Open Source Java Database Engines/Servers

Firebird RDBMS offers ANSI SQL features & runs on Linux, Windows & several Unix platforms. Features excellent concurrency & performance & powerful stored procedure & trigger language. More information and downloads at https://firebirdsql.org/, issue tracker is https://github.com/FirebirdSQL/firebird/issues

HSQLDB is a relational database engine written in Java, with a JDBC driver, conforming to ANSI SQL:2023. A small, fast, multithreaded engine and server with memory and disk tables, LOBs, transaction isolation, multiversion concurrency and ACID.

The Lemur Project develops search engines, browser toolbars, text analysis tools, and data resources that support research and development of information retrieval and text mining software, including the Indri search engine in C++, the Galago search engine research framework in Java, the RankLib learning to rank library, ClueWeb09 and ClueWeb12 datasets and the Sifaka data mining application.

Mondrian is an OLAP (online analytical processing) engine written in Java. It reads from JDBC data sources, aggregates data in a memory cache, and implements the MDX language and the olap4j and XML/A APIs.

Conductor is an open-source, durable workflow orchestration engine designed to manage distributed systems, microservices, and AI-driven applications at internet scale. Originally built at Netflix and actively maintained by Orkes and the open-source community, Conductor simplifies complex workflow management by handling retries, failures, state persistence, and long-running process execution automatically. The platform enables developers to orchestrate AI agents, APIs, human approvals, and microservices using flexible JSON-based workflows with dynamic runtime execution. Conductor supports durable execution, deterministic orchestration, workflow replayability, and horizontal scalability, making it suitable for enterprise-grade applications and mission-critical automation. It offers polyglot worker support across languages such as Java, Python, JavaScript, Go, C#, Ruby, and Rust, allowing teams to integrate workflows into virtually any technology stack.

The BSBM (Berlin SPARQL Benchmark) suite for benchmarking RDF stores or RDB-2-RDF wrappers over the SPARQL protocol. Pure RDBMS benchmarking via SQL is also possible. Visit http://www4.wiwiss.fu-berlin.de/bizer/BerlinSPARQLBenchmark for more details.

OpenRefine is a powerful Java-based tool designed to work with messy data and improve it. With OpenRefine you can load data, understand it, clean it up, transform it, reconcile it, and augment it with web services and external data. It allows you to do this all from a web browser and in the convenience and privacy of your own computer. OpenRefine keeps all data securely in your computer by running a small server on it, using your web browser to interact with it. When you're ready, then that is the only time the data will be shared outside of your computer. OpenRefine is available in over 15 languages, is cross-platform and part of the Code for Science & Society.

Make AI-driven decisions using your data, in real-time. At any scale, with unbeatable performance. Vespa is a full-featured text search engine and supports both regular text search and fast approximate vector search (ANN). This makes it easy to create high-performing search applications at any scale, whether you want to use traditional techniques or a modern vector-based approach. You can even combine both approaches efficiently in the same query, something no other engine can do. Recommendation, personalization and targeting involves evaluating recommender models over content items to select the best ones. Vespa lets you build applications which does this online, typically combining fast vector search and filtering with evaluation of machine-learned models over the items. This makes it possible to make recommendations specifically for each user or situation, using completely up to date information.