A persistent object can be in one of several states:
- Unsaved
The object has been created but not saved in a data manager.
In this state, the _p_changed attribute is non-None and false
and the _p_jar attribute is None.
- Saved
The object has been saved and has not been changed since it was saved.
In this state, the _p_changed attribute is non-None and false
and the _p_jar attribute is set to a data manager.
- Sticky
This state is identical to the saved state except that the
object cannot transition to the ghost state. This is a special
state used by C methods of persistent objects to make sure that
state is not unloaded in the middle of computation.
In this state, the _p_changed attribute is non-None and false
and the _p_jar attribute is set to a data manager.
There is no Python API for detecting whether an object is in the
sticky state.
- Changed
The object has been changed.
In this state, the _p_changed attribute is true
and the _p_jar attribute is set to a data manager.
- Ghost
the object is in memory but its state has not been loaded from
the database (or its state has been unloaded). In this state,
the object doesn't contain any application data.
In this state, the _p_changed attribute is None, and the _p_jar
attribute is set to the data manager from which the object was
obtained.
In all the above, _p_oid (the persistent object id) is set when
_p_jar first gets set.
The following state transitions are possible:
- Unsaved -> Saved
This transition occurs when an object is saved in the
database. This usually happens when an unsaved object is added
to (e.g. as an attribute or item of) a saved (or changed) object
and the transaction is committed.
- Saved -> Changed
Sticky -> Changed
Ghost -> Changed
This transition occurs when someone sets an attribute or sets
_p_changed to a true value on a saved, sticky or ghost object. When
the transition occurs, the persistent object is required to call the
register() method on its data manager, passing itself as the
only argument.
Prior to ZODB 3.6, setting _p_changed to a true value on a ghost object
was ignored (the object remained a ghost, and getting its _p_changed
attribute continued to return None).
- Saved -> Sticky
This transition occurs when C code marks the object as sticky to
prevent its deactivation.
- Saved -> Ghost
This transition occurs when a saved object is deactivated or
invalidated. See discussion below.
- Sticky -> Saved
This transition occurs when C code unmarks the object as sticky to
allow its deactivation.
- Changed -> Saved
This transition occurs when a transaction is committed. After
saving the state of a changed object during transaction commit,
the data manager sets the object's _p_changed to a non-None false
value.
- Changed -> Ghost
This transition occurs when a transaction is aborted. All changed
objects are invalidated by the data manager by an abort.
- Ghost -> Saved
This transition occurs when an attribute or operation of a ghost
is accessed and the object's state is loaded from the database.
Note that there is a separate C API that is not included here.
The C API requires a specific data layout and defines the sticky
state.
About Invalidation, Deactivation and the Sticky & Ghost States
The sticky state is intended to be a short-lived state, to prevent
an object's state from being discarded while we're in C routines. It
is an error to invalidate an object in the sticky state.
Deactivation is a request that an object discard its state (become
a ghost). Deactivation is an optimization, and a request to
deactivate may be ignored. There are two equivalent ways to
request deactivation:
- call _p_deactivate()
- set _p_changed to None
There are two ways to invalidate an object: call the
_p_invalidate() method (preferred) or delete its _p_changed
attribute. This cannot be ignored, and is used when semantics
require invalidation. Normally, an invalidated object transitions
to the ghost state. However, some objects cannot be ghosts. When
these objects are invalidated, they immediately reload their state
from their data manager, and are then in the saved state.
"""
_p_jar = Attribute('The data manager for the object.\n\n The data manager implements the IPersistentDataManager interface.\n If there is no data manager, then this is None.\n ')
_p_oid = Attribute("The object id.\n\n It is up to the data manager to assign this.\n The special value None is reserved to indicate that an object\n id has not been assigned. Non-None object ids must be non-empty\n strings. The 8-byte string '\x00'*8 (8 NUL bytes) is reserved to\n identify the database root object.\n ")
_p_changed = Attribute("The persistent state of the object.\n\n This is one of:\n\n None -- The object is a ghost.\n\n false but not None -- The object is saved (or has never been saved).\n\n true -- The object has been modified since it was last saved.\n\n The object state may be changed by assigning or deleting this\n attribute; however, assigning None is ignored if the object is\n not in the saved state, and may be ignored even if the object is\n in the saved state.\n\n At and after ZODB 3.6, setting _p_changed to a true value for a ghost\n object activates the object; prior to 3.6, setting _p_changed to a\n true value on a ghost object was ignored.\n\n Note that an object can transition to the changed state only if\n it has a data manager. When such a state change occurs, the\n 'register' method of the data manager must be called, passing the\n persistent object.\n\n Deleting this attribute forces invalidation independent of\n existing state, although it is an error if the sticky state is\n current.\n ")
_p_serial = Attribute('The object serial number.\n\n This member is used by the data manager to distiguish distinct\n revisions of a given persistent object.\n\n This is an 8-byte string (not Unicode).\n ')
def __getstate__():
'''Get the object data.
The state should not include persistent attributes ("_p_name").
The result must be picklable.
'''
pass
def __setstate__(state):
'''Set the object data.
'''
pass
def _p_activate():
'''Activate the object.
Change the object to the saved state if it is a ghost.
'''
pass
def _p_deactivate():
'''Deactivate the object.
Possibly change an object in the saved state to the
ghost state. It may not be possible to make some persistent
objects ghosts, and, for optimization reasons, the implementation
may choose to keep an object in the saved state.
'''
pass
def _p_invalidate():
'''Invalidate the object.
Invalidate the object. This causes any data to be thrown
away, even if the object is in the changed state. The object
is moved to the ghost state; further accesses will cause
object data to be reloaded.
'''
pass
class IPersistentNoReadConflicts(IPersistent):
def _p_independent():
'''Hook for subclasses to prevent read conflict errors.
A specific persistent object type can define this method and
have it return true if the data manager should ignore read
conflicts for this object.
'''
pass
class IPersistentDataManager(Interface):
'''Provide services for managing persistent state.
This interface is used by a persistent object to interact with its
data manager in the context of a transaction.
'''
def setstate(object):
"""Load the state for the given object.
The object should be in the ghost state. The object's state will be
set and the object will end up in the saved state.
The object must provide the IPersistent interface.
"""
pass
def oldstate(obj, tid):
"""Return copy of 'obj' that was written by transaction 'tid'.
The returned object does not have the typical metadata (_p_jar, _p_oid,
_p_serial) set. I'm not sure how references to other peristent objects
are handled.
Parameters
obj: a persistent object from this Connection.
tid: id of a transaction that wrote an earlier revision.
Raises KeyError if tid does not exist or if tid deleted a revision of
obj.
"""
pass
def register(object):
'''Register an IPersistent with the current transaction.
This method must be called when the object transitions to
the changed state.
A subclass could override this method to customize the default
policy of one transaction manager for each thread.
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