Drools

Drools is a business rule management system (BRMS) and rules engine written in Java by JBoss, now part of Red Hat. It provides a production rule system for modeling and executing business rules and supports complex event processing. At its core, Drools evaluates rules defined in the Drools Rule Language (DRL) against a set of facts represented by Java objects. Rules are organized into a knowledge base and executed through stateless or stateful sessions. The engine uses a Rete-based algorithm to determine rule activations and an agenda to fire rules in a controlled order.

Drools supports decision tables, domain-specific languages (DSLs), and various editors to author rules, and can ingest rules in DRL as well as guided or business-friendly formats. It includes components such as Drools Expert (the rules engine) and Drools Fusion (complex event processing), with historical Drools Flow for process/workflow integration. The Guvnor web-based governance tool provides knowledge-base management and versioning. In modern releases, these capabilities are organized under the KIE (Knowledge Is Everything) group as part of KIE Workbench and the KIE API, with integration pathways to jBPM for business processes.

Drools is designed to be embedded in Java applications and supports both stateless and stateful sessions, as well as query support and event streams. It emphasizes modularity and interoperability, allowing rules to be maintained separately from application code and combined with processes when needed. Drools is released under the Apache License 2.0, reflecting its open-source model and community development.