The oslo_db.sqlalchemy.session Module¶

Session Handling for SQLAlchemy backend.

Recommended ways to use sessions within this framework:

• Use the enginefacade system for connectivity, session and transaction management:

  from oslo_db.sqlalchemy import enginefacade
  
  @enginefacade.reader
  def get_foo(context, foo):
      return (model_query(models.Foo, context.session).
              filter_by(foo=foo).
              first())
  
  @enginefacade.writer
  def update_foo(context, id, newfoo):
      (model_query(models.Foo, context.session).
              filter_by(id=id).
              update({'foo': newfoo}))
  
  @enginefacade.writer
  def create_foo(context, values):
      foo_ref = models.Foo()
      foo_ref.update(values)
      foo_ref.save(context.session)
      return foo_ref
  

  In the above system, transactions are committed automatically, and are shared among all dependent database methods. Ensure that methods which “write” data are enclosed within @writer blocks.

  Note

  Statements in the session scope will not be automatically retried.

• If you create models within the session, they need to be added, but you do not need to call model.save():

  @enginefacade.writer
  def create_many_foo(context, foos):
      for foo in foos:
          foo_ref = models.Foo()
          foo_ref.update(foo)
          context.session.add(foo_ref)
  
  @enginefacade.writer
  def update_bar(context, foo_id, newbar):
      foo_ref = (model_query(models.Foo, context.session).
                  filter_by(id=foo_id).
                  first())
      (model_query(models.Bar, context.session).
                  filter_by(id=foo_ref['bar_id']).
                  update({'bar': newbar}))
  

  The two queries in update_bar can alternatively be expressed using a single query, which may be more efficient depending on scenario:

  @enginefacade.writer
  def update_bar(context, foo_id, newbar):
      subq = (model_query(models.Foo.id, context.session).
              filter_by(id=foo_id).
              limit(1).
              subquery())
      (model_query(models.Bar, context.session).
              filter_by(id=subq.as_scalar()).
              update({'bar': newbar}))
  

  For reference, this emits approximately the following SQL statement:

  UPDATE bar SET bar = '${newbar}'
      WHERE id=(SELECT bar_id FROM foo WHERE id = '${foo_id}' LIMIT 1);
  

  Note

  create_duplicate_foo is a trivially simple example of catching an exception while using a savepoint. Here we create two duplicate instances with same primary key, must catch the exception out of context managed by a single session:

  @enginefacade.writer
  def create_duplicate_foo(context):
      foo1 = models.Foo()
      foo2 = models.Foo()
      foo1.id = foo2.id = 1
      try:
          with context.session.begin_nested():
              session.add(foo1)
              session.add(foo2)
      except exception.DBDuplicateEntry as e:
          handle_error(e)
  

• The enginefacade system eliminates the need to decide when sessions need to be passed between methods. All methods should instead share a common context object; the enginefacade system will maintain the transaction across method calls.

  @enginefacade.writer
  def myfunc(context, foo):
      # do some database things
      bar = _private_func(context, foo)
      return bar
  
  def _private_func(context, foo):
      with enginefacade.using_writer(context) as session:
          # do some other database things
          session.add(SomeObject())
      return bar
  

• Avoid with_lockmode('UPDATE') when possible.

  FOR UPDATE is not compatible with MySQL/Galera. Instead, an “opportunistic” approach should be used, such that if an UPDATE fails, the entire transaction should be retried. The @wrap_db_retry decorator is one such system that can be used to achieve this.

Enabling soft deletes:

• To use/enable soft-deletes, SoftDeleteMixin may be used. For example:

  class NovaBase(models.SoftDeleteMixin, models.ModelBase):
      pass
  

Efficient use of soft deletes:

• While there is a model.soft_delete() method, prefer query.soft_delete(). Some examples:

  @enginefacade.writer
  def soft_delete_bar(context):
      # synchronize_session=False will prevent the ORM from attempting
      # to search the Session for instances matching the DELETE;
      # this is typically not necessary for small operations.
      count = model_query(BarModel, context.session).\
          find(some_condition).soft_delete(synchronize_session=False)
      if count == 0:
          raise Exception("0 entries were soft deleted")
  
  @enginefacade.writer
  def complex_soft_delete_with_synchronization_bar(context):
      # use synchronize_session='evaluate' when you'd like to attempt
      # to update the state of the Session to match that of the DELETE.
      # This is potentially helpful if the operation is complex and
      # continues to work with instances that were loaded, though
      # not usually needed.
      count = (model_query(BarModel, context.session).
                  find(some_condition).
                  soft_delete(synchronize_session='evaulate'))
      if count == 0:
          raise Exception("0 entries were soft deleted")
  

oslo_db.sqlalchemy.session.EngineFacade¶

alias of LegacyEngineFacade

oslo_db.sqlalchemy.session.create_engine(sql_connection, sqlite_fk=False, mysql_sql_mode=None, mysql_enable_ndb=False, idle_timeout=3600, connection_debug=0, max_pool_size=None, max_overflow=None, pool_timeout=None, sqlite_synchronous=True, connection_trace=False, max_retries=10, retry_interval=10, thread_checkin=True, logging_name=None, json_serializer=None, json_deserializer=None)¶

Return a new SQLAlchemy engine.

oslo_db.sqlalchemy.session.get_maker(engine, autocommit=True, expire_on_commit=False)¶

Return a SQLAlchemy sessionmaker using the given engine.

class oslo_db.sqlalchemy.session.Query(entities, session=None)¶

Bases: sqlalchemy.orm.query.Query

Subclass of sqlalchemy.query with soft_delete() method.

soft_delete(synchronize_session='evaluate')¶

update_on_match(specimen, surrogate_key, values, **kw)¶

Emit an UPDATE statement matching the given specimen.

This is a method-version of oslo_db.sqlalchemy.update_match.update_on_match(); see that function for usage details.

update_returning_pk(values, surrogate_key)¶

Perform an UPDATE, returning the primary key of the matched row.

This is a method-version of oslo_db.sqlalchemy.update_match.update_returning_pk(); see that function for usage details.

class oslo_db.sqlalchemy.session.Session(bind=None, autoflush=True, expire_on_commit=True, _enable_transaction_accounting=True, autocommit=False, twophase=False, weak_identity_map=True, binds=None, extension=None, info=None, query_cls=<class 'sqlalchemy.orm.query.Query'>)¶

Bases: sqlalchemy.orm.session.Session

oslo.db-specific Session subclass.