z3c.saconfig 0.9.1

Minimal SQLAlchemy ORM session configuration for Zope

z3c.saconfig

Introduction

This aim of this package is to offer a simple but flexible way to configure SQLAlchemy’s scoped session support using the Zope component architecture. This package is based on zope.sqlalchemy, which offers transaction integration between Zope and SQLAlchemy.

We sketch out two main scenarios here:

• one database per Zope instance.

• one database per site (or Grok application) in a Zope instance (and thus multiple databases per Zope instance).

GloballyScopedSession (one database per Zope instance)

The simplest way to set up SQLAlchemy for Zope is to have a single thread-scoped session that’s global to your entire Zope instance. Multiple applications will all share this session. The engine is set up with a global utility.

We use the SQLAlchemy sqlalchemy.ext.declarative extension to define some tables and classes:

>>> from sqlalchemy import *
>>> from sqlalchemy.ext.declarative import declarative_base
>>> from sqlalchemy.orm import relation

>>> Base = declarative_base()
>>> class User(Base):
...     __tablename__ = 'test_users'
...     id = Column('id', Integer, primary_key=True)
...     name = Column('name', String(50))
...     addresses = relation("Address", backref="user")
>>> class Address(Base):
...     __tablename__ = 'test_addresses'
...     id = Column('id', Integer, primary_key=True)
...     email = Column('email', String(50))
...     user_id = Column('user_id', Integer, ForeignKey('test_users.id'))

So far this doesn’t differ from the zope.sqlalchemy example. We now arrive at the first difference. Instead of making the engine directly, we can set up the engine factory as a (global) utility. This utility makes sure an engine is created and cached for us.

>>> from z3c.saconfig import EngineFactory
>>> engine_factory = EngineFactory(TEST_DSN)

You can pass the parameters you’d normally pass to sqlalchemy.create_engine to EngineFactory.

We now register the engine factory as a global utility using zope.component. Normally you’d use either ZCML or Grok to do this confirmation, but we’ll do it manually here:::

>>> from zope import component
>>> from z3c.saconfig.interfaces import IEngineFactory
>>> component.provideUtility(engine_factory, provides=IEngineFactory)

Note that setting up an engine factory is not actually necessary in the globally scoped use case. You could also just create the engine as a global and pass it as bind when you create the GloballyScopedSession later.

Let’s look up the engine by calling the factory and create the tables in our test database:

>>> engine = engine_factory()
>>> Base.metadata.create_all(engine)

Now as for the second difference from zope.sqlalchemy: how the session is set up and used. We’ll use the GloballyScopedSession utility to implement our session creation:

>>> from z3c.saconfig import GloballyScopedSession

We give the constructor to GloballyScopedSession the parameters you’d normally give to sqlalchemy.orm.create_session, or sqlalchemy.orm.sessionmaker:

>>> utility = GloballyScopedSession(twophase=TEST_TWOPHASE)

GlobalScopedSession looks up the engine using IEngineFactory if you don’t supply your own bind argument. GloballyScopedSession also automatically sets up the autocommit, autoflush and extension parameters to be the right ones for Zope integration, so normally you wouldn’t need to supply these, but you could pass in your own if you do need it.

We now register this as an IScopedSession utility:

>>> from z3c.saconfig.interfaces import IScopedSession
>>> component.provideUtility(utility, provides=IScopedSession)

We are done with configuration now. As you have seen it involves setting up two utilities, IEngineFactory and IScopedSession, where only the latter is really needed in this globally shared session use case.

After the IScopedSession utility is registered, one can import the Session class from z3c.saconfig. This Session class is like the one you’d produce with sessionmaker from SQLAlchemy. z3c.saconfig.Session` is intended to be the only Session class you’ll ever need, as all configuration and Zope integration is done automatically for you by z3c.saconfig, appropriate the context in Zope where you use it. There is no need to create Session classes yourself with sessionmaker or scoped_sesion anymore.

We can now use the Session class to create a session which will behave according to the utility we provided:

>>> from z3c.saconfig import Session
>>> session = Session()

Now things go the usual zope.sqlalchemy way, which is like SQLAlchemy except you can use Zope’s transaction module:

>>> session.query(User).all()
[]
>>> import transaction
>>> session.add(User(name='bob'))
>>> transaction.commit()

>>> session = Session()
>>> bob = session.query(User).all()[0]
>>> bob.name
u'bob'
>>> bob.addresses
[]

Events

When a new engine is created by an EngineFactory, an IEngineCreatedEvent is fired. This event has an attribute engine that contains the engine that was just created:

>>> from z3c.saconfig.interfaces import IEngineCreatedEvent
>>> @component.adapter(IEngineCreatedEvent)
... def createdHandler(event):
...     print "created engine"
>>> component.provideHandler(createdHandler)
>>> event_engine_factory = EngineFactory(TEST_DSN1)
>>> engine = event_engine_factory()
created engine

Let’s get rid of the event handler again:

>>> sm = component.getSiteManager()
>>> sm.unregisterHandler(None,
...   required=[IEngineCreatedEvent])
True

SiteScopedSession (one database per site)

In the example above we have set up SQLAlchemy with Zope using utilities, but it did not gain us very much, except that you can just use zope.sqlalchemy.Session to get the correct session.

Now we’ll see how we can set up different engines per site by registering the engine factory as a local utility for each one.

