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
DBconstructor 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
connectionfunction:
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.
Containers and search¶
BTrees provide the core scalable containers and indexing facility for ZODB. There are different families of BTrees. The most general are OOBTrees, which have object keys and values. There are specialized BTrees that support integer keys and values. Integers can be stored more efficiently, and compared more quickly than objects and they’re often used as application-level object identifiers. It’s critical, when using BTrees, to make sure that its keys have a stable ordering.
ZODB doesn’t provide a query engine. The primary way to access objects in ZODB is by traversing (accessing attributes or items, or calling methods) other objects. Object traversal is typically much faster than search.
You can use BTrees to build indexes for efficient search, when necessary. If your application is search centric, or if you prefer to approach data access that way, then ZODB might not be the best technology for you. Before you turn your back on the ZODB, it may be worth checking out the up-and-coming Newt DB [6] project, which combines the ZODB with Postgresql for indexing, search and access from non-Python applications.
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.