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2.1 Overview

Elephant is a Persistence Metaprotocol and Database for Common Lisp. It provides the ability for users to define and interact with persistent objects and to transparently store ordinary lisp values. Persistent objects are CLOS instances that overload the ordinary slot access semantics so that every write to a slot is passed through and written to disk. Non-persistent lisp objects and values can be written to slots and will be automatically persisted. In addition, Elephant provides a persistent index which maintains an ordered collection of lisp values or persistent object references.

The use of persistent objects makes coding concise, convenient, and powerful, and makes persistence almost invisible to the programmer. However, Elephant also allows the same basic data dictionary of key/value retrieval that BerkeleyDB provides.

When someone says "database," most people think of SQL Relational Data Base Management Systems (e.g. Oracle, Postgresql, MySql). Those systems store data in statically typed tables with unique shared values to connect rows in separate tables. Objects can be mapped into these tables in an object-relational mapping that assigns objects to rows and slot values to columns in a row's table. If a slot references another type of object, a unique ID can be used to reference that object's table. CL-SQL, for example, provides facilities for this kind of object-relational mapping and there are many systems for other languages that do the same (i.e. Hibernate for Java).

While Elephant can use either RDBMSs or Berkeley DB as a data store, the model it supports is that of objects stored in persistent indices. Unlike systems such as Hibernate for Java, the user does not need to construct or worry about a mapping from the object space into the database. Elephant relies on LISP rather than SQL for its data manipulation language. Elephant is designed to be a simple and convenient tool for the programmer.

Elephant consists of a small universe of basic concepts:

There are a set of more advanced concepts you will learn about later, but these basic concepts will serve to acquaint you with Elephant.

If you do not already have Elephant installed and building correctly, read the Installation section of this manual and then move on to Getting Started.