zope.app.locking 3.4.0

Detailed Dcoumentation

Object Locking

This package provides a framework for object locking. The implementation is intended to provide a simple general-purpose locking architecture upon which other locking applications can be built (WebDAV locking, for example).

The locking system is purely advisory - it provides a way to associate a lock with an object, but it does not enforce locking in any way. It is up to application-level code to ensure that locked objects are restricted in a way appropriate to the application.

The Zope 3 locking model defines interfaces and a default implementation that:

• allows for a single lock on an object, owned by a specific principal

• does not necessarily impose inherent semantic meaning (exclusive vs. non-exclusive, write vs. read) on locks, though it will provide fields that higher-level application components can use to implement and enforce such semantics

• can potentially be used to build more ambitious locking mechanisms (such as WebDAV locking equivalent to Zope 2)

• supports common use cases that have been uncovered in several years of development of real-world applications (such as reporting all of the objects locked by a given user)

The Zope3 locking architecture defines an ILockable interface and provides a default adapter implementation that requires only that an object be adaptable to IKeyReference. All persistent objects can be adapted to this interface by default in Zope 3, so in practice all persistent objects are lockable.

The default ILockable adapter implementation provides support for:

• locking and unlocking an object

• breaking an existing lock on an object

• obtaining the lock information for an object

Locking operations (lock, unlock, break lock) fire events that may be handled by applications or other components to interact with the locking system in a loosely-coupled way.

Lock information is accessible through an object that supports the ILockInfo interface. The ILockInfo interface implies IAnnotatable, so that other locking implementations (superseding or complementing the default implementation) can store more information if needed to support extended locking semantics.

The locking architecture also supports an efficient method of lock tracking that allows you to determine what locks are held on objects. The default implementation provides an ILockTracker utility that can be used by applications to quickly find all objects locked by a particular principal.

Locking essentials

Normally, locking is provided by the default locking implementation. In this example, we’ll create a simple content class. The content class is persistent, which allows us to use the default locking adapters and utilities.

>>> import persistent

>>> class Content(persistent.Persistent):
...     """A sample content object"""
...
...     def __init__(self, value):
...         self.value = value
...
...     def __call__(self):
...         return self
...
...     def __hash__(self):
...         return self.value
...
...     def __cmp__(self, other):
...         return cmp(self.value, other.value)

Now we will create a few sample objects to work with:

>>> item1 = Content("item1")
>>> item1.__name__ = "item1"

>>> item2 = Content("item2")
>>> item2.__name__ = "item2"

>>> item3 = Content("item3")
>>> item3.__name__ = "item3"

It is possible to test whether an object supports locking by attempting to adapt it to the ILockable interface:

>>> from zope.app.locking.interfaces import ILockable
>>> from zope.app.locking.interfaces import ILockInfo

>>> ILockable(item1, None)
<Locking adapter for...

>>> ILockable(42, None)

There must be an active interaction to use locking, to allow the framework to determine the principal performing locking operations. This example sets up some sample principals and a helper to switch principals for further examples:

>>> class FauxPrincipal:
...    def __init__(self, id):
...        self.id = id

>>> britney = FauxPrincipal('britney')
>>> tim = FauxPrincipal('tim')

>>> class FauxParticipation:
...     interaction = None
...     def __init__(self, principal):
...         self.principal = principal

>>> import zope.security.management
>>> def set_principal(principal):
...     if zope.security.management.queryInteraction():
...         zope.security.management.endInteraction()
...     participation = FauxParticipation(principal)
...     zope.security.management.newInteraction(participation)

>>> set_principal(britney)

Now, let’s look at basic locking. To perform locking operations, we first have to adapt an object to ILockable:

>>> obj = ILockable(item1)
>>> from zope.interface.verify import verifyObject
>>> verifyObject(ILockable, obj)
True

We can ask if the object is locked:

>>> obj.locked()
False

If it were locked, we could get the id of the principal that owns the lock. Since it is not locked, this will return None:

>>> obj.locker()

Now let’s lock the object. Note that the lock method return an instance of an object that implements ILockInfo on success:

>>> info = obj.lock()
>>> verifyObject(ILockInfo, info)
True

>>> obj.locked()
True

>>> obj.locker()
'britney'

Methods are provided to check whether the current principal already has the lock on an object and whether the lock is already owned by a different principal:

>>> obj.ownLock()
True

>>> obj.isLockedOut()
False

If we switch principals, we see that the answers reflect the current principal:

>>> set_principal(tim)
>>> obj.ownLock()
False

>>> obj.isLockedOut()
True

A principal can only release his or her own locks:

>>> obj.unlock()
Traceback (most recent call last):
  ...
LockingError: Principal is not lock owner

If we switch back to the original principal, we see that the original principal can unlock the object:

>>> set_principal(britney)
>>> obj.unlock()

There is a mechanism for breaking locks that does not take the current principal into account. This will break any existing lock on an object:

>>> obj.lock()
<...LockInfo...>

>>> set_principal(tim)
>>> obj.locked()
True

>>> obj.breaklock()
>>> obj.locked()
False

Locks can be created with an optional timeout. If a timeout is provided, it should be an integer number of seconds from the time the lock is created.

