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Transactions provide hooks to allow data objects (i.e. inventory weaves or
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the revision-history file) to be placed in a registry and retrieved later
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during the same transaction. Transactions in bzr are not atomic - they
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depend on data ordering of writes, so we do not have commit or rollback
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facilities at the transaction level.
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during the same transaction. This allows for repeated read isolation. At
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the end of a transaction, a callback is issued to each registered changed
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item informing it whether it should commit or not. We provide a two layer
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facility - domain objects are notified first, then data objects.
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Read only transactions raise an assert when objects are listed as dirty
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against them - preventing unintended writes.
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against them - preventing unintended writes. Once all the data storage is
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hooked into this facility, it might be nice to have a readonly transaction
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that just excepts on commit, for testing or simulating of things.
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Write transactions preserve dirty objects in the cache, though due to the
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write ordering approach we use for consistency 'dirty' is a misleading term.
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A dirty object is one we have modified.
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Write transactions queue all changes in the transaction (which may in the
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future involve writing them to uncommitted atomic files in preparation
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for commit - i.e. on network connections where latency matters) and then
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notify each object of commit or rollback.
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Both read and write transactions *may* flush unchanged objects out of
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memory, unless they are marked as 'precious' which indicates that
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repeated reads cannot be obtained if the object is ejected, or that
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the object is an expensive one for obtaining.
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memory, unless they are marked as 'preserve' which indicates that
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repeated reads cannot be obtained if the object is ejected.
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class ReadOnlyTransaction(object):
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"""A read only unit of work for data objects."""
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"""ReadOnlyTransactions cannot commit."""
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raise errors.CommitNotPossible()
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"""Clean up this transaction."""
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"""Clean up this transaction
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This will rollback on transactions that can if they have nto been
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def __init__(self):
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super(ReadOnlyTransaction, self).__init__()
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self._precious_objects = set()
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def is_clean(self, an_object):
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"""Return True if an_object is clean."""
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return an_object in self._clean_objects
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def register_clean(self, an_object, precious=False):
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"""Register an_object as being clean.
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"""Register an_object as being dirty."""
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raise errors.ReadOnlyObjectDirtiedError(an_object)
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"""Let people call this even though nothing has to happen."""
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def set_cache_size(self, size):
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"""Set a new cache size."""
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"""Read only transactions do not allow writes."""
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class WriteTransaction(ReadOnlyTransaction):
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"""A write transaction
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- caches domain objects
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- clean objects can be removed from the cache
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- dirty objects are retained.
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"""Clean up this transaction."""
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for thing in self._dirty_objects:
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callback = getattr(thing, 'transaction_finished', None)
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if callback is not None:
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super(WriteTransaction, self).__init__()
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self._dirty_objects = set()
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def is_dirty(self, an_object):
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"""Return True if an_object is dirty."""
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return an_object in self._dirty_objects
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def register_dirty(self, an_object):
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"""Register an_object as being dirty.
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Dirty objects are not ejected from the identity map
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until the transaction finishes and get informed
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when the transaction finishes.
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self._dirty_objects.add(an_object)
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if self.is_clean(an_object):
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self._clean_objects.remove(an_object)
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del self._clean_queue[self._clean_queue.index(an_object)]
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"""Write transactions allow writes."""
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class PassThroughTransaction(object):
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"""A pass through transaction
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- nothing ever gets into the identity map.
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- all actions are committed immediately.
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- rollback is not supported.
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- commit() is a no-op.
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"""PassThroughTransactions have nothing to do."""
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def finish(self):
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"""Clean up this transaction."""
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for thing in self._dirty_objects:
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callback = getattr(thing, 'transaction_finished', None)
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if callback is not None:
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"""Clean up this transaction
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This will rollback on transactions that can if they have nto been
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def __init__(self):
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super(PassThroughTransaction, self).__init__()
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self.map = NullIdentityMap()
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self._dirty_objects = set()
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def register_clean(self, an_object, precious=False):
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"""Register an_object as being clean.
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def register_dirty(self, an_object):
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"""Register an_object as being dirty.
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Dirty objects get informed
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when the transaction finishes.
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self._dirty_objects.add(an_object)
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"""Register an_object as being dirty."""
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"""Cannot rollback a pass through transaction."""
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raise errors.AlreadyCommitted
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def set_cache_size(self, ignored):
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"""Do nothing, we are passing through."""
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"""Pass through transactions allow writes."""
added
removed
bzrlib/transactions.py
