Neutron Database Layer

This section contains some common information that will be useful for developers that need to do some db changes.

Difference between ‘default’ and ‘server_default’ parameters for columns

For columns it is possible to set ‘default’ or ‘server_default’. What is the difference between them and why should they be used?

The explanation is quite simple:

  • default - the default value that SQLAlchemy will specify in queries for creating instances of a given model;
  • server_default - the default value for a column that SQLAlchemy will specify in DDL.

Summarizing, ‘default’ is useless in migrations and only ‘server_default’ should be used. For synchronizing migrations with models server_default parameter should also be added in model. If default value in database is not needed, ‘server_default’ should not be used. The declarative approach can be bypassed (i.e. ‘default’ may be omitted in the model) if default is enforced through business logic.

Database migrations

For details on the neutron-db-manage wrapper and alembic migrations, see Alembic Migrations.

Tests to verify that database migrations and models are in sync

class neutron.tests.functional.db.test_migrations._TestModelsMigrations

Test for checking of equality models state and migrations.

For the opportunistic testing you need to set up a db named ‘openstack_citest’ with user ‘openstack_citest’ and password ‘openstack_citest’ on localhost. The test will then use that db and user/password combo to run the tests.

For PostgreSQL on Ubuntu this can be done with the following commands:

sudo -u postgres psql
postgres=# create user openstack_citest with createdb login password
postgres=# create database openstack_citest with owner

For MySQL on Ubuntu this can be done with the following commands:

mysql -u root
>create database openstack_citest;
>grant all privileges on openstack_citest.* to
 openstack_citest@localhost identified by 'openstack_citest';

Output is a list that contains information about differences between db and models. Output example:

  Table('bat', MetaData(bind=None),
        Column('info', String(), table=<bat>), schema=None)),
  Table(u'bar', MetaData(bind=None),
        Column(u'data', VARCHAR(), table=<bar>), schema=None)),
  Column('data', Integer(), table=<foo>)),
  Column(u'old_data', VARCHAR(), table=None)),
   {'existing_server_default': None,
   'existing_type': INTEGER()},
  • remove_* means that there is extra table/column/constraint in db;

  • add_* means that it is missing in db;

  • modify_* means that on column in db is set wrong type/nullable/server_default. Element contains information:

    • what should be modified,
    • schema,
    • table,
    • column,
    • existing correct column parameters,
    • right value,
    • wrong value.

The Standard Attribute Table

There are many attributes that we would like to store in the database which are common across many Neutron objects (e.g. tags, timestamps, rbac entries). We have previously been handling this by duplicating the schema to every table via model mixins. This means that a DB migration is required for each object that wants to adopt one of these common attributes. This becomes even more cumbersome when the relationship between the attribute and the object is many-to-one because each object then needs its own table for the attributes (assuming referential integrity is a concern).

To address this issue, the ‘standardattribute’ table is available. Any model can add support for this table by inheriting the ‘HasStandardAttributes’ mixin in neutron.db.model_base. This mixin will add a standard_attr_id BigInteger column to the model with a foreign key relationship to the ‘standardattribute’ table. The model will then be able to access any columns of the ‘standardattribute’ table and any tables related to it.

The introduction of a new standard attribute only requires one column addition to the ‘standardattribute’ table for one-to-one relationships or a new table for one-to-many or one-to-zero relationships. Then all of the models using the ‘HasStandardAttribute’ mixin will automatically gain access to the new attribute.

Any attributes that will apply to every neutron resource (e.g. timestamps) can be added directly to the ‘standardattribute’ table. For things that will frequently be NULL for most entries (e.g. a column to store an error reason), a new table should be added and joined to in a query to prevent a bunch of NULL entries in the database.