The persistence package defines guidelines for interacting with
an abstract data storage system. It does not provide any kind of
implementation for an actual database, but instead specifies a generic
storage and retrieval interfaces for working with objects. The key
interface is PersistentStore. The store specifies methods
for schema definition and evolution, saving and deleting objects, making
queries, as well as lazy object graph expansion. The Entity
class is a generic container for object data. It is used to house the
data which will be stored and retrieved. It must contains a reference to
its EntityDefinition.

The bdb package provides an implementation for the persistence interface
for BerkeleyDB. See PSWeb for use cases.
 
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