>>> # fake time function to avoid a time.sleep in tests!
>>> import time
>>> def faketime():
...    return time.time() + 3600.0

>>> obj.lock(timeout=10)
<...LockInfo...>

>>> obj.locked()
True

>>> import zope.app.locking.storage
>>> zope.app.locking.storage.timefunc = faketime
>>> obj.locked()
False

(Note that we undo our time hack in the tearDown of this module.)

Finally, it is possible to explicitly get an ILockInfo object that contains the lock information for the object. Note that locks that do not have a timeout set have a timeout value of None.

>>> obj = ILockable(item2)
>>> obj.lock()
<...LockInfo...>

>>> info = obj.getLockInfo()
>>> info.principal_id
'tim'
>>> info.timeout

It is possible to get the object associated with a lock directly from an ILockInfo instance:

>>> target = info.target
>>> target.__name__ == 'item2'
True

The ILockInfo interface extends the IMapping interface, so application code can store extra information on locks if necessary. It is recommended that keys for extra data use qualified names following the convention that is commonly used for annotations:

>>> info['my.namespace.extra'] = 'spam'
>>> info['my.namespace.extra']
'spam'
>>> obj.unlock()
>>> obj.locked()
False

Lock tracking

It is often desirable to be able to report on the currently held locks in a system (particularly on a per-user basis), without requiring an expensive brute-force search. An ILockTracker utility allows an application to get the current locks for a principal, or all current locks:

>>> set_principal(tim)
>>> obj = ILockable(item2)
>>> obj.lock()
<...LockInfo...>

>>> set_principal(britney)
>>> obj = ILockable(item3)
>>> obj.lock()
<...LockInfo...>

>>> from zope.app.locking.interfaces import ILockTracker
>>> from zope.app.zapi import getUtility
>>> util = getUtility(ILockTracker)
>>> verifyObject(ILockTracker, util)
True

>>> items = util.getLocksForPrincipal('britney')
>>> len(items) == 1
True

>>> items = util.getAllLocks()
>>> len(items) >= 2
True

These methods allow an application to create forms and other code that performs unlocking or breaking of locks on sets of objects:

>>> items = util.getAllLocks()
>>> for item in items:
...     obj = ILockable(item.target)
...     obj.breaklock()

>>> items = util.getAllLocks()
>>> len(items)
0

The lock storage utility provides further capabilities, and is a part of the standard lock adapter implementation, but the ILockable interface does not depend on ILockStorage. Other implementations of ILockable may not use ILockStorage. However, if used by the adapter, it provides useful capabilties.

>>> from zope.app.locking.interfaces import ILockStorage
>>> util = getUtility(ILockStorage)
>>> verifyObject(ILockStorage, util)
True

Locking events

Locking operations (lock, unlock, break lock) fire events that can be used by applications. Note that expiration of a lock does not fire an event (because the current implementation uses a lazy expiration approach).

>>> import zope.event

>>> def log_event(event):
...     print event

>>> zope.event.subscribers.append(log_event)

>>> obj = ILockable(item2)
>>> obj.lock()
LockedEvent ...

>>> obj.unlock()
UnlockedEvent ...

>>> obj.lock()
LockedEvent ...

>>> obj.breaklock()
BreakLockEvent ...

TALES conditions based on locking

TALES expressions can use a named path adapter to get information about the lock status for an object, including whether or not the object can be locked. The default registration for this adapter uses the name “locking”, so a condition might be expressed like “context/locking:ownLock”, for example.

For objects that aren’t lockable, the adapter provides information that makes sense:

>>> from zope.component import getAdapter
>>> from zope.traversing.interfaces import IPathAdapter

>>> ns = getAdapter(42, IPathAdapter, "locking")
>>> ns.lockable
False

>>> ns.locked
False

>>> ns.lockedOut
False

>>> ns.ownLock
False

Using an object that’s lockable, but unlocked, also gives the expected results:

>>> ns = getAdapter(item1, IPathAdapter, "locking")
>>> ns.lockable
True

>>> ns.locked
False

>>> ns.lockedOut
False

>>> ns.ownLock
False

If we lock the object, the adapter indicates that the object is locked and that we own it:

>>> ob = ILockable(item1)
>>> ob.lock()
LockedEvent for ...

>>> ns.lockable
True

>>> ns.locked
True

>>> ns.lockedOut
False

>>> ns.ownLock
True

3.4.0 (2007-10-25)

• Initial release independent of the main Zope tree.