ZODB

The ZODB is a persistence system for Python objects. It is included as part of the Zope web application server, but can also be used independently of Zope. 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. ZODB manages moving objects in and out of memory for you. The unit of storage is the persistent object. When users 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. The maximum number of objects or bytes in memory is configurable. The ZODB is a hierarchical database. There is a root object, initialized when a database is created. The root object is used like a Python dictionary and it can contain other objects (which can be dictionary-like themselves). To store an object in the database, it’s enough to assign it to a new key inside its container. The ZODB provides a very simple mechanism to roll back any committed transaction. This feature is possible because ZODB keeps track of the database state before and after every transaction. This makes it possible to undo the changes in a transaction, even if more transactions have been committed after it. ZODB uses a special storage for blobs, which makes it feasible to easily handle large files up to a few hundred megabytes without performance problems.