Zope Object Database

The Zope Object Database (ZODB) is an object-oriented database for transparently and persistently storing Python objects. It is included as part of the Zope web application server, but can also be used independently of Zope.

Features of the ZODB include: transactions, history/undo, transparently pluggable storage, built-in caching, multiversion concurrency control (MVCC), and scalability across a network (using ).

History

* Created by Jim Fulton of Zope Corporation in the late 90s.
* Started as simple Persistent Object System (POS) during Principia development (which later became Zope)
* ZODB 3 was renamed when a significant architecture change was landed.
* ZODB 4 was a short lived project to re-implement the entire ZODB 3 package using 100% Python.

Implementation

Basics

ZODB stores Python objects using an extended version of Python's built-in object persistence (pickle). A ZODB database has a single root object (normally a dictionary), which is the only object directly made accessible by the database. All other objects stored in the database are reached through the root object. Objects referenced by an object stored in the database are automatically stored in the database as well.

ZODB supports concurrent transactions using MVCC and tracks changes to objects on a per-object basis. Only changed objects are committed. Transactions are non-destructive by default and can be reverted.

Example

For example, say we have a car described using 3 classes Car, Wheel and Screw. In Python, this could be represented the following way:

class Car: ..class Wheel: ..class Screw: ..
myCar = Car()
myCar.wheel1 = Wheel()
myCar.wheel2 = Wheel()
for wheel in (myCar.wheel1, myCar.wheel2):
wheel.screws = crew(), Screw()