1 Introduction

Elephant is a persistent object protocol and database for Common Lisp. The persistent protocol component of elephant overrides class creation and standard slot accesses using the Meta-Object Protocol (MOP) to render slot values persistent. Database functionality includes the ability to persistently index and retrieve ordered sets of class instances and ordinary lisp values. Elephant has an extensive test suite and the core functionality is becoming quite mature.

The Elephant code base is available under the LLGPL license. Data stores each come with their own, separate license and you will have to evaluate the implications of using them yourself.

1.1 History

Elephant was originally envisioned as a lightweight interface layer on top of the Berkeley DB library, a widely-distributed embedded database that many unix systems have installed by default. Berkeley DB is ACID compliant, transactional, process and thread safe, and fast relative to relational databases.

Elephant has been extended to provide support for multiple backends, specifically a relational database backend based on CL-SQL which has been tested with Postgres and SQLite 3, and probably support other relational systems easily. It supports, with some care, multi-repository operation and enables convenient migration of data between repositories.

The support for relational backends and migration to the LLGPL was to allow for broader use of Elephant in both not-for-profit and commercial settings. Several additional backends are planned for future releases including a native Lisp implementation released under the LLGPL.

Elephant's current development focus is to enhance the feature set including a native lisp backend, a simple query language, and flexible persistence models that selectively break one or more of the ACID constraints to improve performance.

1.2 Elephant Goals

• Transparency: most Lisp values are easy to persist without significant effort or special syntax. You can interact with the DB entirely from Lisp. There is no requirement to use domain-specific languages, such as SQL, to access persistent resources. Elephant loads via ASDF and requires no external server (except for some SQL backends like Postgres).
• Simplicity: a small library with few surprises for the programmer. Lisp and Berkeley DB together are an excellent substrate; Elephant tries to leverage their features as much as possible. Support for additional backends are load-time options and more or less transparent to the user.
• Safety: ACID, transactions. Concurrent with good multi-user (BDB) and multi-threaded semantics (BDB/SQL), isolation, locking and deadlock detection. (Deadlock detection does require an external process to be launched for Berkeley DB)
• Performance: leverage Berkeley DB performance and/or Relational database reliability. In addition to fast concurrent / transactional modes, elephant will (eventually) offer an accelerated single-user as well as pure in-memory mode that should be comparable to prevalence style solutions, but employ a common programmer interface.
• Historical continuity: Elephant does not try to innovate significantly over prior Lisp persistent object stores such as AllegroStore (also based on Berkeley DB), the new AllegroCache, the Symbolics system Statice and PLOB. Anyone familiar with those systems will recognize the Elephant interface.
• License Flexibility: Elephant is released under the LLGPL. Because it supports multiple implementation of the backend, one can choose a backend with licensing and other features appropriate to your needs.

1.3 More Information

Join the Elephant mailing lists to ask your questions and receive updates. You can also review archives for past discussions and questions. Pointers can be found on the Elephant website at

http://www.common-lisp.net/project/elephant.

Installation instructions can be found in the Installation section. Bugs can be reported via the Elephant Trac system at

http://trac.common-lisp.net/elephant/.

This also serves as a good starting point for finding out what new features or capabilities you can contribute to Elephant. The Trac system also contains a wiki with design discussions and a FAQ.