Tutorial

This tutorial is intended to guide developers with a step-by-step introduction of how to develop an application which stores its data in the ZODB.

Introduction

To save application data in ZODB, you’ll generally define classes that subclass persistent.Persistent:

# account.py

import persistent

class Account(persistent.Persistent):

    def __init__(self):
        self.balance = 0.0

    def deposit(self, amount):
        self.balance += amount

    def cash(self, amount):
        assert amount < self.balance
        self.balance -= amount

This code defines a simple class that holds the balance of a bank account and provides two methods to manipulate the balance: deposit and cash.

Subclassing Persistent provides a number of features:

  • The database will automatically track object changes made by setting attributes [1].

  • Data will be saved in its own database record.

    You can save data that doesn’t subclass Persistent, but it will be stored in the database record of whatever persistent object references it.

  • Objects will have unique persistent identity.

    Multiple objects can refer to the same persistent object and they’ll continue to refer to the same object even after being saved and loaded from the database.

    Non-persistent objects are essentially owned by their containing persistent object and if multiple persistent objects refer to the same non-persistent subobject, they’ll (eventually) get their own copies.

Note that we put the class in a named module. Classes aren’t stored in the ZODB [2]. They exist on the file system and their names, consisting of their class and module names, are stored in the database. It’s sometimes tempting to create persistent classes in scripts or in interactive sessions, but if you do, then their module name will be '__main__' and you’ll always have to define them that way.

Installation

Before being able to use ZODB we have to install it. A common way to do this is with pip:

$ pip install ZODB

Creating Databases

When a program wants to use the ZODB it has to establish a connection, like any other database. For the ZODB we need 3 different parts: a storage, a database and finally a connection:

import ZODB, ZODB.FileStorage

storage = ZODB.FileStorage.FileStorage('mydata.fs')
db = ZODB.DB(storage)
connection = db.open()
root = connection.root

ZODB has a pluggable storage framework. This means there are a variety of storage implementations to meet different needs, from in-memory databases, to databases stored in local files, to databases on remote database servers, and specialized databases for compression, encryption, and so on. In the example above, we created a database that stores its data in a local file, using the FileStorage class.

Having a storage, we then use it to instantiate a database, which we then connect to by calling open(). A process with multiple threads will often have multiple connections to the same database, with different threads having different connections.

There are a number of convenient shortcuts you can use for some of the commonly used storages:

  • You can pass a file name to the DB constructor to have it construct a FileStorage for you:

    db = ZODB.DB('mydata.fs')
    

    You can pass None to create an in-memory database:

    memory_db = ZODB.DB(None)
    
  • If you’re only going to use one connection, you can call the connection function:

    connection = ZODB.connection('mydata.fs')
    memory_connection = ZODB.connection(None)
    

Storing objects

To store an object in the ZODB we simply attach it to any other object that already lives in the database. Hence, the root object functions as a boot-strapping point. The root object is meant to serve as a namespace for top-level objects in your database. We could store account objects directly on the root object:

import account

# Probably a bad idea:
root.account1 = account.Account()

But if you’re going to store many objects, you’ll want to use a collection object [3]:

import account, BTrees.OOBTree

root.accounts = BTrees.OOBTree.BTree()
root.accounts['account-1'] = Account()

Another common practice is to store a persistent object in the root of the database that provides an application-specific root:

root.accounts = AccountManagementApplication()

That can facilitate encapsulation of an application that shares a database with other applications. This is a little bit like using modules to avoid namespace colisions in Python programs.

Transactions

You now have objects in your root object and in your database. However, they are not permanently stored yet. The ZODB uses transactions and to make your changes permanent, you have to commit the transaction:

import transaction

transaction.commit()

Now you can stop and start your application and look at the root object again, and you will find the data you saved.

If your application makes changes during a transaction and finds that it does not want to commit those changes, then you can abort the transaction and have the changes rolled back [4] for you:

transaction.abort()

Transactions are a very powerful way to protect the integrity of a database. Transactions have the property that all of the changes made in a transaction are saved, or none of them are. If in the midst of a program, there’s an error after making changes, you can simply abort the transaction (or not commit it) and all of the intermediate changes you make are automatically discarded.

Memory Management

ZODB manages moving objects in and out of memory for you. The unit of storage is the persistent object. When you access attributes of a persistent object, they are loaded from the database automatically, if necessary. If too many objects are in memory, then objects used least recently are evicted [5]. The maximum number of objects or bytes in memory is configurable.

Summary

You have seen how to install ZODB and how to open a database in your application and to start storing objects in it. We also touched the two simple transaction commands: commit and abort. The reference documentation contains sections with more information on the individual topics.

[1]You can manually mark an object as changed by setting its _p_changed__ attribute to True. You might do this if you update a subobject, such as a standard Python list or set, that doesn’t subclass Persistent.
[2]Actually, there is semi-experimental support for storing classes in the database, but applications rarely do this.
[3]

The root object is a fairy simple persistent object that’s stored in a single database record. If you stored many objects in it, its database record would become very large, causing updates to be inefficient and causing memory to be used ineffeciently.

Another reason not to store items directly in the root object is that doing so would make adding a second collection of objects later awkward.

[4]A caveat is that ZODB can only roll back changes to objects that have been stored and committed to the database. Objects not previously committed can’t be rolled back because there’s no previous state to roll back to.
[5]Objects aren’t actually evicted, but their state is released, so they take up much less memory and any objects they referenced can be removed from memory.