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#
# Copyright (c) 2001, 2002 Zope Corporation and Contributors.
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#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL). A copy of the ZPL should accompany this distribution.
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"""Persistence Interfaces
$Id$
"""
try:
from zope.interface import Interface
from zope.interface import Attribute
except ImportError:
# just allow the module to compile if zope isn't available
class Interface(object):
pass
def Attribute(s):
return s
class IPersistent(Interface):
"""Python persistent interface
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 up-to-date 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, currently, no official way to detect 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 has been unloaded). In this state, the object
doesn't contain any data.
The following state transactions 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
This transition occurs when someone sets an attribute or sets
_p_changed to a true value on an up-to-date or sticky
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.
- Saved -> Sticky
This transition occurs when C code marks the object as sticky to
prevent its deactivation and transition to the ghost state.
- Saved -> Ghost
This transition occurs when an saved object is deactivated, by:
calling _p_deactivate, setting _p_changed to None, or deleting
_p_changed.
- Sticky -> Saved
This transition occurs when C code unmarks the object as sticky to
allow its deactivation and transition to the ghost state.
- Changed -> Saved
This transition occurs when a transaction is committed.
The data manager affects the transaction by setting _p_changed
to a true value.
- Changed -> Ghost
This transition occurs when a transaction is aborted.
The data manager affects the transaction by deleting _p_changed.
- 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 that is used to prevent object deactivation while in C
routines.
"""
_p_jar=Attribute(
"""The data manager for the object
The data manager implements the IPersistentDataManager interface.
If there is no data manager, then this is None.
""")
_p_oid=Attribute(
"""The object id
It is up to the data manager to assign this.
The special value None is reserved to indicate that an object
id has not been assigned.
""")
_p_changed=Attribute(
"""The persistent state of the object
This is one of:
None -- The object is a ghost. It is not active.
false -- The object is saved (or has never been saved).
true -- The object has been modified.
The object state may be changed by assigning this attribute,
however, assigning None is ignored if the object is not in the
up-to-date state.
Note that an object can change to the modified state only if
it has a data manager. When such a state change occurs, the
'register' method of the data manager is called, passing the
persistent object.
Deleting this attribute forces deactivation independent of
existing state.
Note that an attribute is used for this to allow optimized
cache implementations.
""")
_p_serial=Attribute(
"""The object serial number
This is an arbitrary object.
""")
_p_atime=Attribute(
"""The integer object access time, in seconds, modulus one day
XXX When does a day start, the current implementation appears
to use gmtime, but this hasn't be explicitly specified.
XXX Why just one day?
""")
def __getstate__():
"""Get the object state data
The state should not include persistent attributes ("_p_name")
"""
def __setstate__(state):
"""Set the object state data
Note that this does not affect the object's persistent state.
"""
def _p_activate():
"""Activate the object
Change the object to the up-to-date state if it is a ghost.
"""
def _p_deactivate():
"""Deactivate the object
If possible, change an object in the up-to-date state to the
ghost state. It may not be possible to make some persistent
objects ghosts.
"""
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.
"""
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 deactivated (ghost) state.
The object's state will be set and the object will end up
in the up-to-date state.
The object must implement the IPersistent interface.
"""
def register(object):
"""Register a IPersistent with the current transaction.
This method provides some insulation of the persistent object
from details of transaction management. For example, it allows
the use of per-database-connection rather than per-thread
transaction managers.
A persistent object should not register with its data manager
more than once during a single transaction. XXX should is too
wishy-washy; we should probably guarantee that this is true,
and it might be.
"""
def mtime(object):
"""Return the modification time of the object.
The modification time may not be known, in which case None
is returned.
"""
class ICache(Interface):
"""In-memory object cache
The cache serves two purposes. It peforms pointer swizzling, and
it keeps a bounded set of recently used but otherwise unreferenced
in objects to avoid the cost of re-loading them.
Pointer swizzling is the process of converting between persistent
object ids and Python object ids. When a persistent object is
serialized, its references to other persistent objects are
represented as persitent object ids (oids). When the object is
unserialized, the oids are converted into references to Python
objects. If several different serialized objects refer to the
same object, they must all refer to the same object when they are
unserialized.
A cache stores persistent objects, but it treats ghost objects and
non-ghost or active objects differently. It has weak references
to ghost objects, because ghost objects are only stored in the
cache to satisfy the pointer swizzling requirement. It has strong
references to active objects, because it caches some number of
them even if they are unreferenced.
The cache keeps some number of recently used but otherwise
unreferenced objects in memory. We assume that there is a good
chance the object will be used again soon, so keeping it memory
avoids the cost of recreating the object.
An ICache implementation is intended for use by an
IPersistentDataManager.
"""
def get(oid):
"""Return the object from the cache or None."""
def set(oid, obj):
"""Store obj in the cache under oid.
obj must implement IPersistent
"""
def remove(oid):
"""Remove oid from the cache if it exists."""
def invalidate(oids):
"""Make all of the objects in oids ghosts.
`oids` is an iterable object that yields oids.
The cache must attempt to change each object to a ghost by
calling _p_deactivate().
If an oid is not in the cache, ignore it.
"""
def clear():
"""Invalidate all the active objects."""
def activate(oid):
"""Notification that object oid is now active.
The caller is notifying the cache of a state change.
Raises LookupError if oid is not in cache.
"""
def shrink():
"""Remove excess active objects from the cache."""
def statistics():
"""Return dictionary of statistics about cache size.
Contains at least the following keys:
active -- number of active objects
ghosts -- number of ghost objects
"""