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## Preface

The Envers project aims to enable easy auditing of persistent classes. All that you have to do is annotate your persistent class or some of its properties, that you want to audit, with `@Audited`. For each audited entity, a table will be created, which will hold the history of changes made to the entity. You can then retrieve and query historical data without much effort.

Similarly to Subversion, the library has a concept of revisions. Basically, one transaction is one revision (unless the transaction didn't modify any audited entities). As the revisions are global, having a revision number, you can query for various entities at that revision, retrieving a (partial) view of the database at that revision. You can find a revision number having a date, and the other way round, you can get the date at which a revision was commited.

The library works with Hibernate and requires Hibernate Annotations or Entity Manager. For the auditing to work properly, the entities must have immutable unique identifiers (primary keys). You can use Envers wherever Hibernate works: standalone, inside JBoss AS, with JBoss Seam or Spring.

Some of the features:

1. auditing of all mappings defined by the JPA specification

2. auditing of Hibernate mappings, which extend JPA, like custom types and collections/maps of "simple" types (Strings, Integers, etc.) (see also Chapter 10, Mapping exceptions)

3. logging data for each revision using a "revision entity"

4. querying historical data

Copyright © 2004 Red Hat Inc.

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