db4o

db4o is an object-oriented database management system that provides persistence for Java and .NET objects without requiring an object-relational mapping layer. It is designed to be embedded inside applications, storing object graphs directly to disk or in memory. db4o emphasizes transparent persistence, meaning that application code can continue to work with native objects while the database handles storage details.

Key features include in-process operation (no separate database server), a schema-free data model, and support for multiple query approaches such as query by example, native queries, and a variant of SODA-style queries. The database uses an object index to speed up retrieval and supports cascading saves and deletes to persist whole object graphs. Transactions provide atomicity and consistency for operations.

db4o stores data in a single database file or memory structures and integrates with applications via client libraries for Java and .NET. It is often used in scenarios where rapid development is needed and where the object model maps naturally to the domain, including mobile and embedded environments.

History and status: The project originated in the early 2000s as an alternative to ORM-based persistence for Java and .NET. Development continued through the 2000s, with various commercial and open-source components. In the 2010s, official development slowed, and the project moved toward community forks and archived releases; several forks continue in use in niche applications, though active mainstream development has diminished.