In order to make this work, we’ll set up SiteScopedSession instead of GloballyScopedSession. We need to subclass SiteScopedSession first because we need to implement its siteScopeFunc method, which should return a unique ID per site (such as a path retrieved by zope.traversing.api.getPath). We need to implement it here, as z3c.saconfig leaves this policy up to the application or a higher-level framework:

>>> from z3c.saconfig import SiteScopedSession
>>> class OurSiteScopedSession(SiteScopedSession):
...   def siteScopeFunc(self):
...      return getSite().id # the dummy site has a unique id
>>> utility = OurSiteScopedSession()
>>> component.provideUtility(utility, provides=IScopedSession)

We want to register two engine factories, each in a different site:

>>> engine_factory1 = EngineFactory(TEST_DSN1)
>>> engine_factory2 = EngineFactory(TEST_DSN2)

We need to set up the database in both new engines:

>>> Base.metadata.create_all(engine_factory1())
>>> Base.metadata.create_all(engine_factory2())

Let’s now create two sites, each of which will be connected to another engine:

>>> site1 = DummySite(id=1)
>>> site2 = DummySite(id=2)

We set the local engine factories for each site:

>>> sm1 = site1.getSiteManager()
>>> sm1.registerUtility(engine_factory1, provided=IEngineFactory)
>>> sm2 = site2.getSiteManager()
>>> sm2.registerUtility(engine_factory2, provided=IEngineFactory)

Just so we don’t accidentally get it, we’ll disable our global engine factory:

>>> component.provideUtility(None, provides=IEngineFactory)

When we set the site to site1, a lookup of IEngineFactory gets us engine factory 1:

>>> setSite(site1)
>>> component.getUtility(IEngineFactory) is engine_factory1
True

And when we set it to site2, we’ll get engine factory 2:

>>> setSite(site2)
>>> component.getUtility(IEngineFactory) is engine_factory2
True

We can look up our global utility even if we’re in a site:

>>> component.getUtility(IScopedSession) is utility
True

Phew. That was a lot of set up, but basically this is actually just straightforward utility setup code; you should use the APIs or Grok’s grok.local_utility directive to set up local utilities. Now all that is out of the way, we can create a session for site1:

>>> setSite(site1)
>>> session = Session()

The database is still empty:

>>> session.query(User).all()
[]

We’ll add something to this database now:

>>> session.add(User(name='bob'))
>>> transaction.commit()

bob is now there:

>>> session = Session()
>>> session.query(User).all()[0].name
u'bob'

Now we’ll switch to site2:

>>> setSite(site2)

If we create a new session now, we should now be working with a different database, which should still be empty:

>>> session = Session()
>>> session.query(User).all()
[]

We’ll add fred to this database:

>>> session.add(User(name='fred'))
>>> transaction.commit()

Now fred is indeed there:

>>> session = Session()
>>> users = session.query(User).all()
>>> len(users)
1
>>> users[0].name
u'fred'

And bob is still in site1:

>>> setSite(site1)
>>> session = Session()
>>> users = session.query(User).all()
>>> len(users)
1
>>> users[0].name
u'bob'

Engines and Threading

>>> engine = None
>>> def setEngine():
...     global engine
...     engine = engine_factory1()

Engine factories must produce the same engine:

>>> setEngine()
>>> engine is engine_factory1()
True

Even if you call it in a different thread:

>>> import threading
>>> engine = None
>>> t = threading.Thread(target=setEngine)
>>> t.start()
>>> t.join()

>>> engine is engine_factory1()
True

Unless they are reset:

>>> engine_factory1.reset()
>>> engine is engine_factory1()
False

Even engine factories with the same parameters created at (almost) the same time should produce different engines:

>>> EngineFactory(TEST_DSN1)() is EngineFactory(TEST_DSN1)()
False

Configuration using ZCML

A configuration directive is provided to register a database engine factory using ZCML.

>>> from cStringIO import StringIO
>>> from zope.configuration import xmlconfig
>>> import z3c.saconfig
>>> xmlconfig.XMLConfig('meta.zcml', z3c.saconfig)()

Let’s try registering the directory again.

>>> xmlconfig.xmlconfig(StringIO("""
... <configure xmlns="http://namespaces.zope.org/db">
...   <engine name="dummy" url="sqlite:///:memory:" />
... </configure>"""))

>>> component.getUtility(IEngineFactory, name="dummy")
<z3c.saconfig.utility.EngineFactory object at ...>

This time with a setup call.

>>> xmlconfig.xmlconfig(StringIO("""
... <configure xmlns="http://namespaces.zope.org/db">
...   <engine name="dummy2" url="sqlite:///:memory:"
...           setup="z3c.saconfig.tests.engine_subscriber" />
... </configure>"""))
got: Engine(sqlite:///:memory:)

The session directive is provided to register a scoped session utility:

>>> xmlconfig.xmlconfig(StringIO("""
... <configure xmlns="http://namespaces.zope.org/db">
...   <session name="dummy" engine="dummy2" />
... </configure>"""))

>>> component.getUtility(IScopedSession, name="dummy")
<z3c.saconfig.utility.GloballyScopedSession object at ...>

>>> from z3c.saconfig import named_scoped_session
>>> factory = component.getUtility(IEngineFactory, name="dummy2")
>>> Session = named_scoped_session('dummy')
>>> Session().bind is factory()
True

z3c.saconfig

0.9.1 (2009-08-14)

• Include documentation on PyPI.

• Small documentation tweaks.

0.9 (2009-08-14)

• Initial public release.