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Savepoints
==========
Savepoints provide a way to save to disk intermediate work done during
a transaction allowing:
- partial transaction (subtransaction) rollback (abort)
- state of saved objects to be freed, freeing on-line memory for other
uses
Savepoints make it possible to write atomic subroutines that don't
make top-level transaction commitments.
Applications
------------
To demonstrate how savepoints work with transactions, we'll show an example.
>>> import ZODB.tests.util
>>> db = ZODB.tests.util.DB()
>>> connection = db.open()
>>> root = connection.root()
>>> root['name'] = 'bob'
As with other data managers, we can commit changes:
>>> import transaction
>>> transaction.commit()
>>> root['name']
'bob'
and abort changes:
>>> root['name'] = 'sally'
>>> root['name']
'sally'
>>> transaction.abort()
>>> root['name']
'bob'
Now, lets look at an application that manages funds for people.
It allows deposits and debits to be entered for multiple people.
It accepts a sequence of entries and generates a sequence of status
messages. For each entry, it applies the change and then validates
the user's account. If the user's account is invalid, we role back
the change for that entry. The success or failure of an entry is
indicated in the output status. First we'll initialize some accounts:
>>> root['bob-balance'] = 0.0
>>> root['bob-credit'] = 0.0
>>> root['sally-balance'] = 0.0
>>> root['sally-credit'] = 100.0
>>> transaction.commit()
Now, we'll define a validation function to validate an account:
>>> def validate_account(name):
... if root[name+'-balance'] + root[name+'-credit'] < 0:
... raise ValueError('Overdrawn', name)
And a function to apply entries. If the function fails in some
unexpected way, it rolls back all of it's changes and
prints the error:
>>> def apply_entries(entries):
... savepoint = transaction.savepoint()
... try:
... for name, amount in entries:
... entry_savepoint = transaction.savepoint()
... try:
... root[name+'-balance'] += amount
... validate_account(name)
... except ValueError, error:
... entry_savepoint.rollback()
... print 'Error', str(error)
... else:
... print 'Updated', name
... except Exception, error:
... savepoint.rollback()
... print 'Unexpected exception', error
Now let's try applying some entries:
>>> apply_entries([
... ('bob', 10.0),
... ('sally', 10.0),
... ('bob', 20.0),
... ('sally', 10.0),
... ('bob', -100.0),
... ('sally', -100.0),
... ])
Updated bob
Updated sally
Updated bob
Updated sally
Error ('Overdrawn', 'bob')
Updated sally
>>> root['bob-balance']
30.0
>>> root['sally-balance']
-80.0
If we give provide entries that cause an unexpected error:
>>> apply_entries([
... ('bob', 10.0),
... ('sally', 10.0),
... ('bob', '20.0'),
... ('sally', 10.0),
... ])
Updated bob
Updated sally
Unexpected exception unsupported operand type(s) for +=: 'float' and 'str'
Because the apply_entries used a savepoint for the entire function,
it was able to rollback the partial changes without rolling back
changes made in the previous call to apply_entries:
>>> root['bob-balance']
30.0
>>> root['sally-balance']
-80.0
If we now abort the outer transactions, the earlier changes will go
away:
>>> transaction.abort()
>>> root['bob-balance']
0.0
>>> root['sally-balance']
0.0