:

            BackendNetwork.objects.create(backend=backend, network=self)

    def get_pool(self):

        if not self.pool_id:

            self.pool = IPPoolTable.objects.create(available_map='',

                                                   reserved_map='',

                                                   size=0)

            self.save()

        return IPPoolTable.objects.select_for_update().get(id=self.pool_id)\

                                                      .pool

    def reserve_address(self, address):

        pool = self.get_pool()

        pool.reserve(address)

        pool.save()

    def release_address(self, address):

        pool = self.get_pool()

        pool.put(address)

        pool.save()

    class InvalidBackendIdError(Exception):

        def __init__(self, value):

            self.value = value

        def __str__(self):

            return repr(self.value)

    class InvalidBackendMsgError(Exception):

        def __init__(self, opcode, status):

            self.opcode = opcode

            self.status = status

        def __str__(self):

            return repr('<opcode: %s, status: %s>' % (self.opcode,

                    self.status))

    class InvalidActionError(Exception):

        def __init__(self, action):

            self._action = action

        def __str__(self):

            return repr(str(self._action))

    class DeletedError(Exception):

        pass

    class BuildingError(Exception):

        pass

class BackendNetwork(models.Model):

    OPER_STATES = (

        ('PENDING', 'Pending'),

        ('ACTIVE', 'Active'),

        ('DELETED', 'Deleted'),

        ('ERROR', 'Error')

    )

    # The list of possible operations on the backend

    BACKEND_OPCODES = (

        ('OP_NETWORK_ADD', 'Create Network'),

        ('OP_NETWORK_CONNECT', 'Activate Network'),

        ('OP_NETWORK_DISCONNECT', 'Deactivate Network'),

        ('OP_NETWORK_REMOVE', 'Remove Network'),

        # These are listed here for completeness,

        # and are ignored for the time being

        ('OP_NETWORK_SET_PARAMS', 'Set Network Parameters'),

        ('OP_NETWORK_QUERY_DATA', 'Query Network Data')

    )

    # The operating state of a Netowork,

    # upon the successful completion of a backend operation.

    # IMPORTANT: Make sure all keys have a corresponding

    # entry in BACKEND_OPCODES if you update this field, see #1035, #1111.

    OPER_STATE_FROM_OPCODE = {

        'OP_NETWORK_ADD': 'PENDING',

        'OP_NETWORK_CONNECT': 'ACTIVE',

        'OP_NETWORK_DISCONNECT': 'PENDING',

        'OP_NETWORK_REMOVE': 'DELETED',

        'OP_NETWORK_SET_PARAMS': None,

        'OP_NETWORK_QUERY_DATA': None

    }

    network = models.ForeignKey(Network, related_name='backend_networks')

    backend = models.ForeignKey(Backend, related_name='networks')

    created = models.DateTimeField(auto_now_add=True)

    updated = models.DateTimeField(auto_now=True)

    deleted = models.BooleanField('Deleted', default=False)

    mac_prefix = models.CharField('MAC Prefix', max_length=32, null=False)

    operstate = models.CharField(choices=OPER_STATES, max_length=30,

                                 default='PENDING')

    backendjobid = models.PositiveIntegerField(null=True)

    backendopcode = models.CharField(choices=BACKEND_OPCODES, max_length=30,

                                     null=True)

    backendjobstatus = models.CharField(choices=BACKEND_STATUSES,

                                        max_length=30, null=True)

    backendlogmsg = models.TextField(null=True)

    backendtime = models.DateTimeField(null=False,

                                       default=datetime.datetime.min)

    class Meta:

        # Ensure one entry for each network in each backend

        unique_together = (("network", "backend"))

    def __init__(self, *args, **kwargs):

        """Initialize state for just created BackendNetwork instances."""

        super(BackendNetwork, self).__init__(*args, **kwargs)

        if not self.mac_prefix:

            # Generate the MAC prefix of the BackendNetwork, by combining

            # the Network prefix with the index of the Backend

            net_prefix = self.network.mac_prefix

            backend_suffix = hex(self.backend.index).replace('0x', '')

            mac_prefix = net_prefix + backend_suffix

            try:

                utils.validate_mac(mac_prefix + ":00:00:00")

            except utils.InvalidMacAddress:

                raise utils.InvalidMacAddress("Invalid MAC prefix '%s'" % \

                                               mac_prefix)

            self.mac_prefix = mac_prefix

class NetworkInterface(models.Model):

    FIREWALL_PROFILES = (

        ('ENABLED', 'Enabled'),

        ('DISABLED', 'Disabled'),

        ('PROTECTED', 'Protected')

    )

    machine = models.ForeignKey(VirtualMachine, related_name='nics')

    network = models.ForeignKey(Network, related_name='nics')

    created = models.DateTimeField(auto_now_add=True)

    updated = models.DateTimeField(auto_now=True)

    index = models.IntegerField(null=False)

    mac = models.CharField(max_length=32, null=False, unique=True)

    ipv4 = models.CharField(max_length=15, null=True)

    ipv6 = models.CharField(max_length=100, null=True)

    firewall_profile = models.CharField(choices=FIREWALL_PROFILES,

                                        max_length=30, null=True)

    dirty = models.BooleanField(default=False)

    def __unicode__(self):

        return '%s@%s' % (self.machine.name, self.network.name)

class PoolTable(models.Model):

    available_map = models.TextField(default="", null=False)

    reserved_map = models.TextField(default="", null=False)

    size = models.IntegerField(null=False)

    # Optional Fields

    base = models.CharField(null=True, max_length=32)

    offset = models.IntegerField(null=True)

    objects = ForUpdateManager()

    class Meta:

        abstract = True

    @classmethod

    def get_pool(cls):

        try:

            pool_row = cls.objects.select_for_update().get()

            return pool_row.pool

        except IndexError:

            raise pools.EmptyPool

    @property

    def pool(self):

        return self.manager(self)

class BridgePoolTable(PoolTable):

    manager = pools.BridgePool

class MacPrefixPoolTable(PoolTable):

    manager = pools.MacPrefixPool

class IPPoolTable(PoolTable):

    manager = pools.IPPool

@contextmanager

def pooled_rapi_client(obj):

        if isinstance(obj, VirtualMachine):

            backend = obj.backend

        else:

            backend = obj

        if backend.offline:

            raise ServiceUnavailable