This is a brief introduction to the Python transaction module, which was part of ZODB but now is a standalone package.

1. Introduction to the transaction package Carlos de la Guardia
This is a brief introduction to the Python transaction module, which was part of ZODB
but now is a standalone package.
How transactions work
An introduction to Plone's transaction mechanism
Carlos de la Guardia
Plone architecture
Many in the Plone community know about this:
_____________ ____________ ____________
| | | | | |
| Plone |­­­| Zope |­­­| ZODB |
|_____________| |____________| |____________|
ZODB
The ZODB is a key component of Plone. It offers:
● Transparent persistence for Python objects.
● Scalability with ZEO (and now ZRS).
● Transactions.
Transaction support is not part of the ZODB core. Instead, the support is provided by the
transaction package.
Transaction concepts
● Transaction (duh!)
● Transaction manager
● Data manager
● Two­phase commit
● Savepoint
Transactions in Plone
● In plone, transactions are handled transparently.
● If you use the undo facility, you use transactions.
● Sometimes using the features directly can be useful.
● Transaction can be imported in your plone code.
● Useful for long­running processes like import operations.

2. Import script
import transaction
for seal in seals:
accession_no = seal.strip()
image1 = open('/seals/' + accession_no + '.o.jpeg')
image2 = open('/seals/' + accession_no + '.r.jpeg')
images += 1
construct(seals_folder,
id=accession_no,
title=accession_no,
obverseImage=image1,
reverseImage=image2)
print "Created %s seals" % images
transaction.commit()
collective.transmogrifier
import transaction
def __iter__(self):
count = 0
for item in self.previous:
count = (count + 1) % self.every
if count == 0:
transaction.savepoint(optimistic=True)
yield item
Use it outside Zope
Even without the ZODB!
The transaction package can be used to provide full transaction support for your
applications.
Outside Plone
● Install the package:
$ easy_install transaction
● Install your backend (or backends):
$ easy install ZODB
$ easy_install sqlalchemy
● If not using ZODB, install or create a data manager:
$ easy_install zope.sqlalchemy
Using ZODB

3. from ZODB import DB, FileStorage
import transaction
storage = FileStorage.FileStorage('test.fs')
db = DB(storage)
connection = db.open()
root = connection.root()
root['example'] = 'Some data'
transaction.commit()
Using SQLAlchemy
from sqlalchemy import create_engine
engine = create_engine('sqlite:///:memory:')
from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
login = Column(String)
fullname = Column(String)
Base.metadata.create_all(engine)
ZopeTransactionExtension
from sqlalchemy.orm import sessionmaker
from zope.sqlalchemy import ZopeTransactionExtension
Session = sessionmaker(bind=engine,
extension=ZopeTransactionExtension())
session = Session()
import transaction
session.add(User(id=1, login='John', fullname='John Smith'))
transaction.commit()
Aborting transactions
from sqlalchemy.orm import sessionmaker
from zope.sqlalchemy import ZopeTransactionExtension
Session = sessionmaker(bind=engine,
extension=ZopeTransactionExtension())
session = Session()
import transaction
session.add(User(id=2, login='John', fullname='John Smith'))
transaction.abort()

4. Concurrency
● Has to be handled by the developer.
● 'Transient errors' are common.
● If at first you don't succeed, try again.
● Transaction manager looks for should_retry method.
zope.sqlalchemy
def should_retry(self, error):
if isinstance(error, ConcurrentModificationError):
return True
if isinstance(error, DBAPIError):
orig = error.orig
for error_type, test in _retryable_errors:
if isinstance(orig, error_type):
if test is None:
return True
if test(orig):
return True
Transaction aware middleware
● WSGI in general: repoze.tm2
● Pyramid: pyramid_tm
Other transaction features
● Multiple data managers
● Before and after commit hooks
● Synchronizers
● Notes
Thank You!
● Transaction documentation:
http://zodb.readthedocs.org/en/latest/transactions.html
● Contact me:
cguardia on twitter
cguardia@yahoo.com
#pyramid IRC channel