diff --git a/terracotta/db/__init__.py b/terracotta/db/__init__.py
new file mode 100644
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+++ b/terracotta/db/__init__.py
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+# Copyright (c) 2015 - 2016 Huawei Technologies Co., Ltd.
+# 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.
+"""
+DB abstraction for Terracotta
+"""
+
+from terracotta.db.api import *
\ No newline at end of file
diff --git a/terracotta/db/api.py b/terracotta/db/api.py
new file mode 100644
index 0000000..0500aaf
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+++ b/terracotta/db/api.py
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+# Copyright (c) 2015 - 2016 Huawei Technologies Co., Ltd.
+# 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.
+"""
+Defines interface for DB access.
+"""
+
+from oslo_config import cfg
+from oslo_db import concurrency as db_concurrency
+
+
+CONF = cfg.CONF
+
+_BACKEND_MAPPING = {'sqlalchemy': 'terracotta.db.sqlalchemy.api'}
+
+IMPL = db_concurrency.TpoolDbapiWrapper(CONF, backend_mapping=_BACKEND_MAPPING)
+
+
+def select_cpu_mhz_for_vm(uuid, limit):
+ """ Select n last values of CPU MHz for a VM UUID.
+
+ :param uuid: The UUID of a VM.
+ :param limit: The number of last values to select.
+ :return: The list of n last CPU Mhz values.
+ """
+ return IMPL.select_cpu_mhz_for_vm(uuid, limit)
+
+
+def select_last_cpu_mhz_for_vms():
+ """ Select the last value of CPU MHz for all the VMs.
+
+ :return: A dict of VM UUIDs to the last CPU MHz values.
+ """
+ return IMPL.select_last_cpu_mhz_for_vms()
+
+
+def select_vm_id(uuid):
+ """ Select the ID of a VM by the VM UUID, or insert a new record.
+
+ :param uuid: The UUID of a VM.
+ :return: The ID of the VM.
+ """
+ return IMPL.select_vm_id(uuid)
+
+
+def insert_vm_cpu_mhz(data):
+ """ Insert a set of CPU MHz values for a set of VMs.
+
+ :param data: A dictionary of VM UUIDs and CPU MHz values.
+ """
+ return IMPL.insert_vm_cpu_mhz()
+
+
+def update_host(hostname, cpu_mhz, cpu_cores, ram):
+ """ Insert new or update the corresponding host record.
+
+ :param hostname: A host name.
+ :param cpu_mhz: The total CPU frequency of the host in MHz.
+ :param cpu_cores: The number of physical CPU cores.
+ :param ram: The total amount of RAM of the host in MB.
+ :return: The ID of the host.
+ """
+ return update_host(hostname, cpu_mhz, cpu_cores, ram)
+
+
+def insert_host_cpu_mhz(hostname, cpu_mhz):
+ """ Insert a CPU MHz value for a host.
+
+ :param hostname: A host name.
+ :param cpu_mhz: The CPU usage of the host in MHz.
+ """
+ return IMPL.insert_host_cpu_mhz(hostname, cpu_mhz)
+
+
+def select_cpu_mhz_for_host(hostname, limit):
+ """ Select n last values of CPU MHz for a host.
+
+ :param hostname: A host name.
+ :param limit: The number of last values to select.
+ :return: The list of n last CPU Mhz values.
+ """
+ return IMPL.select_cpu_mhz_for_host(hostname, limit)
+
+
+def select_last_cpu_mhz_for_hosts():
+ """ Select the last value of CPU MHz for all the hosts.
+
+ :return: A dict of host names to the last CPU MHz values.
+ """
+ return IMPL.select_last_cpu_mhz_for_hosts()
+
+
+def select_host_characteristics(self):
+ """ Select the characteristics of all the hosts.
+
+ :return: Three dicts of hostnames to CPU MHz, cores, and RAM.
+ """
+ return IMPL.select_host_characteristics()
+
+
+def select_host_id(hostname):
+ """ Select the ID of a host.
+
+ :param hostname: A host name.
+ :return: The ID of the host.
+ """
+ return IMPL.select_host_id(hostname)
+
+
+def select_host_ids():
+ """ Select the IDs of all the hosts.
+
+ :return: A dict of host names to IDs.
+ """
+ return IMPL.select_host_ids()
+
+
+def cleanup_vm_resource_usage(datetime_threshold):
+ """ Delete VM resource usage data older than the threshold.
+
+ :param datetime_threshold: A datetime threshold.
+ """
+ IMPL.cleanup_vm_resource_usage(datetime_threshold)
+
+
+def cleanup_host_resource_usage(sdatetime_threshold):
+ """ Delete host resource usage data older than the threshold.
+
+ :param datetime_threshold: A datetime threshold.
+ """
+ IMPL.cleanup_host_resource_usage()
+
+
+def insert_host_states(hosts):
+ """ Insert host states for a set of hosts.
+
+ :param hosts: A dict of hostnames to states (0, 1).
+ """
+ IMPL.insert_host_states(hosts)
+
+
+def select_host_states():
+ """ Select the current states of all the hosts.
+
+ :return: A dict of host names to states.
+ """
+ return IMPL.select_host_states()
+
+
+def select_active_hosts():
+ """ Select the currently active hosts.
+
+ :return: A list of host names.
+ """
+ return IMPL.select_active_hosts()
+
+
+def select_inactive_hosts():
+ """ Select the currently inactive hosts.
+
+ :return: A list of host names.
+ """
+ return IMPL.select_inactive_hosts()
+
+
+def insert_host_overload(hostname, overload):
+ """ Insert whether a host is overloaded.
+
+ :param hostname: A host name.
+ :param overload: Whether the host is overloaded.
+ """
+ IMPL.insert_host_overload(hostname, overload)
+
+
+def insert_vm_migration(vm, hostname):
+ """ Insert a VM migration.
+
+ :param hostname: A VM UUID.
+ :param hostname: A host name.
+ """
+ IMPL.insert_vm_migration(vm, hostname)