20
20
Transactions provide hooks to allow data objects (i.e. inventory weaves or
21
21
the revision-history file) to be placed in a registry and retrieved later
22
during the same transaction. Transactions in bzr are not atomic - they
23
depend on data ordering of writes, so we do not have commit or rollback
24
facilities at the transaction level.
22
during the same transaction. This allows for repeated read isolation. At
23
the end of a transaction, a callback is issued to each registered changed
24
item informing it whether it should commit or not. We provide a two layer
25
facility - domain objects are notified first, then data objects.
26
27
Read only transactions raise an assert when objects are listed as dirty
27
against them - preventing unintended writes.
28
against them - preventing unintended writes. Once all the data storage is
29
hooked into this facility, it might be nice to have a readonly transaction
30
that just excepts on commit, for testing or simulating of things.
29
Write transactions preserve dirty objects in the cache, though due to the
30
write ordering approach we use for consistency 'dirty' is a misleading term.
31
A dirty object is one we have modified.
32
Write transactions queue all changes in the transaction (which may in the
33
future involve writing them to uncommitted atomic files in preparation
34
for commit - i.e. on network connections where latency matters) and then
35
notify each object of commit or rollback.
33
37
Both read and write transactions *may* flush unchanged objects out of
34
memory, unless they are marked as 'precious' which indicates that
35
repeated reads cannot be obtained if the object is ejected, or that
36
the object is an expensive one for obtaining.
38
memory, unless they are marked as 'preserve' which indicates that
39
repeated reads cannot be obtained if the object is ejected.
46
49
class ReadOnlyTransaction(object):
47
50
"""A read only unit of work for data objects."""
53
"""ReadOnlyTransactions cannot commit."""
54
raise errors.CommitNotPossible('In a read only transaction')
50
"""Clean up this transaction."""
57
"""Clean up this transaction
59
This will rollback on transactions that can if they have nto been
52
63
def __init__(self):
53
64
super(ReadOnlyTransaction, self).__init__()
58
69
self._precious_objects = set()
60
def is_clean(self, an_object):
61
"""Return True if an_object is clean."""
62
return an_object in self._clean_objects
64
71
def register_clean(self, an_object, precious=False):
65
72
"""Register an_object as being clean.
76
83
def register_dirty(self, an_object):
77
84
"""Register an_object as being dirty."""
78
raise errors.ReadOnlyObjectDirtiedError(an_object)
85
raise errors.ReadOnlyError(
86
"Cannot dirty objects in a read only transaction")
89
"""Let people call this even though nothing has to happen."""
80
91
def set_cache_size(self, size):
81
92
"""Set a new cache size."""
113
"""Read only transactions do not allow writes."""
116
class WriteTransaction(ReadOnlyTransaction):
117
"""A write transaction
119
- caches domain objects
120
- clean objects can be removed from the cache
121
- dirty objects are retained.
125
"""Clean up this transaction."""
126
for thing in self._dirty_objects:
127
callback = getattr(thing, 'transaction_finished', None)
128
if callback is not None:
132
super(WriteTransaction, self).__init__()
133
self._dirty_objects = set()
135
def is_dirty(self, an_object):
136
"""Return True if an_object is dirty."""
137
return an_object in self._dirty_objects
139
def register_dirty(self, an_object):
140
"""Register an_object as being dirty.
142
Dirty objects are not ejected from the identity map
143
until the transaction finishes and get informed
144
when the transaction finishes.
146
self._dirty_objects.add(an_object)
147
if self.is_clean(an_object):
148
self._clean_objects.remove(an_object)
149
del self._clean_queue[self._clean_queue.index(an_object)]
153
"""Write transactions allow writes."""
157
125
class PassThroughTransaction(object):
158
126
"""A pass through transaction
161
- nothing ever gets into the identity map.
128
- all actions are committed immediately.
129
- rollback is not supported.
130
- commit() is a no-op.
134
"""PassThroughTransactions have nothing to do."""
164
136
def finish(self):
165
"""Clean up this transaction."""
166
for thing in self._dirty_objects:
167
callback = getattr(thing, 'transaction_finished', None)
168
if callback is not None:
137
"""Clean up this transaction
139
This will rollback on transactions that can if they have nto been
171
143
def __init__(self):
172
144
super(PassThroughTransaction, self).__init__()
173
145
self.map = NullIdentityMap()
174
self._dirty_objects = set()
176
147
def register_clean(self, an_object, precious=False):
177
148
"""Register an_object as being clean.
183
154
def register_dirty(self, an_object):
184
"""Register an_object as being dirty.
186
Dirty objects get informed
187
when the transaction finishes.
189
self._dirty_objects.add(an_object)
155
"""Register an_object as being dirty."""
158
"""Cannot rollback a pass through transaction."""
159
raise errors.AlreadyCommitted
191
161
def set_cache_size(self, ignored):
192
162
"""Do nothing, we are passing through."""
195
"""Pass through transactions allow writes."""
added
removed
bzrlib/transactions.py
